Timber roof framing


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Timber roof framing
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Mixed Material
Dessauer, Peter F.
Peter F. Dessauer
Place of Publication:
Gainesville, Fla.
Publication Date:


General Note:
UF course AE686 : building technology ; Phillip Wisely, instructor

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Source Institution:
University of Florida
Holding Location:
University of Florida
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All rights reserved by the source institution.
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Full Text






The Following contents are an overview of major

historical developments in the technology and technique

of timber roof framing and construction, from Prehis-

toric times up to circa 1800. Emphasis has been made

in stressing the contributions of the Romans and the

master builders of the Gothic Cathedrals.

Limitations: Timber roofs include framework

of heavy wooden beams; no attempt has been made in this

paper to explain the evolution or architecture of

bamboo or light beam structures. Geographical focus

is on Europe, with only brief mention of the ancient

Near East. The closing pages touch upon some of the

European traditions as reproduced in Colonial America.

For further reading pertinent to to trusses,

see the paper written by David Ferro, Technology 686,

Spring 1976.

/^i, 5P- (DCS&/L.

Peter F. Dessauer


This paper is a completion of an outline and

Bibliography compiled by Fred Harley Huddleston in

March 1974.

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page 1

1. Influence of Climate on Timber Roof Construction and
Design: Medieval Structure: The Gothic Vault, by
James H. Acland, p. 58



In the hot, wet jungle, walls disappeared and the
roof became a steeply pitched umbrella.

Builders in the hot, dry desert constructed heavy
insulating walls with small openings. Flat white
roofs reflected the heat of the sun.

In cold, wet (so-called temperate) climates
builders had to compromise between winter and
summer demands. Dry insulating walls became
variable membranes capable of being opened to
light and air.

Builders in the cold, dry northern deserts.used
double walls, double ceilings, and snow covered
roofs to trap air as an insulator.

Timber is one of the most available building materials

and certainly one of the most renewable. Its use and architecture

in different parts of the world and by succeeding civilizations

depends upon this availability and the climate of each locale

which can determine the design of shelters for which the wood

is used.

page 2

2 Simple Eastern Bearer Roof: Flat Timber Arrangement;
Theory and Elements of Architecture, by Robert Atkinson
and Hope Bagenal, p. 195, fig.88.

In the East to-day, wherever timber is to be had, a common roof is made by
first laying logs or beams across from wall to wall of a mud-brick building. The


logs are spaced near together, covered with reeds or mats, and earth is then thrown
upon the top to a considerabledepth (Fig. 88) and rammed and consolidated,

This form of flat roof is typical of simple structures

in arid zones worldwide: including the Near East, North Africa,

and the Southwest U.S.A. and Mexico. The depth of earth on

top of the timber logs is of considerable depth as insolation

from the sun's rays. The frequence and size of structures

built in this manner depended upon the availability of timber

and timber types and lengths.

page 3

3. Cedar Timbers from
Lebanon ;i.Ibid.,p. 196 /

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poplar, ndjuniper.


In ancient times, before the Roman Empire, the great

sources of timber in the Near East and Mesopotamia were

Lebanon, Taurus, and Amanus; these areas became the markets

for the timber trade, servicing the centers of civilization

in Egypt, Babylon, Assyria, Israel, and the Aegean Islands.

The types of wood available from these areas were pine, cypress,

poplar, and' juniper.

However, by the beginning of the present era (A.D.)

most of the primeval forests of the East were exhausted;

eventual deforestation encouraged arch and dome construction,

especially in the case of large monumental buildings requiring

extensive spans- and interiors.

page 4

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...Temple of Thebes and the Minoan 'i -'i'
House; Ibid., p. 199.

Fri. o .-ELEVATrO o r lImI wA
A FALL. (Aftr Evans.)

FLzure 89b: In the case of the.Egyptian Temples and palaces

the roofs were composed of great stone slabs supported iin

multiple giant columns; this was the only roofing system

known to the ancientt Egyptians for span-ning so great a distance

and creating a sense of monumentality.

Fig ure 90: In Crete and the Aegean Islands the climate

was arid. durin t'he sumer miont2hs;. at other times of the year

rain fal. as c .oom. From frescos dep acting house elevations

one -an interpreted their construction thusly: 2 storey houses

with log bea -earth Lid at:a slight fall or pitch; clay

cement on the oof surface acted as waterproofng; the ci.-rcles

n theme d fwing iht suggest the ends of iogs, plastered or clayed

for Fa ter p(roofIng.

page 5


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5. Robert Atkinson and Hope Bagenal, Theory and Elements
of Architecture. "Roofs" Chapter VI, pp. 175-229,
Volume I, Robert M. Mcbride &Company, New York, 1926.

page 178

page 6


According to Atkinson and Bagenal "the History of

Building is largely a record of the effort to roof larger

and ever larger floor spaces unimpeded by supports"1. The

evolution of timber roofing can be defined according to the

same principle as,stated above- the process of seeking new

methods of spanning greater spaces while removing impeding

ground supports from the interior, and even going as far as

opening the structure itself for more ceiling space.

In Figure 77 on the following page Atkinson and Bagenal

have graphically depicted the course of timber roof development,

dividing this into three major construction classes with

corresponding examples:2

I. Roofs designed to be propped in the center,
the rafters thus acting as beams.

II. Roofs not propped in the center but
exerting a "thrust" resisted by the.mass
of the walls alone.

III. Roofs in which "thrust" is resolved in a
true truss.

In type I which is called "Bearer Beam" or "Prop and

Lintel" the rafters and ridge are upheld on posts at two

ends, at slight pitch, creating little or no pressure or

thrust on walls; this construction required intermediate

1. Atkinson and Bagenal, Theory and Elements of Architecture,
p. 177

2. Ibid., p. 178

page 7

support, a center post(s) or walls. An excellent example.

of the "Prop and Lentel" roof was the system utilized by

the Classical Greeks as exemplified by the Parthenon and

the Arsenal at Piraeus (see slide no. 22): the rafter rests

on three points of support- on the "Hypothema", on the

"Cella" wall,and on the entablature. The advantages of

such a construction are in its simplicity and aesthetics,

greatly enhancing the impression of monumentality and

functionalism of separated "Cella" and colonnade.3

In the "Compressive Roofs", type II, pairs of fixed

rafters exert pressure on walls and thrust. Massively thick

walls or walls with buttressing are required to take up the

thrust, as exemplified by some Gothic Cathedrals with flying

buttresses. "It can give on the inside of a building a fine

unimpeded space and a great height, but the necessary

aboutment system on the outside, as in a Gothic Cathedral,
requires ceaseless attention and upkeep.".

With the Truss (Type II) the outward thrust is counter-

acted by the tie beam which pulls the rafters inward; all

members of the truss triangle are pinned together creating

unalterable rigidity. The only disadvantage with the truss

may be the restriction of head room caused by the horizontal

tie beam.

3- Ibid., pp. 178-79, 184-85.
4. Ibid.

page 8

6. First Primitive Pitched Roofs used ground foundations
to insure rigidity in roofs: Acland, op. cit., p.12.

See Type I- ex. 1, Evolution chart, page 5

The special problem which plagued early builders
was the attainment of rigidity within the cage of
linear elements. They quickly found that a simple
linear cube was perilously unstable until some
means had been found to lock the members into s
position. At first the posts were embedded in the f
ground to make the structure resistant to mechani-
cal shock, winds, or earthquake loads. This worked ""
temporarily but the posis had only a short life be-
cause the alternate wetting and drying of the butts ",
in the ground led to rapid decay.
The sloping diagonals of the roof rafters in the
Germanic long house were ideal to brace the rec-
tangular grid of posts and beams into position. With
Their butts resting on the ground and tied at the pur-
lins and at the ridge, these pole rafters converted the
structure into a rigidly braced triangulated figure.
The rectangle can deform by hinging at the joints,
but the triangle is not subject to such deformation.
This is the principle of the truss: a bra-drl configura-
tion of triangulated elements. In the truss the joints '
.can be hinged or free to rotate because the triangles
convert all loading to tension or compression along
the lines of the members. The raftered roof with a
tie at the base is the simplest form of truss. The pair
of rafters, leaning one against the other, can be
taken as the limiting case for the arch, reducing it to
two wedged elements. Acland, p. 12

Wind loads, earthquakes, and mechanical shocks
overturn simple rectangular framed boxes. The early
builders first embedded posts in the ground and
then braced the rafters against these posts in the
earth lodge to ensure rigidity.

page 9

'**.. op o oeo o *

The nomadic Lapps of Finland used a square of
curved whale bones or birch poles to carry the pole
and turf cover. (after Manker)


The earth lodge can be found from England to Fin-
land, across Siberia to the Japanese islands, and in the
great plains of North America. Each of these variants
has a cone of rafters resting on a four-square grid of
posts in a shallow depression. Robert H. Lowie
Indians of the Plains (New York: American Museum
of Natural History 1963) 34-8; Arthur Drexler The "
Architecture of Japan (New York 1955) 18-19; Kaj
Birket-Smith Primitive Man and His Ways (English
translation, Odhams Press 1960, Mentor Books 1963)

-..... -.

In Nwr Mexico the deeply buried earth lodge even-
tually became the ceremonial Kiva, dedicated to the
earth spirits.


The neolithic builders of northern Japan added a
sheltering cap over the smoke vent in a region of.
heavy rain. The extended pole rafters carrying this
cap became a striking element in later Japanese
architecture. (after Drexler)

7. Primitive Timber Structures of
the Neolithic Period were built
into the earth: Ibid., p. 5

In the subpolar regiotis of the open tundra and the
sparse force, of the taiga, the Lapps of Finland
adapted wh\le ribs and jawbones (or, when avail-
able, birch poles) to create four arched braces fram-
ing a smoke vent and carrying an insulating cover of
turf: a form very similar to the Eskimo summer
earth lodge. The Yakut of Siberia devised a deeply
buried earth lodge with revetted walls to shelter
against the cold. Japanese neolithic builders in the
northern islands took this Siberian prototype earth
iodge from Sakhalin to create the Tateana hut. Pro-
jecting the rafters above the basic cone of insulating
turf, they began that emphasis upon intersecting
roof planes which was to become such a major ele-
ment in Japanese architecture.

At Kamchadal, Siberia, the Yakut buried the earth
lodge with pole-walls to contain the earth. (after

page 10

8. Primitive Lash Construction
of pitched rafters on posts
was recognized as the major

Raftered roofs springing directly from the ground
very neatly solved the problem of rigidity, but once
the roof came to be perched on a framed box entirely
above ground level some means had to be found to
lock the joints together. Before iron tools made it'
possible to cut and interlock linear timber elements,
the builders had to rely upon complex cord or hide
lashings to ensure a firm joint. Lashed frame huts
which are faithful echoes of neolithic modes of
construction continue to be built today. A Mayan
hut at Mulchic, Yucatan, built by the caretaker of an
excavation site, shows exactly the problems faced
by the early builders.
. The builder supported his principal rafters on
posts resting on stone sills. This would be unstable
were it not for the wall panels of lashed vertical
cane, plastered on the inner surface, which braced
the posts longitudinally. The rounded ends of the

encouraged the elevation
and walls; the triangle
element of rigidity: Ibid., p.49

hut provided further rigidity. He tied the tops of the
walls together by setting cross beams into sockets
on the tops of the posts. Setting rafters with
crutches on these ties he lashed the rafters securely
to a ridge pole at the top. For further strength a
collar tie was introduced part way down the rafter.
Without the rigidly braced fabric of the roof the
walling would be quite insecure. The weak point in
the design is the link between roof and wall. If a
diagonal brace were introduced between tie beam
and post the entire fabric would become a com-
pletely braced system. But this would reduce head-
room in the hut and would be an unnecessary refine-
ment in a temporary hut. Once the post and cane
wall is fully plastered as is customary in Mayan
huts, it gains more than sufficient rigidity for long

.Primitive builders continue to use poles lashed to
create rigid triangles. The roof of this Mayan hut,
at Mulchic, Yucatan, sheds rain but allows the free
movement of air to keep the hut dry.

The lashed-pole, thatched-roof assembly

page 11

When agricultural techniques and animal hus-
bandry were introduced into Europe about 2500 BC,
the early farmers soon found that they needed extra
space to store grain and crops. Though the circular
earth lodge sheltered animals as well as men, it was
a form difficult to expand. Size was limited by the
length of poles available for rafters. The Neolithic
farmers met this new need by repeating the central
square of supports used in the earth lodge, to create
an elongated regular plan. The section through this
long house of the north is identical to that through
the earth lodge, but the repetitive bays allowed in-
definite expansion for storage. The frame was, as in
the past, made of lashed poles. Earth berms were
banked up against the low walls of split logs or
interwoven wattle.1 The builders set the butt ends
of the long tapering poles into the berms and braced
them over the two rows of posts. Over the rafters
they placed a close mesh of light horizontal purlins
to carry the thatch, turf or bark roof.


9. Timber Pole Lash Construction
in Northern Europe: Ibid., p. 9
This is another example of the"Bearer"
Roof system.

In this elongated plan, farmers could add as many
bays as were needed for food storage or animal
shelter. The lashed heavy poles of the basic frame
carried a thatched roof. First century farmhouse,
Feddersen Wierde, near Bremerhaven. (after Horn)

page 12


10. Lashed Timber Construction,
Japan, circa 200 A.D.;
Ibid.,p. 10

"Bearer Beam Roof System"

1 Walter Horn 'On the Origins of the Medieval Bay
System' Journal of the Society of Architectural
Historians 17, 2 (1958)
2 Arthur Drexler The Architecture of Japan (New York
1955) 18--19

Over the long reaches of prehistoric time the
stubborn conservatism of Neolithic builders en-
sured a continental dispersal of a few building
forms. Their hard-won competence was based upon
a very few technical devices. The lashed pole,
woven cane, and tied thatch structures could be
adapted to a wide.range of climates and social de-
mands. In Kyushu and the southern Japanese
islands early builders copied the elevated frame
huts of the islands of the South China Sea to create
huts with high pitched roofs to shed the unceasing
rain, platforms above the water and wet ground
which ensured a dry living space, and walls left por-
ous to allow the movement of air. As in Europe, the
structural elements of these walls and roofs became
the decorative motifs for a later monumental archi-
tecture. In Japan rocks and logs tied over the roof to
hold down the thatch against high winds were trans-
lated in the Shinto temples into decorative roof
combs called katsuogi and projecting rafters de-
veloped into pierced chigi.

Around the South China Sea, the lashed frame house
perched on a platform was dry and airy in a hot and
humid climate. Lashed frame Takayuka Hut, Kyu-
shu, Japan, AD 200. (after Drexler)

page 13

11. Primitive Hut on the Palatine Hill: Conjectural
Reconstruction; The Golden House of Nero, by Axel Boethius,
fig. 2, page 4.

Drawing of a primitive B.C. "Bearer-Beam Roof"

with central post supporting the ridge.

12. Exterior of Welsh Folk Cottage or Animal Shelter; this
primitive construction with thatch roof continued into
the last century from prehistoric times; The Welsh House,
by Peate, p. 125.

13. Interior of the Primitive Welsh Cottage: Showing the
unhewn timber poles laid or lashed together rather
roughly: Ibid., p. 124

Many peasant shelters of Northern Europe were of

rough construction- the best techniques reserved for

barns and grain storage; this Welsh cottage is crudely

built, on the principle of the "Bearer-Beam" roof, or

Prop and Lentel, since the rafters are supported by

two posts in turn connected by a lentel. Such crude

architecture was common in areas were large timbers were

scarce, the soil rocky, and livelihood depended upon

grazing flocks.

14. Rough Timber Roof of Peasant Smithery: Farm Houses, Manor
Houses, Minor Chateaux, Small Churches, in Normandy and
Brittainy, Wenzel and Krakow, p. 2

The underside of the roof hood reveals a crude timber

pole arrangement.

page 114.

15. Scientific Carpentry:
Mortise and Tenon:
Acland, p. 123

With iron tools builders could cut and shape timbers
to interlock the elements in a rigid cage. At Biskupin,
Poland, about 700 BC they grooved the heavy posts
to take tenoned logs.

The tightly lashed shell of poles carrying a protec-
tion of thatch or turf gave adequate insulation in
the north but was subject to rapid decay. Moisture
penetrated into the rafters, lashings rotted away,
and the bark or sheathing became damp and mouldy
in a short time. With only a central vent as a flue
and one entry opening, the long house of the north
tended to be dark, damp, smoky, and uncomfort-
able. The solution was to use the same basic plan
but construct it of heavier timbers so that openings
could be placed between the supports. To build a
true frame of this type required better tools than the
bronze axes and polished stone celts of the Neo-
lithic. Only about 700 BC when iron tools became
generally available in Europe, do we begin to see the
emergence of carefully fitted framed structures in
The peasant farmers exploited the new efficiency
of iron axes and adzes to shape and model whole
logs and develop a new form of structure. They
set up vertical posts, cut with grooves into which
were hammered tongued logs, and thus created a
rigidly braced grid wall.' No longer was it necessary
to tie the building fabric together with complex and
delicate lashings. Shaped timber connectors did the
job better and were more lasting. Later builders
learned to dispense with the heavy log or plank
infills between the posts. They put up an open
framed grid of timbers, mortised and tenoned at the
joints to ensure rigidity. The apertures could then
be left open as windows or doors or they could be
filled with panels of woven wattle and daubed clay,
effectively 'separat; re structural f i.r A~ ,. heI tl-

page 15

16. Sophisticated Timber Construction:
Mortise and Tenon Joints:
Acland, p. 50

About 1230 carpenters used alternating braced
frame and arched and braced crucks to ensure
rigidity in the Tithe Barn of the Abbey of Beaulieu
at Great Coxwell, Berkshire. (after Horn and Born)

It is instructive to compare a tithe barn built in
the thirteenth century with the lashed assemblies of
early houses and barns. A thousand years of build-
ing evolution have left the basic form essentially
unchanged a hall with aisles on either side, and a
steeply pitched timber roof tying the three elements
together under a single gable but there has been a
marked improvement in the craft of carpentry. In
place of lashed poles, butt joints, and rough and
ready framing, the medieval tithe barn is con-
structed of carefully shaped and interlocked tim-
hers, keyed together to create a rigidly braced con-
linuu m.
The tithe barn of the Abbey of Beaulieu at Great brace or
Coxwell, Berkshire, was built about AD 1230 with raking strut
rigid frames of oak di-posed in large square bays
along an interior 152 feet in length.' The new em-
phasis upon a rigid articulation of the elements into
braced and keyed frames by the use of diagonal
braces is most evident in the care taken to avoid
deflection or movement of the posts. Because -the
posts were set on pedestals of masonry to protect
them from moisture, there was a special need to
guard against overturning. Longitudinally the posts cruck or arched br
were braced by double tiers of raking struts, cut
with mortises and pegged with dowels to the roof raking strut
plate. sole piece
Transversely across the span the posts were wall plate
joined by tie beams with a lap dovetail joint in
which the wood is cut to lock together, and then tie beam--
raking struts were socketed into the angle to create
rigid portal frames. Finally the posts were braced brace
against the external fieldstone wall pieces. Because post 152" xi5%"_
the major frames were roughly 20 feet apart, some walliece
means had to be found to introduce subsidiary sup-
ports. Between each frame the carpenters introduced corbel
crucks or long arched braces of timber inserted into
the fieldstone wall and wedged against the roof
plate. Long raking struts were then introduced to
key the wall plates to the posts and comparatively
slender roof rafters, 4 inches by 5' inches, locked
the entire assembly into a rigid unit. Finally the
roofers attached the stone slates with wooden pegs
over the laths and rafters of the roof.

page 16

17. Swedish Barns- Prop and Lentel Roof Construction:
BykRadskonsten I Sverge Under Medeltiden 1000-1400,
p. 1.61.

The construction here is the same as the last

large detailed drawing, slide 16, page 15; the section

and plan shows how the rafters act as beams resting

upon an interior system of posts and lentels which

are braced and raftered. This was a very common barn

technique throughout Northern Europe and is prevalent

among old barns in New York and New Jersey, called

"Dutch Barns", having come there via the influence

of the Colonial Dutch. (See slide 67)

18. Scandinavian Timber Barns: Middle Ages: Ibid., p.161

This illustration shows a variety of hewn timber

frame styles of Scandinavian tradition, evolving during

the Middle Ages. The Construction is half-timber: post

and beam skeleton with flushboard exterior; the roof

system is "Bearer-Beam" as depicted in example I-2 of

the Atkinson-Bagenal Chart, page 5 (slide 5).

19. Central Main Shrine, Ise Shrine, Naigu: Living Architecture,
Japanesse, photo #23.

In many aspects this shrine resembles the Scandinavian

timber barns shown in the last slide 18, and has the same


page 17

20. Transverse Section- Kondo od the Toshodai-ji Temple,
Nara: Ibid., p. 122.

The section shows the structural system of the

Japanesse shrine, also practiced by the Chinesse; the

long structural beams were supported by two inner posts;

the rafters were propped on lentels actihg -ast.purlins,

themselves'spanning from post to post; the roof system

is Prop and Lentel, not greatly different in principle

from that of the Greeks.

21. Haida House, Northwest Coast: Acland, op. cit., p.11

Along the Northwest Coast of North America with its

humid marine climate, timbers of immense size made such

dwellings possi e for the When they obtained iron tools, the Haida on the
west coast of North America built plank houses in
Haida. which they used the characteristic square of internal
posts of the earth lodge,

page 18

22. The Arsenal at Piraeus; Classical Greek Roof Type-
Prop and Lentel or Bearer-Beam; Atkinson and Bagenal, o2.. cit.
p. 205
The Greeks took the shape of this gabled oblong and made of it aesthetically
something unsurpassed in history, but from the point of view of construction
they left it quite undeveloped. The roof was given a pitch of about 15 degrees,
and the tiles and the gutters were developed as we shall see (vii. i) to the highest
water-proofing efficiency. But beneath the marble tiles the carpentry is really.
as elementary as the primeval bearer roof we have been discussing. This can best
be illustrated by quoting the roof clauses from Philo's specification for the arsenal
of the Piraeus (4), which owing to Choisy's researches can be clearly followed.
The arsenal differs constructionally from the ordinary Greek temple only in
having its piers inside and cella wall outside; but the result of this is that the

CLAUSES OF PHILO'S SPECIFICATION. (From Choisy's Etudes Epigraphiques.)

lintels which are of wood and the roof timbers above can be the more clearly
compared with primitive origins. The roof clauses begin at line 45. They can
be best followed with reference to the section, Fig. 93.

The rafters rest or are !"propped".on three different

points of support: the exterior wall, the lentel which

spans the interior columns, and the "Hypothema" at the


page 19

23. Reconstruction Drawing from Vitruvius' description
of an Etruscan Temple: A Handbook of Greek and Roman
Architecture, by D.S. Robertson, Fig. 89, p. 197.

Fig. 89. Vitruvius' Etruscan temple (Wiegand's restoration). A = trabes com-
pactiles; M=mutuli; C= Cantherii; F= column; T= temple. (From La
Glyptoddque Ny-Carlsberg, F. Bruckmann, Munich, 1896-1912)

In the Etruscan temple the rafters rested on heavy

purlin beams which spanned the gable ends; these purlins

or "Mutuli" were stepped by degrees at the typanums so as

to create the pitch of the roof. This system was somewhat

similar to the Greek being a method of Prop and Lentel;

however, a difference should be noted here- that the Greek

temple was peristyle in nature, the cella in the center

surrounded by a peripteral, while the Etruscan temple,

as exemplified above, was a-prostyle arrangement',,having

a d6ubie row6'of columns on the front serving as a colonnaded

portico for the cella.

page 20

24, The Vitruvius Truss: Atkinson-Bagenal, op. cit., p. 208

The Roman builders with their instincts for fine construction by no means
confined themselves to the bearer type of roof characteristic of the Greek. We do



not hear of any Roman building that had to remain un-roofed because of its wide
span. Vitruvius recommends two roofs (Fig. 95), one for small, one for large spans.
In the smaller the purlins span from wall to wall and are really loaded beams;
the principle of construction is precisely the same as that of the Piraeus arsenal

A true truss is a triangle or series of triangles

all members connected and in stress, all pinned together

as to be unalterable. The King-Post truss which Vitruvius

described was probably developed by the Hellenistic Greeks

(4-1 centuries B.C.) and was used in Roman Basilicas

spanning widths of 50-60 feet. With the truss the purlins

are not beams in stress but simply framing for the sawn

timber placed vertically for roof covering.

page 21

25. Roman Basilica at Cosa: Roman Architecture, by Frank
Brown, p. 65

The cross section shows the king-post truss

spanning the nave, resting on an-upper.storey:co-

lonnade serving also as clerestory. The side aisles

are covered by prop shed roofs.

26. Three Types of Trusses used by the Later Greeks and
Romans: Histoire des Charpentiers, by Antoine Moles,
page 11.

Figure 24. The simplest type of truss- a pinned

triangle, the tie beam pulling on the "Capreoli"

negating thrust against the walls; the restriction here

is that the ridge beam must span from one gable end to

another, thus limiting the extent of this system.to

whatever timber lengths are available.

Figure 25. This truss has the advantage of the king

post and struts which carry the compressive load of the

roofing material to the tie beam, making it possible to

have fewer trusses spread farther apart with short.ridge

beams spanning the-king posts. Compression on the tie

beam (also in tension) is amply countered by the heavy

joists which span the -width of the peripteral.

Figure 26. By far the strongest truss system: the

King Post is in tension, being pinned underneath the

tie beam; a collar beam in tension assists the tie beam,

equalizing the compressive forces.

page 22

27. Advantage of the King Post
Truss over the Queen Post
Truss: Atkinson and Bagenal,
Theory and Elements of
Architecture, 183, Fig.81

The King Post Truss is a true

truss because by its triangu-

larity all members are in direct

stress; the Queen Post system

in the illustration is not a

truss, it is not a network of

interacting triangles, nor are

all members unalterable,

immovable or fixed rigid.



Truss- An assemblage of members, such as beams, bars,

rods, etc., forming a rigid framework; the basic

design for rigidity is the triangle.

King-post- A post standing on a tie or collar beam and

rising to the apex of the roof where it supports

a ridge-piece.

Queen-posts- A pair of upright posts placed symmetrically

on a tie beam (or collar-beam), connecting

it with the rafters above.

page 23

28. Basilica de Fano, designed
by Vitruvius: Ibid., p. 209,
Figure 95a.

This is a plan and .. H .

interior scene showing

the timber roof trusses, -

supported on a peristyle, '

spanning an open space

of 60'. Other examples

of unvaulted but

trussed basilicas in

Rome were Julia, Trajan,

and Constatine's nave

of St. Peter's (80').

(After H. L. Warren.)
Vitruvius himself built a wooden-roofed basilica in the time of Augustus,
at Fano, which he has described (Book V. Chap. I.) having galleries as a kind of
promenade. The Roman basilica was a building of great importance in Roman
_city life. It adjoined the forum and was used not only as a Hall of Justice but
also as Market and Exchange. As such the aisles with the galleries over them
were of considerable use. An illustration is given of Vitruvius' design (Fig. 95a)
after Mr Warren's reconstruction, in Morgan and Warren's edition of Vitruvius.
Its span between the nave columns was 60 Roman feet, something less than
the same dimension in English feet. Vitruvius especially mentions the agreeable
effect of the view of the roof both outside and inside, and at the same time
mentions the cheapness and simplicity of the design. There is no mention of
roof thrust or of problems arising from roof thrust, which suggests that the
"transtra" did their proper work. They are seen in the figure. In the roofs
of the Basilica Ulpia and of the Pantheon portico (6) bronze members were used
(Fig. 95b). This of itself suggests
the recognition of tensile stress
although doubtless they were also
used for resisting fire.

page 24

29. Later Development of the Roman truss: Introduction
of Bronze Members and Double Tie-Beams; Ibid., p. 210,
Fig. 95b.


Figure 1. Like the roofs of the Basilica Ulpia, the Pantheon

Portico was roofed by a bronze truss, spanning 41' over the

nave; as the section shows, the roof framing is a combination

of truss and bearer-beam.

In the cases of old St. Peters and St Pauls, both

3. JfPaul/ outride the Wa/l/

Figure 2.

page 25

Basilica plans, the truss King-posts were in tension owing

to the pinning beneath the tie beam. In St. Peters the tie

beams were doubled, gripping the King-post between them.

"In the St. Paul's roof the Queen-posts are doubled with

the other members and the King-post hangs free, except at

the bottom where it is pinned to the tie-beams. These trusses

covered spans greater than Gothic vaults and endured for


30..Cross Section Perspective showing the Truss of St. Pauls:
Medieval Architecture, Arthur Kinsley Porter, il. 43, page 68.

Built in the 4/th century A.D.

31. Interior of Saint Demetrius,Thessalonika: Early Christian and
Byzantine Architecture, Macdonald, photo 28.

Built in 412 A.D., with later additions.

32. Romanesque Interior, St. Etienne, Vignory, France: Medieval
Architecture, by Howard Saalman, photo 54.

Early 11/th century truss roof.

33. Romanesque Church in Lombardy, Italy: Byggnadskonsten I Sverige
Under Medeltiden 1000-1400, p. 37

Old traditions die hard, for in this church, built ca. 1025.

the roof is a-bearer-beam construction, the purlins acting as

beams in stress, supporting the roofing materials. The purlins

are divided into separate spans, from one arched bay to another.

5. Atkinson and Bagenal, Theories and Elements of Architecture,
p. 210

page 26

34. Illustrating the Cruck Tradition, Popular in England:
Medieval Structure, The Gothic Vault, by James Acland,
p. 54

Before the thirteenth century medieval builders
emphasized long straight timber members in their
trusses.1 This was perfectly logical because the
straight compression strut is best suited to withstand
buckling or torsion. But in the alternate bays of the
Great Coxwell tithe barn the carpenters used long
curved blades or 'crucks' for their braces. There is
a certain structural illogic to this because the curved
member in compression has to be made heavier for
a given load than a straight strut.
In this instance they appear to have looked back
to the curved members of cruck barns. With the new
pointed-arch detailing of Gothic masonry every-
where triumphant, they seized upon this element,
the curved cruck or arch-brace, as the ideal device to
unify the design of masonry arches and timber
trusses into a consistent vocabulary of design, pull-

ing the work of the carpenter and of the mason into
aesthetic unity. The cruck, the curved brace of tim-
ber characteristic of construction in the north and
west of England, developed from the curved mem-
ber of the Welsh hafod or summer house used by
sheep herders.2 Similar in its basic form to the
Ukrainian cruck huts of Russia, the hafod was a
demountable hut constructed of saplings embedded
in the ground and tied together at the apex.
In time this technique was translated into a per-
manent structure with heavy curved blades resting
on masonry footings. A collar tied the arches to-
gether at the apex and a mortised tie beam projected
through the curved arches to secure a framed verti-
cal wall. This fusion of framed and arch-braced ele-
ments became a characteristic English contribution
to timber construction in the Middle Ages.

The long curved bents or blades used in arched The'cruck' had long been used in primitive barns
braced timber roofs were derived from the frequent and shelters throughout Europe to replace straight
use of 'crucks' or curved braces in barns, rafters, as in this Ukrainian peasant hut.

Atkinson and Bagenal classify this tradition as an

intermediate stagabetween the Bearer-Beam and Compressive

types of roofs, as shown on their chart- page 5.


page 27

35. The English Cruck Tradition:
Atkinsin and Bagenal, op. cit.,
p. 220, fig. 102

The idea was for the cruck to

carry the stresses down to the

foundation or into the walls;

the roofing rafters and purlins

simply lay on the cruck, propped uni/eGu
7erejord re.
to the ridge.

The disadvantage of the cruck

was the fact that it required

a pair of arched beams thicker

and more cumbersome than the Noron Cn'0U1c
vertical posts that could have
(Afier Inm entU.)

36. English. Bearer Beam Roof, Swardestone,Church, Norfolk:
Ibid., p. 219, fig. 101

In this example which is not a truss, a beam

spans the width of the nave and receives the compression

load of the ridge and rafters.through the King-post,

in this case in compression. Actually, the Bearer-Beam

and King-post are working as substitutes for an interior

central post or column.

page 28

37. Two Types of the Early
French Truss-Carolingian
and Romanesque develop-
ments in France:
Ibid., p.215, fig. 97a

C/,) cTedM ueedG'cS d.Be

clj)ecum usedcj ac Tie


(i) The beam is a tie beam in tension, counteracting

the thrust of the rafters, and, at the same time,

bearing weight in compression of the strut(s).

(ii) This is a radical departure from the example

i. above; the strut conducts stress to the King-post

and not the beam.

Viollet-le-Duc measured trusses of these types,

finding timber members 20" thick or more to span a

distance of 50'. Such roof framing was possible as

long as there was a large reserve of timber available.

rom Violdle-l-Due,

page 29


THATCH. o ..




FI .7 5

The development of the timber roof in this country
exceeds that of any other and, in spite of the perishable
nature of the material and the restorations of the last
century, there is still a considerable number of examples
extant, especially in parish churches.
There is little distinctive evolution in the timber roof,
and the various types, with the exception of that of the
hammer-beam, were used indiscriminately. The greatest
changes are in the inclination of the external roof and for
these changes a suggested explanation is given later.
Many are the varieties of the English open timber roofs.
The different kinds may be reduced, broadly speaking, to
the following classifications:
(a) Tie-beam roofs. (b) Collar-beam or trussed rafter
roofs. (c) Hammer-beam roofs. (d) Aisle roofs.
The tie-beam roof is the simplest form of cover for a
building in this country where the climatic conditions
necessitate a distinct slope for throwing off rain and snow.
Its main construction consists of two principal rafters
rising from the walls and prevented from spreading at their
bases by a tie-beam which extends transver-ally from wall
to wall, thus forming a large triangle. The tie-beam,
although used to some extent throughout all the periods,
had four disadvantages: (1) A tall tree was necessary to
produce it. (2) It was likely to sag. (3) Its transport from
where it had grown to the building was difficult. (4) It was
no higher than the top of the walls and therefore gave
limited head room. To protect stone vaults timber roofs
were necessary, and the tie-beam, because of this lack of
head-room, was seldom used for this purpose.

38. Early English Truss and
Tie Beams:
James A. Davidson, An
Outline of_ Medieval
Architecture, page 68



The tendency to sag was partly corrected by cambering
the beam upwards towards the centre. This was often
neutralised by the use of a king-post which rested on the
centre of the beam and extended to the apex of the roof.
For some unknown reason the builders of the Middle Ages
thought that the king-post helped to support the rafters
The modern king-post hangs from the apex and helps to
support the tie-beam.
Embedded in the tops of the walls are wall-plates. To
these the tie-beams are joined. Underneath the tie-beams,
simple, or arched braces are often connected with the wall
to give added support. The principal rafters resting on the
tie-beam are fixed at their apex by the ridge-piece. On the
principal rafters one to three purlins are placed. The
purlins support the common rafters which in turn support
the outer covering of the roof.
The Early English roof had a high pitch which became
lower in later times possibly because the ends of the rafters
rotted and were cut off. Another reason for lowering the
pitch would be the greater use of tiles, slates and lead as an
outer covering. Thatch and shingles was more usual in the
earlier Middle Ages and they required a greater slope. In
the Perpendicular period, where in a low-pitched roof the
tie-beam 'sometimes serves as the principal rafter, the
camber decides the pitch.
The king-post sometimes had the addition of struts to
strengthen the framework. Where two or more posts rest
on the tie-beam they are called queen-posts. (See Figs. 75
and 76.)

page 30

39. Timber Elements in Medieval English Domestic Construction:
Building I En land, by Salzman, p.197


The trusses R and Q in the above illustration are the

same as those described in slides 36 and 37 respectively.

(see pages 27 and 28)

page 31


40. The Arched Brace Roof 6
:and -th"S6cissor Truss
in English Medieval3
Architecture: Davidson,
m. cit., p. 71 DECORATED. COLLARP-fEA/v

Ft 6.77



FI'C 78

Collar-beam or trussed rafter roofs had no tie-beam and
the collar-beam being placed higher up gave more head-
room. The collar-beam being shorter modified the difficulty
of transport and the tall tree necessary for the longer tie-
beam. It also had less tendency to sag. Its use was made
possible by the trussed structure at the lower ends of the
principal rafters. This structure is formed by a sole-piece
put at right angles on the wall-plate and often extending
beyond the outside of the wall thus forming dripping eaves.
To the outer end of the sole-piece is fixed the principal
rafter and from its inner end the side-post extends vertically
to the rafter; thus a triangle is formed which gives sound
support to the rafter. The collar-beam, like the tie-beam,
is often given additional support with simple or arched
braces. Where the arched braces extend from the sole-
pieces to the centre of the collar-beam it is called an arched-
braced roof. If the arch-braced roof is covered on the lower
side with boards, which are sometimes covered with
plaster, it becomes a barrel roof. Scissor-beams are used
instead and sometimes in conjunction with collar-beams.
This kind of roof is seldom found in England. (See Figs.
77 and 78.)

page 32

I Pt. q I It- TI

FI'l G 79 F 80o

41. English Timber Hammer
Beam Roofs of the
Middle Ages: 7,
Ibit-.': p. 2 2'

The hammer-beam roof is, in all probability, a develop-
ment of the trussed-rafter roof. It evolved about the end
of the fourteenth century and is typically English. The
hammer-beam is a lengthened sole-piece of which the pro-
jecting part is supported by a curved brace from the wall-
post, and in its turn it supports a vertical strut, which is
really the side-post moved farther inwards from the base
angle formed by the principal rafter and the sole-piece, an
arrangement which gives greater support to the rafter.
Amongst the varieties of the hammer-beam roof are:
(1) those with hammer-beams, side-posts, curved braces and
collar-beams; (2) those in which there is no collar-beam
and curved braces are carried to the apex; (3) those in
which hammer-beams support curved braces instead of
side-posts, with collar-beams above; (4) those with an arched
rib which, with rising from the wall-post to the collar-
beam, gives additional strength; (5) double hammer-beam
roofs, which have a second range of hammer-beams higher
up to act as additional support to the principal rafters.
The hammer-beam construction continued into the period
of the Renaissance and is one of the chief glories of our
craftsmanship. (See Figs. 79 to 81.)

page 33

42. Cruck and Hammer Beam
Combination for Westminster
Hall: Theory and Elements
of Architecture, by
Atkinson-Bagenal, p. 221,
fig. 104.


Westminster Hall, designed for King Richard II by

Hugh Herland, incorporates three different!timber roof

construction traditions practiced in the British Isles;

this Medieval late 14/th century hall has a roof which

consists of hammer-beams, cruck, and truss, the intention

being to open the interior to create a high and free space.

Thrust from the wall beams and posts, and from the cruck

were countered by the thick walls; buttressing was re-

quired to counteract the thrust of the rafters. Such a

combination was a hybrid of unexcelled aethetics and an

example of the genius of English Medieval carpentry.

page 34

43. Roof Construction- Hammer Beam, Cruck, and Truss- of
Westminster Hall: Theory and Elements of Architecture,
Atkinson and Bagenal,page 222, fig. 10. -

See illustration of the next page, 35; explanation below:

When Herland had to span the same width without intermediate
columns, he could find no timbers long enough either to -reach from wall
to wall as bearers, or to form principal rafters in one piece. He had, there-
fore, to build up a framed truss out of timber lengths. He designed this
truss as it were in three steps. First, starting some 21 feet below the wall-
plate (Fig. 1o4a) he built out two cantilever structures on the hammer-beam
principle, out of hammer-beam C, hammer-post D, wall-post A, and the lower
principal rafter E. The feet of these lower principals, E, with their load acting
vertically downwards pin down the ends towards the wall of the hammer-beams,
C, which at their other ends are strutted up and kept horizontal by means of
bracket pieces, B. These two cantilever structures carry first the trussed purlins, Q.
These trussed purlins consist each of three timbers and have to bridge 19 feet
between the principals. Over them and resting upon them is the upper portion
of the roof truss.
This upper portion consists of a frame in which the cross beam, H, takes the
feet of the upper principal rafters, I, which are also stiffened by struts L and J.
At the same time by means of crown-post, K, .and branching-struts, M, the cross-
beam, H, acting as a bearer supports a ridge-piece, N.
The third step in the design was to introduce two huge cracks or arch braces,
F, serving a double purpose: they both stiffen the lower or cantilever members
and at the same time help to support the upper framed structure at its centre
point. These crucks, primitive in their origin, and serving the primitive purpose
of supporting the roof at its centre are the real feature of the Westminster roof.
The structure began to fail by the decay of joints upon which, as we have seen,
the hammer-beam roof largely depends. Also the purlins, 0, spanning some
19 feet between trusses were too small in section for their load. But for centuries
Westminster Hall has stood a monument to the empiric English craftsman, actually
covering a wider unimpeded space than any English stone vault or, with a few
exceptions, than any vault in the world. Westminster is wider, for instance, than
Albi nave spanning 60 feet, and exceeds the span of Cologne vault by some
24 feet. To surpass it we have to refer to the 70 and 80 feet spans of the Roman
concrete vaults and the true trusses over early Christian basilicas. One effect,
however, of the huge timbers is that the interior does not look its size. The
walls are now bare and there'is'little to give the scale. But this unemphasised
grandeur has its own value and, as a setting for national functions, when myriads
of human figures are present and give a unit of measurement, the building is
unsurpassed.? See footnote 1 under illustration on the -
followinr pape, 35.

page 35

SfCn Y IJ e action XX.
t o 1 s 10 1i oreef 1':o i 2 1 4 s loreet P
k /for 7..u 'i ..

(After Report by Sir Frank Baines, H.M. Office of Works.)

Above illustration taken from Theory and Elements of Architecture,
by Atkinson and Bagenal, page 222.

The length of the hall is 240 feet, the width originally 69 feet. The height to the ridge piece
above the original floor is over 90 feet. The building is divided into twelve bays, about 19 feet 6 inches
centre to centre, with two smaller end bays. 'The roof is of oak throughout, obtained from the king's
wood of Pettelewode in Sussex, from the king's park at Odiham, from the wood of the abbot of St Alban's
at Burnham, and from a wood near Kingston-on-Thames. The oak as originally worked was not
seasoned but allowed to season in situ. Oak pegs were used at first in all joints. The arch brace is of
three members, each 9 x 12 inches, the "collar" beam consists of two members, each 19 x r2 inches,
and +o feet long. The principal rafters are out of 17 x 12 inches. The hammer beam is out of
22.x 21 inches, and 18 feet long. (See sections in Fig. ro4a.) Oaks having diameters of 4 feet to
5 feet, with limbs from 25 feet to 40 feet at this diameter, must have been used. The stone used was
Caen stone and Reigate ashlar." Lead for the roof was had from the High Peak in the County of
Derby. (Blue Book, Westminster Hall, Report by Sir F. Baines, H.M. Office of Works).
I Ellis, Practical Carpentry, 1906, p. 83.
From Atkinson and Bagenal, op. cit., footnote, pages 223-224

JTuncton [e&tien 7russed

a ^


page 36

Medieval English timbermen spent considerable effort inventing
roof trusses that would eliminate the tie beams obstructing the
view of the east and west windows. The solution was the invention
of the hammer beam truss which served the primary purpose
of permitting an unobstructed view of the interior, although it
is not a statically perfect truss. The hammer beam made poss-
ible the construction of beautiful timber roofs, such as the one
in Westminster Hall where the art of timbering reached skillful
refinements that have never since been excelled.

Taken from Man the Builder, by Gosta E. Sandstrom, p.117

page 37

01 lu


-" .""



44. Vo'ab6ldary' for Timber
Roofs of the Hafmer Beam
and Truss Specie:
James A. Davidson,
An Outline of Medieval
Architecture, p. 74.


Aisle roots were sometimes formed by the continuation
of the nave rafters, often with the addition of arch-braces
and wall-posts. In some cases the tie-beam of the aisle
roof was carried through the nave wall to form a corbel
for the wall-post of the nave roof, thus adding to the
strength of the whole construction.
Anglo-Saxon roofs were sloped, but to what degree it is
difficult to state. In illuminated manuscripts their outer
covering seems to have been thatch, slates, tiles or shingles.
Norman roofs were also sloped, in some cases to a very
steep pitch, but in others the ridge was formed at about a
right angle. The framing may have been open to view or
covered with boards and ornamented with paintings. The
outer covering in some cases was lead but was probably
more often of cheaper material.
Early English roofs were usually made with a steep pitch.
Towards the end of the period the pitch began to decrease.
Thatch was often used as a covering, especially in small
parish churches. If the district could produce slates or other
covering they would probably be used. (See Fig. 75.)
Decorated roofs had a similar construction to that of the
Early English, but were often more ornamented with
carving and with a lower pitch. The king-post, when used,
is commonly octagonal with a moulded capital and base.
A type of roof often met with in this style is the arch-
braced, sometimes with or without the aid of a collar-beam.
(See Figs. 75 and 77.)
Perpendicular roofs are, in most examples, of a still lower
pitch than that of the preceding style. This period saw the
development of the hammer-beam type of which there are
many extant. They are often richly ornamented with carved
figures of angels and tracery. During the latest part of the
period, roofs became almost flat and lead is generally used
as an outer covering. (See Figs. 76, 79 to 81.)

page 38



Bays. Compartments into which the roof of a building
is divided.
Brace. A piece of timber fixed underneath the end of a
beam to prevent it swerving. The simple brace is a straight
piece of timber placed diagonally between the beam and
the wall. The arched brace is a curved piece of timber
serving the same purpose, but which is stronger and more
Collar-beam. A beam for tying rafters. It is placed
higher up than the tie-beam.
Common Rafters. These rafters run parallel to the
principal rafters and are supported by the purlins.
Corbel. (Latin: corbis, a basket.) A projection on the
wall supporting a beam or a wall-post.
Hammer-beam. An extended sole-piece which enables
the side-post to be placed farther from the base angle of
the truss, thus giving greater leverage to the rafter. A
second hammer-beam may be placed higher up and partly
resting on the top of the side-post to give added support
to the rafter.
King-post. A vertical post extending from the centre of
the tie-beam to the ridge. The mediaeval type rested on
the beam to help to support the rafters. The modern type
hangs from.the apex and helps to support the beam. The
king-post may be strengthened by struts.
Parapet. A wall around the base of the roof which may
be placed directly on the top of the walls or partly sup-
ported by a corbel-table or cornice.

Principal Rafters. These form the main portions of the
framing; they are, in most cases, placed at regular intervals
dividing the bays. The angle they form determines the
pitch of the roof.
Purlins. The pieces of timber which rest longitudinally
on the principal rafters and support the common rafters.
Queen-posts, When two or more vertical posts are'used,
with or without a king-post and performing a similar
function, they are called queen-posts.
Ridge-piece. A piece of timber running along the upper
angle of a roof, against which the upper ends of the rafters
Tie-beam. A beam which rests on the walls and connects
a pair of principal rafters.
Truss. (Latin: torqueo, twist.) A framed assemblage of
timbers for fastening or binding a rafter.
Trussed Roof. This has a framed construction to support
the principal rafters.
Scissor-beams. A pair of beams crossed scissor-like.
Shingle. A wooden tile made of cleft oak.
Side-post. A strut of timber fixed to the upper and inner
end of the sole-piece or hammer-beam and extending
vertically to the principal rafter, which it helps to support.
Sole-piece. This is a strut of timber which rests trans-
versally on the top of the wall and fixed to the wall-plate:
it forms the base of the truss. On its outer end rests the
rafter and the inner end supports the side-post.
Wall-plate. A beam of wood embedded in the top of
the wall to which is fixed the tie-beam, the sole-piece or
the hammer-beam.
Wall-post. A strut of timber fixed. vertically to the wall

6. James A. Davidson, An Outline of Medieval Architecture,
"Timber Roofs" Chapter IX, Cassell And Company, Ltd.,
London, 1952.


FIGURE 402-Romanesque and Gothic timber roofs: (A) early Romanesque (Italian): rafters resting on
extrados of vault; (B) Romanesque (French): tie-beam introduced to relieve vault; (c) Romanesque (French):
collar-beam used instead of tie-beam; (D) Gothic (French): diagonal braces used instead of re-beanm; (E) early
Gothic roof of Peterborough cathedral with wooden ceiling; (F) Gothic (English): trussed raftr roof with
collar-beam; (G) Gothic (English): curved-brace roof with collar-beam; (H) late Gothic (Englsh): so-called
tie-beam roof. Note low pitch; (i) late Gothic (English): hammer-beam roof

'45. A Variety of Timber Roof Systems Contrived and Used
During the Middle Ages: A History of Technology, by Singer,
Volume II, p. 441.

This shows an array of roofing systems: A-D are

solutions for covering vaults; E-F are open timber

methods with allowance for extra headroom, since no

bottom tie beam is used; E and F are scissor trusses;

G, H, and I are combinations of truss, cruck, and


page 39

page 40

46. Variety of Roof Types for Gothic Churches:
Histoire des Charpentiers, by Antoine Moles, p. 171.

In specific Cathedrals various timber roof

systems were built: such as cruck and central post,

hammer-beam, bearer-beam of rafters and central post,

and many truss types with struts.

47. The Difference Between English and French Timber
Farming in the Middle Ages: The Theory and Elements
of Architecture, by Atkinson and Bagenal, p. 40, fig. 14

The French and English Cathedral builders used

timbers for their roofs from the same species of Oak-

the pedunculate and the sessile. In England the

pedunculate is common the south and has curved and

irregular branches; the sessile, more common in the

north of England, has a straighter stem with higher

branches than the pedunculate.

The difference between the English and French

timber-roof traditions was undoubtedly influenced by

the differences in oak quality harvested in each country.

The English cruck and hammer-bean traditions were perhaps

necessary results due to the irregular and crooked

growth of their woodland oaks; the French on the otherhand

practiced scientific tree farming, planting their oaks

close together, forcing them to grow straight, tall, and

lean, making perfect timbers for trusses and rafters.7

7. Atkinson and Bagenal, Theory and Elements of Architecture,
p. 39 and footnote #1, p. 39

page 41

48. Assembly of
High Gothic
Timber Roofs

Ibid., p.216-- 77e


s of ts te alo s enturyo


tie-beam, K, spanning fromwall plate to wall plate; this tie-beam is supported at three points by beig
Jec^X.V. J C .y

[From Viod-le-Duc.
Trusses of this type also survived as late as the nineteenth century at Paris and Rouen, and were
examined by Viollet-le-Duc and by Choisy.
The following is a rough analysis of the stresses in this truss (Fig. 98). Principal rafters, E, are
stiffened against bending under load in their upper half by a series of horizontal braces, P, R, t, and
angle-pieces, S, T, andhin their lower half by an additional member, G, which increases the effective
depth. These principals, strengthened thus against dead load, are prevented from spreading by a long
tie-beam, K, spanning from wall plate to wall plate; this tie-beam is supported at three points by being
strapped up to the ends of a series of vertical posts, A, H, which play a complex part in the structure.
Resistance to distortion by wind pressure is provided by coupled braces, I, resembling principals in
appearance, running from wall plate to centre post. One ormore of these extra couples is provided in
each principal truss; they are double timbers and grip all the members they cross, so that their stiffening
value is considerable. Intermediate rafters are in the same plane as the principal rafters, and are
strengthened by struts, N, carried upon a bearer, B, and by the purlins, V; they are stiffened near the top
by braces, 0. The horizontal braces, P, R, Q and 0, are not collars" in the modern sense, that is
to say their function is not to help tie in the principals and prevent spreading; on the contrary they are
in compression, and in conjunction with the angle-pieces, S, T, probably act as a kind of web.

page 42

49. Actual Assembly of Cathedral Roof Truss: Cathedral, by
David Macaulay, p. 40.

The roof was made up of a series of triangular frames or trusses. The car-
penters first assembled each individual truss on the ground. The timbers were
fastened together by the mortice-and-tenon method; holes called mortices
were cut, into which the tongues or tenons of other pieces would then fit. After
test assembling every part the truss was dismantled and hoisted piece by piece
to the top of the walls. There it was reassembled and the entire frame was
locked together with oak pegs. Nails were not used by the carpenters in the
construction of the roof frame.
The first few beams were hoisted to the tops of the walls using pulleys hung
from the scaffolding.

4' I"E~(II \1~~f~~ ~ V L

I, -8k~~t BMM b


I .I4 -"

/ ~ 'SP~~E -tfW~ntL -~---~--r

/~ ~~ ~~~ I~3l,;;f ,fBMt~~~~

1 '

r ii

I ~ ~-:i--~ V -

Daae 44

51. Extrados: The Vaults of the Cathedral were built after
the roof timbers were erected; wooden bow trusses served
as supports for the vaults as the mbarar cured.
Ibid., p.56

When the mortar in the webbing had set, a four-inch layer of concrete was
poured over the entire vault to prevent any cracking between the stones. Once
the concrete had set, the lagging was removed and the centering was lowered
and moved onto the scaffolding of the next bay. The procedure was repeated
until eventually the entire choir was vaulted.

52. Illustrating the Technique
of Erecting a High Pitched
Roof of a Gothic Cathedral:
Acland, op. cit., p.51

architect, in co-ordinating-the construction of a
dievat cathedral church, had to draw upon the
Ils of many trades and the technical capacities of
Kialist artisans.

page 46


- Chceur

B Choeur

53- Intrados et extrados
des voiltes

Le Style Gothique de L'oust de La France
by Andre Mussat

D Nef

page 47

54. Le
A Saint-Serge : vue d'ensemble 54 by
B Saint-Serge : nervure pdentrante ",
C Chapelle Saint-Jean 55
D Salle des Malades a St-Jean : nervure pendtrante

Style Gothique de L'oust de la France:
Andre Mussat


Slide 55

Slide 56

page 48

57. Interior of various Extrados: Histoire des Charpentiers,
by Antoine Moles, p. 179, Fig. 216

Forests of timbers characterize the extrados-

space above the vaults where exist the skeleton structure

for the roof. To the lower left hand corner is the

interior structure for a tower roof.

58. Cross Section Perspective of the Extrados over Fan Vaults:
The Master Builders, by Harvey, p. 128, Fig. 121

59. Conical Tower Roof: Byggnadskonstein I Sverige Under
Medeltiden 1000-1400, Erik Lundberg, p. 263

The conical shaped towers and turrets so popular

in the Middle Ages were constructed by a system tw-

of perimeter rafters all leaning towards a central post,

this being well braced by a mass of struts near the apex;

struts and cross pieces held the rafters rigid to the

spiral floor joists and to the central posts,

60. Timber Foundation of Cathedral Spire: Antoine Moles,
op. cit., p.263

The stone of the Gothic Cathedral spiresand

fleches were facing for the actual heavy timber frame-

work underneath. This slide shows the before and after

of restoration- the nude timber skeleton and the exterior

stone skin.

page 49

61. Renaissance Italy- Wooden Truss Supports for Domes:
A History of Technology, by Singer, Vol.III, P.249,
fig. 161.

Brunelleschl's Dome 42Bi420-34) ron the Duomoi in

Florence marked- the beginningg: of an era' in'Etropean

Architecture when-Architects began to design such

features in buildings of monumental proportions.

Domes were usually of double design- an interior

shellqof brick or stone and an exterior of slate

supported on a timber network of bow trusses.

The arched wooden roof of Santa Maria dei Miracoli at Venice (1480) has a
coffered ceiling, also arched; and the timber members of the roof are built up.
in sections (figure 161). As already explained (vol II, p 442), a roof needs a tie-
beam at its springing to prevent it from spreading and thus overturning the
supporting walls, while a collar-beam half-way up its height is only a palliative
and is often ineffective. At Santa Maria dei Miracoli, iron tie-bars have been
inserted across each bay of the church. Such tie-bars may be seen across arches
in the courtyards of many Italian palaces.
Curved roofs with built-up timbers are also found on the town-halls of Padua
(1306) and Vicenza (1560), and in sundry other buildings in north Italy. In
France this type seems to have been evolved independently but concurrently.
Similarly the double-slope or mansard roof (p 253) appeared in Italy and Eng-

------ 4--32- O ---

FIGURE I6x-Renaissance carpentry in Italy. (Left) Centring for the main arches of St Peter's, Rome;
(right) roof of the church of Santa Maria dei Miracoli, Venice.

land at least as early as in France. The designer of the Paduan roof is said to
have copied it from drawings he had made of a palace in India.

page 50

During the sixteenth century a few small domes had been built in French
churches. There were (a) a dome covered with slates over the Church of the
Visitation, later the Calvinist Church, in Paris (1632-4); (b) a high dome over
the church of SS Paul and Louis (1625-41) close by; (c) another over Guarini's
Theatine Church of Ste-Anne-la-Royale (begun in 1662 and since destroyed),
which had the Greek-cross plan that Wren tried to use at St Paul's in 1673; and
(d) yet another over the chapel of the huge Hospital of the Salpetriere (1657),
also in Paris. All these Wren could study, but more important than any of them
is the dome of the Church of the Sorbonne in Paris (1635-56), which is double,
like those in Rome and Florence (p 246). The outer dome is of timber and slated;
the inner, or structural dome, of stone. The lantern is of timber. The inner
dome is about 44 ft in internal diameter and 94 ft above the pavement at its
springing (figure 165).
At the abbey church of Val-de-Grice in Paris (begun in 1645), Wren could
have seen an even finer dome, actually in course ofconstruction. Like that at the
Sorbonne, it is double, with a timber lantern; and, again like the Sorbonne,
the stone attic (parapet stage) of the outer drum rises as high as the crown of the
inner structural dome of stone, thereby helping to resist its outward thrust. This
dome is about 56 ft in diameter, and its springing is 105 ft above the floor
(figure 165).
The much loftier dome of the second Church of the Invalides in Paris
(figure 165) has some affinity with Wren's work, but in this case any traffic of
ideas must have been in the opposite direction, for J. Hardouin-Mansart (1646-
1708), grand-nephew of Frangois Mansart, did not begin it till 1693, when Wren
was hard at work on St Paul's; it is possible that Mansart may have seen engrav-
ings of Wren's 'Rejected Design' (1673), and certainly both architects must
have borrowed ideas from the dome of St Peter's, Rome. Mansart's dome differs
in many respects from the dome actually built by Wren at St Paul's.

62. Timber in French Renaissance
Dome Construction:
A History of Technology,
Singer, Vol.III, p. 225,
Fig. 165

c.44FT- -c 901- -
FIGURE 165-French Renaissance dome-construction. (Left) Double dome of the Church of the Sorbonne,
Paris, 1635-56; (centre) double dome of the Church of Val-de-GrIce, Paris, begun in 1645; (right) triple
dome of the second Church of the Invalides, Paris, 1693-1706. Drawn to uniform scale.

The internal diameter is about 90 ft and the height from the floor to the springing
is over 140 ft. It is of triple construction, like Wren's. The two lower domes are
of masonry, the lowest being pierced by a very large circular opening through
which one can see the underside of the middle dome from beneath. The outer
dome is of timber covered with lead, like Wren's; but the lantern is of wood,
whereas Wren's is of stone and' therefore so heavy that his ingenious brick cone
was neederl to crrv it

page 51

63. The Mansard Roof: Ibid., p.252, Fig. 163

II. FRANCE'S Contribution to European Renaissance Architecture:
The influence of the Renaissance began to make itself felt upon French
building construction in the first years of the sixteenth century. As in England
at the same period, it was confined at first to ornamental details in churches and
to the country mansions and palaces erected by monarchs and nobles. As in
England again, there was a new demand for more spaciousness, light, and
dignity in the homes of the wealthy. The change of taste can in large measure be
traced to the French military campaigns in Italy between 1495 and 1559, which
familiarized the invaders with the novel fashions prevailing there. The actual
design and execution of the buildings in France were carried out partly by
Italian architects and decorators, and partly by native builders and craftsmen
who began to visit Italy for study even before 1500.
Gothic architecture was far more firmly rooted in France than in Italy. Thus
it was natural that the first buildings in the new style retained many medieval
characteristics, such as steeply pitched roofs, prominent chimneys, gables, and
mullioned windows. Vitruvius's work was translated into French and soon
became an authority, as in Italy, but it took longer for the introduction of the
Roman 'orders' to produce drastic changes of design or structure. The principal
innovations in building construction were
in roofs, domes, and windows.
For houses and churches alike, the
steep medieval roof died hard in France.
Indeed, it survived for centuries in a modi-
fied form, that of the so-called mansard
roof, named- from the architect Frangois
Mansart or Mansard (1598-1666) who
did much to popularize it. This mansard
roof, with its double slope (figure 163),
was in fact a good deal older than its
name would imply. It was used in Italy at
least as early as in France, and in Eng-
land for the great hall at Hampton Court
Palace (p 265 and figure 173), constructed
between 1530 and 1540. In France it was FIGE 163-Mansard roofs. (Above) Simple type,
employed in the portion of the Louvre with parapet gutter on left side, and with dormer and
designed by Lescot in 549. eaves gutter on right side; (below) trussed type with
designed by Lescot in 1549. parapet gutter.
The invention of this type of roof,
which is still common throughout Europe,
had a functional origin. When the Renais-
sance reached France, there was a vogue
for very high and steep roofs. These provided inadequate space for attics,
whereas the mansard, in conjunction with dormers, allowed of good attics with
vertical sides, giving an additional habitable room with a considerable saving on
masonry walls. For this reason the mansard has remained popular and has led to
the semi-bungalow type ofhouse. In America it came to be called a'gambrel' roof.

page 52

64. Timber Framing For A Roof Dormer Window: L'Architecture
Rurale Et Bourgeoise En France, Doyon et Hubrecht,
p. 219.

Another feature which became prominent intdomestic

architecture with the Renaissance was the dormer window,

treated with mouldings and detail. The cross section

shows the timber skeleton.

Such a feature, which is by definition a window

placed vertically in a sloping roof and with a roof of

its own, is called a dormer in English from the fact

that it usually serves as a sleeping quarters. In

French the dormer is known as "Lucarne".

65. Ogee and Belcot Form Roof Shapes, Constructed of Timbers:
Histoire des Charpentiers, by Antoine Moles; p. 224,
fig. 281

Besides domes and Mansard roofs, Renaissance

carpentry advanced in other roof styles, such as the

ogee shape roof shown in the slide. The Ogee is defined

by sima recta and sima reverse curves. The slide shows

restoration in progress by charpentiers who are repair-

ing the timber skeleton of an ogee roof.

page 53

66. The Composite Roof of the Sheldonian Theatre: A
History of Technology, by Singer, Vol. III, p.265.
fig. 174

The English as well as other countri-es in the

far north of Europe responded rather retardedly to the

impulses of innovation from the Renaissance of Italy.

Up until the last decades of the 17/th century English

Architecture withthe exception of a few royal monuments,

Was steadfastly Gothic and Medieval in tradition. It

was perhaps the Great London Fire of 1666 which gave

some English Architects the opportunity to rebuild in
the continental styles.



FloGUR 174-Design for the composite roof-trusses of the Sheldonian Theatre, Oxford, by Wren. (Above)
Elevation of a single truss; (below) cross-bracing between the trusses.

Except for the low-pitched roofs introduced from Italy by
Inigo Jones, which might have been designed by Vitruvius
himself, there was no great change in English roof-design
before Wren. Gothic trusses with curved braces continued to
be made, as seen in a Yorkshire farmhouse of 1579 (figure 172),
which also has lateral ornamental wind-bracing of the type
often found in Gothic churches. Hammer-beam roofs, too,
were occasionally built as late as the seventeenth century. That
at Hampton Court (figure 173) was erected in 1531-5 and is
essentially medieval in construction, with Renaissance ornament
in its spandrels, pendants, and corbels. It also has the double
slope associated with the mansard truss.

page 54


Timber roof traditions of Colonial America

were imports from the various Mother-countries which

furnished settlers for the New World. The vast majority

of colonists were farmers,and thus, most Colonial

Architecture is simple, reflective of the agrarian

lifestyle, comprised of half timber houses and barns.

England served as the primary example for North America,

although French influence was predominate in the Miss-

issippi Valley and Canada, and the Dutch in the Hudson.

A variety of medieval timber traditions took root in the

Colonies, exhibiting qualities of pragmaticism and

conservatism. Only in a few exceptions- capitol buildings

and specific houseof the rich- did American Colonial

Architecture come close to emulating the academic,

classical, and Georgian styles of Europe.

67 Cross Section of New World Dutch Barn: The New World
Dutch Barn, by John Fitchen, p. 153, plate 18

This bearer-beam roof construction is an ancient

European tradition brought to America (specifically

parts of New York State and New Jersy) by the Dutch

and is exactly the same as exemplified in slides 16,

17, 18- pages 15 and 16. It is believed that most barns
of this style were built between 1680 and 1720.

page 55

68. Timber System for Gable Roof: Earl Domestic Architecture
of Connecticut, by Kelly, p. 46, fig. 52

The typical New England domestic roof was a

frame of paired purlins held rigid by transverse


69. Cross Section of the New England Colonial Lean-to:
Ibid.. p. 53. fig. 61.

The cross section and detail show how the timber

rafters were laid to produce the Lean-to design.

70. Timber Frame for the Gambrel Roof: Ibid., p. 60, fig.70

The gambrel is not a queen-post truss but a prop

and lentel system with the rafters at two pitches; in

this example roof boarding running perpendicular to the

direction of the rafters would act as stabilizers, no

purlins being necessary.

71. New England Meeting House: Ealy Connecticut Meeting
Houses. Kelly, p. xrxv.

These drawings by Asher Benjamin exhibit a

cross section of the roof- a combination of truss

and prop and lentel, hidden by ceiling surface.

72. Sections of Meetinghouses: Ibid., pxlii

Comparison of truss roof with collar beam-prop

and lentel combination which allows more ceiling height.

73. 74. Ogee Timber Framing. Taylor Office. Williamsburg,Va.:
Historic Buildings of Williamsburg. by Whiffen, fig.87

page 56

88 'ROOFS. [slct.v.

that when the height is one-fourth of the span, or when the angle with the horizon is 26i
The pediments of the Greek temples made an angle of from 12 to 16 degrees with the
horizon; the latter, corresponds nearly with one-seventh of the span. The pediments of
the Roman buildings vary from 23 to 24 degrees: 24 degress is nearly two-ninths of the
span; and is the angle Palladio-recommends for roofs in Italy.*
The kinds of covering used for timber roofs are copper, lead, iron, tinned iron, zinc,
slates of different kinds, tiles, shingles,t reeds, straw, and heath. Taking the angle for
slates to be 264 degrees, the following table will show the degree of inclination that 'may
be given for other materials. .

Kind of covering. Inclination to the hori. Eight of roof in parts Weight upon a square
zrgon in degrees. of span. foot of roofing.

deg. min.
Copper or lead . . .. . 3 50 p er 1 ':00,bs
Slates, large . . . . . 22, 0 1120 -
Ditto, ordinary .. . . . ... 26 ,33 from 00 -
Stone slate .... . . . . . 29 ,,41 2380 -
Plain tiles . . . . . . 29 41 1780-
Pan-tiles ............ .24 0 650 -
Thatch of straw, reeds, or heath 45 0 straw 680 -
Force of. wind does not exceed about 40*00 -

Of the Forms of Roofs for different Spans. .

164.-A roof for a span of from 20 to 30 feet may have a truss of the form shown in
Plate V. fig. 49. Within the limits above stated the purlins do not become too' wide
apart, nor the points of support for the tie beam; and in the table, No. 5, at the end of the
volume, the scantlings of the timbers are given according to the length and bearing of the
different parts. The figure (49) is drawn with a parapet on -one side and eaves on the
other side.
165.--For spans exceeding 30 feet, and under 45 feet, the truss, exhibited in Plate V.

Architect book i. chap. 29.
t Shingles are now very little used in this country, though they appear to have been much used formerly; (see
Neve's Builder's Dictionary, art. Shingle; Britton's Archit. Antiq. Vol. II. p. 79;> and are still much in use in the
West Indies, and in America.

^ e

0 4

I C *(6


A Cd

rO 0$ 50


P. 4.9. ar,-A 16

----------------------- 2je- ------------- ----

fi. 5.. art, 75.


- -
. . __ i. ; . :

I 00*

page 58

saCT. Iv.1 ROOFS. 89

fg. 50, is extremely well adapted; each purlin -is supported, consequently there are no cross
strains on the principal rafters ; and the points of support divide the tie beam into three
comparatively short bearings. The scantlings may be had from the table, No. 6, at the end
of the volume.
The sagging which usually takes place from the shrinking of the heads of the qteen
posts may be avoided by letting the end of the principal rafter abut against the end of the
straining beam S; and notching pieces and bolting them together in pairs at each joint.
The side marked D of the figure, is supposed to be done in this manner. This is further
illustrated in Sect. IX.. See art. 306. The same method is applicable to any other roof,
therefore it appeared bette. to explain it under the head of Joints than under any particular
166.-When the span exceeds 45 feet, and is not more than 60 .feet, the truss shown
in Plate VI. fig. 51, is sufficiently strong-for the purpose, and 14~.s a considerable
degree of free space in the middle. For this span the tie beam will most likely require
to be scarfed, and as the bearing of that portion of the tie beam between a and b is short,
the scarf should be made there. The middle part of the tie beam may be made stronger
by bolting.the straining sill s to it. The scantlings may be got from the table, :No. 7,
at the.-end of the volume; and the principles of scarfing and joints are detailed in Sect.
167.-A truss for a roof from 75 to 90 feet span is shown by fig. 52, Plaie VI. In this
truss the straining sill s should be tabled or keyed, and bolted to the tie beam in the manner
that has already been proposed for increasing the depth of girders; (see Sect. III. art. 143.)
This truss nearly resembles the roof of the Birmingham Theatre.*
168.-By omitting, or rather reducing, the upper part of the truss in fig. 52 to the same
form as that infig. 51, the truss would answer for a bearing of from 60 to 75 feet. The-
scantlings may be had from theAable, No. 8, at the end of the volume.
But when the span is so very wide, unless the building be of a proportional height, the
roof exhibits such an immense mass of plain surface, that it destroys the architectural effect
of the building; besides it is difficult to light the large space in the roof in any way that
would not be objectionable, on account of the external appearance.
169.-To avoid a large expanse of roof, the truss may be of the forin shown in
Plate VII. fig. 53, from Price's British Carpenter. A roof of this form is called an M
roof. This roof would do for a span of from 35 to 65 feet; but it wotild be better to
make the top flat, and cover it with lead, and adopt the truss represented in Plate VI.
fig. 52, as the space gained in the roof would amply repay the expense of the lead flat.

The roof of the Birmingham Theatre is described, with the scantlings of the- timbers, in Nicholson's Carpenter's
Assistant, p. 61, plate Ixxiii. 2d edit


FA;. J52. arte,-JW 17

RiWSelAziz A a Sy z Carey aasitre.


t /2-f

. r_

'--- _____ _ _ -- -, -. 40 r~ct -- -- --- -- -- -- -- -- ------ -- -- ---- - ----- -- -- -- ~-!~ -- -- -- ~;



page 60

90 liOOFS. [sECT. Tv.

The scantlings of the M roof may be got from the table, No. 9, at the end of the
170.-In spans that exceed 65 feet, the truss that was adopted in the. construction of
Drury Lane Theatre, in 1793, is, in respect to form, perhaps one of the best that.catb:.e'
devised of its kind.* Plate VII. fig. 54, shows a roof on the same principle, of which' the
scantlings may be obtained from the table, No. 10, at the end of the volume. One part ot
the principal truss is shown with a queen post, the other with suspending pieces, as described
in art. 506, Sect. IX. The middle part of the principal tie beam is supposed to be built as
a girder.
171.-There is a considerable degree of difficulty in executing a roof when there are a
great number of joints, and the timbers of large dimensions; and the shrinkage of the king
or queen posts often produces"considerable derangements in the truss. It is obvious, that to
make principal rafters in a continued series of pieces abutting end to end against one another
would remedy these defects. These pieces would then form a kind of curve, and according
to the degree of neatness required might be made regular, or left with projecting angles, as
is shown by fig. 55, Plate VIII. These pieces might either be bolted, or mortised and put
together with wooden keys, as represented in fig. 56. The length of the pieces would be
determined by the form of the curve; crooked timber would be preferable for the ribs where
it could be procured, as the joints should be as few aspossible, and they should be crossed,
like the'joints in stone-work.
Plate VIII. fig. 5,7, shows a roof constructed in this manner. Each of the supports for
the tie.beam marked S, S, &c., consists of two pieces, one put on'each side of the rib, and
notched both to the rib and to the tie beam. ''The pieces are bolted together, as is shown
by a section to a larger scale, through one of these pairs of suspending pieces, in fig. 58.
This mode of construction admits of a much firmer connection with the tie beam'than is
procured by the ordinary mode, and the number of suspending pieces may be increased at
pleasure. The best situation for the suspending pieces is at the joints of the curved rib.
The weight of the roof being very nearly uniformly distributed, the form of the curved
rib should be a parabola (see Sect. I. art. 57); and as this curve is easily described with
sufficient accuracy for this purpose, it is best to adopt it; because, in that case, the strain
frp!n the weight of the roof and ceiling will have no tendency whatever to derange the form
of the rib; and its depth will always be sufficient to withstand any partial force to which a
roof is ever likely to be exposed. Consequently, when the rib is of a parabolic form, diago-
nal braces will be unnecessary; nevertheless they may be added if thought necessary, as is
shpwn by the lines in the figure. But it may be remarked, that these braces will increase
the strength to resist any partial strain in a very considerable degree.

A description of the roof of Drury Lane Theatre is givcn in Nicholson's Carpenter's Assistant, p. 60, plate lxxi.

t- J ". "S3. art. -6...




page 62

SECT. Iv.) ROOFS. 91

To construct the parabola, let AB, fig. 59, be drawn for the upper side of the tie beam
Sattd AC,.CB, for the under side of the common or small rafters. Then divide AC and CB
.each into the same number of equal parts (an even number is to be preferred;) and join the
- rpdlitis and 1 2 and 2, &c.; then the curve formed by these intersecting lines will be the
paralola required.
But it will be found that this curve scarcely differs from a circular arc that rises half the
height of the roof: therefore either may be used.
If a lantern or any other structure be to be raised on the top, a hyperbolic curve should
be adopted; which admits of a considerable increase of pressure at the crown.*
The scantlings of the curved ribs are given in table No. 12, at the end of the volume.
The tie beam will require to be scarfed for large spans, and would be best made in two
thicknesses, and joined so that the scarfs should not be opposite one another.
172.-Smaller roofs might be constructed in a similar manner, at a comparatively
small expense. But in these, instead of forming the rib of short pieces, it might be
bent by a method somewhat similar to that proposed by Mr. Hookey for bending ship
If the depth of a piece of timber does not exceed about a hundred and twentieth part of
its length, it may be bent into a curve that will rise about one-eighth of the span without
impairing its elastic force. And if two such pieces be laid one upon the other, and then
bent together by means of a rope fixed at the ends, they may be easily bent to the form of
the required curve, by twistirig the rope as a stone sawyer tightens his saw, or as a common
bow-saw is tightened. The pieces may then be bolted together; and if this operation be
performed in a workman-like manner, the pieces will spring very little when the rope is
gently slacked; and it is advisable to do it gradually, that the parts may take theii proper
bearing without crippling.
Otherwise, a piece of about one-sixtieth part of the span in thickness may be sawn along
the middle of its depth, with a thin saw, from each end towards the middle of the length,
leaving a part of about 8 feet in the middle of the length uncut. The pieces may then be
bent to the proper curve, and bolted as before.
In either case the rise of the ribs should be half the height of the roof; and-they should
be bent about one-fourth more, to allow for the springing back when the rope is taken off.
A roof of this kind for a 30 feet span is shown by Plate VIII. fig. 60. The suspending
pieces are notched on each side, in pairs, and bolted or strapped together, as shown by
fig. 58.

The most simple method of describing this curve is given by Nicholson, in his Carpenter's Guide, p, 11 which
work I suppose every carpenter to be in possession of.
t Transactions of the Society of Arts, Vol. XXXII. p. 95.


. art>.e 7

Ae. 0. are.]M.

P4y. 58-

o -




"The advantages of this roof consist in the- small number of joints in the truss, in
being able to support the tie beam at any number of points, in admitting of a firm and
simple connection with the tie beam, and in avoiding the ill effects attending the shrink-
ing of king or queen posts. The scantlings are given in table No. 11, at the end of the
173.--In the construction of modern roofs a continued tie at the foot of the rafters is
always necessary, though sometimes it has been omitted, for in general the lightness of the
walls renders them incapable of sustaining much lateral pressure; and this pressure is
entirely removed by a straight tie beam.
As leaving out the tie beam gains only a very small space in height, and which might
generally be obtained without injury to,the external effect of the building, by raising the
walls a little higher, I will endeavour to show the defects of roofs without tie beams.
In the first place, let fig. 61, Plate IX. represent a roof with a collar beam Cc,;the
whole weight of such a roof is sustained by the parts of the rafters AC, and Be, and when
the roof has the weight of the covering upon ft, it.will settle in proportion to this weight,
in consequence of the lower parts of the rafters bending at C, c, which will press out the
walls. The reader who has thoroughly considered the effect of weight in producing
flexure, will readily see that outward pressure against the walls with some spreading of the
feet cannot be avoided in this construction, though it may be lessened, by making the rafters
very strong at the lower part. I have often observed failures from adopting this form for a
174.-In wider spans another mode of construction has been employed, which, though
better, is not a good one, from the powerful strains that are excited in it by the oblique
disposition of the beams. To show the nature of these strains, I have taken fig. 62 from
Mr. Price's work,* who has said much in praise of it, without being capable of investiga-
ting its construction according to the principles of mechanics. The essential parts of this
roof are contained in fig. 8, Plate I.; and by comparing the strains produced by the
weight in that figure, with the strains when CA is in a horizontal position, it will be found
the strains are more than doubled by the oblique position of CA. Returning again to the
section of the roof in Plate IX. Let the vertical line aE be drawn, fig. 62, and let a b
upon this line represent the weight of half the roof; also draw c b parallel to AC, and c a
parallel to, AD, Then the weight and pressures will be measured by a b, bc, and a e.
But if there .had been a tie beam AB, the pressures produced by the same weight would

British'Carpenter, plate K, fig. L. In the later attempts to give strength to roofs of this form the beams AC, BC,
cross each other, and are connected to the principal rafters; the construction is not so improved by the alterations as
to prevent the application of everyobjection here urged against the roof, for the objections are to the principle, and
apply to a roof formed out of one piece of iron as well as to a wooden one; but of course the more imperfect the
construction, the more the walls will be forced out by the flexure and settlement.

page 65


have been only b d and a d; hence it appears, that they are nearly doubled, while the space
gained in the middle in height amounts only to about one-ninth of the span.. To gain this
small advantage, we encounter the difficulty of making a firm connection of the ties at C,
with the-certainty of a considerable change of figure from flexure, the settlement from the
number of the joints, and the magnitude of the strains. It also must be remembered, that
whatever degree of settlement takes place will tend to move out the walls, and that the
same degree of settlement will produce a greater effect in thrusting out the walls in
proportion as CE is greater. Having thus pointed out the defects of this kind of roof, I
shall leave the reader to judge for himself on the propriety of adopting it.
S175.-The centre aisle of churches being often higher that the side ones, the same effect
as when the tie continues through may be produced by connecting the lower beams, by
means of braces, to the upper one, so that the whole may be as a single beam. .To
illustrates this principle I have drawn the roof in fig. 63, which is for a church similar, to
St. Martin's in London. Here the lower ties, B, B', are so connected to the principal tie
beam AA', by means of the braces b, b', that the foot of the principal rafters P, P', cannot
spread without stretching AA'. The iron rods, a, a', perform the office of king post to the
ties B, B'; and are better than timber, because the shrinkage of timber ones would be
particularly objectionable in that situation.' The oblique positions d d' will render them
effectual in opposing the spread of the rafters.
F(q. 64, Plate IX. is a sketch of the roof of Westminster School, from Smith's Speci-
mens of Ancient Carpentry.* It shows the most usual form of the roofs of Gothic halls, as
they differ more in the tracery and ornaments than in the essential parts of the framing.
The timbers are so disposed as to throw the pressures a considerable way down the walls,
and at the same time nearly in a vertical direction. Indeed, considering the effect that was
intended to be produced, and the massive walls for abutments, the arrangement of the parts
is worthy of much praise.

On Proportioning the Parts of Roofs.

176.-The proportions of the timbers depend so much on the design of the framing of a
roof, that it would be impossible to furnish rules that would apply directly to all cases;
nevertheless, by considering a few combinations, the method that may be adopted will
be seen and consequently may be applied to designs made on other principles Ihan those
already shown. In roofs, as in floors, I have taken the constant' number in each case
from a comparison of roofs already executed, and known to stand.

Plate viii. Smith's Specimens would have been valuable if they had been accompanied with dimensions, and' a
short description of each.

art& 173

art: 17S

u3 I- -I


iBeP-s9I~9 E



.f. 639. art. 175.




Pi. 6'2. awn 74..

-- ---- - -- - - -

page 67



195.-A DoME or cupola is a roof, the base of which is a circle, an ellipsis, or a polygon;
and its vertical section a curve line, concave towards the interior. Hence, domes are called
circular, elliptical, or polygonal, according to the figure of the base.
The most usual form for a dome is the spherical, in which case its plan is a circle, the
section a segment of a circle.
.The top of a large dome is often finished with a lantern, which is supported by the fram-
ing of the dome.
:196.-The interior and exterior forms of a dome are not often alike, and in the space
between, a staircase to the lantern is generally made. According to the space left between
the external and internal domes, the framing must, be designed. Sometimes the framing
may be trussed with ties across the opening; but often the interior dome rises so high that
ties cannot be inserted: in the latter case, the observations made on the equilibrium of domes
in Sect. I. (art. 62-66,) should be attended to.
Accordingly, the construction of domes may be divided into two cases; each of which it
will be my next object to make a few remarks upon.

On the Construction of Domes which admit of Horizontal Ties.

197.-A truss for a dome where horizontal ties can be inserted is shown byfig. 71,'Plate
XII. In this figure AA is the tie; BB posts, which may be continued to form the lantern;
C, C, are continued curbs in two thicknesses, with the joints crossed and bolted together; DD,
a curved rib to support the rafters. This design is calculated for a span of about 60 feet,
and may be extended to 120 feet.
Two principal trusses may be placed across the opening parallel to each other, and at a
distance equal to the diameter of the lantern apart, as AB, CD, fig. 72; with a sufficient
number of half trusses to reduce the bearing of the rafters to a convenient length.
Or, the two principal trusses may cross each other at right angles in the centre of the
dome, the one being placed so much higher than the other as to prevent the ties interfering.
This disposition is represented in fig. 73; and is the same that is adopted for the Dome des
Invalids, at Paris, of which the external diameter is nearly 90 English feet.

page 68

SECT. v.] DOMES. 106

As the dimensions of the parts must depend chiefly on the weight of the lantern, it is
scarcely possible to fix upon any, unless some particular design had been described, which
would not have been satisfactory ; besides being likely to mislead, as there are few cases'
that are similar. The dimensions of the timbers may, however, be easily ascertained to any
particular design, from the rules and principles laid down in Sect. I. and II.

On the Construction of Domes without Horizontal Ties.

198.-The construction of domes without horizontal cross ties is not difficult when there
is a sufficient tie round the base. The most simple method, and one which is particularly
useful in small domes, is to place a series of curved ribs so that the lower ends of those ribs
stand upon the curb at the base, and the upper ends meet at the top, with diagonal struts
between the ribs.
When the. pieces are so long, and so much curved that they cannot be cut out of timber
without being cut across the grain so much as to weaken them, they should be put together
in tlicknesses, with the joints crossed, and well nailed together : or, in very large domes,
they should be bolted or keyed together. The manner of forming these ribs has been
already described, as applied to roofs, (see Sect. IV. art. 171.) This method of making
curved ribs in thicknesses has been used in. the construction of centres for arches from the
earliest period of arch building; and it was first applied to the construction of domes by
Philibert de Lorme,* who gives the following scantlings for different sized domes:

For domes of 24 feet diameter 8 inches by 1 inch.
36 10 .. 1
60 13 2
90 .. .. .13 2
108 13 . 3

These ribs are formed of two thicknesses, of the scantlings given above, and are placed
about two feet-apart at the base. The rafters are notched upon them for receiving the
boarding, and also horizontal ribs are notched on the inside, which gives a great degree of
stiffness to the whole.t Fig. 74 is a section of a dome constructed in this manner; and fig.
75 a projection of a part of the dome, with the rafters and inside ribs.
If the dome be of considerable magnitude, the curve of equilibrium should pass through

See his Nouvelles Inventions pour bien Batir ( Petits Frais, 1561.
t Mr. Price proposes a similar mode of forming bridges and domes in his British Carpenter, p. 26 and 28,

page 69

106 DOMES. [sECT. v.

the middle of the depth of the ribs, particularly if a heavy lantern rests upon them. The
curve in either case will be found by means of art. 64 or 65, Sect. I. Otherwise the curve
must fall within the curve of equilibrium, and struts must be placed between the ribs, to
prevent them bending in. Or, if it be necessary for the external appearance of the dome,
that the curvature of the ribs should'be without the curve of equilibrium, then an iron hoop
may be put round about one-third of the height to prevent the dome bursting outwards.
This latter method was adopted in the external dome of the church de la Salute at Venice,
the outside dimensions of.which are 80 feet diameter, 40.5 feet high, and the lantern 39-5
feet high ; but the lantern is supported by a brick dome, which is considerably below the
wooden one. The ribs of this dome are 96 in number, and each rib is in four thicknesses;
the four together make 5-5 inches, so that each rib is 8-5 inches by 5-5 inches. The iron
hoop is 4-5 inches wide, and half an inch in thickness, and is placed at one-third of the
height of the dome.
199.-When a dome is intended to support a heavy lantern, it may require the principal
ribs to be stronger than can be obtained out of a piece of timber; but the framing may
always be made sufficiently strong by using two ribs, Wiith braces between, and tied together
by radial pieces across from rib to rib. A truss of this form is shown by fig. 76, which
would sustain a very heavy lantern if the curve of equilibrium were to pass in the middle,
between the ribs, as the dotted line does in the figure. The proper form for the curve
will be found by the equations, in art. 65, Sect. I.
200.-Where a light dome is wanted, without occupying much space, the ribs may be
placed so near to each other that the boards may be fixed to them without rafters, or short
struts may be put between the ribs, as shown by fig. 77.
In a splendid collection of specimens of carpentry, published by M. Krafft, there are.
methods of finding the position of the principal timbers in domes and roofs shown, which
were proposed by M. Stierme, a carpenter of Wirtemburg.* Krafft's work contains no
explanation of these methods, and they appear to me to be destitute of anything like sound
principle, particularly as applied to domes, and are only noticed as a caution to the young

* Recueil de Charpente, par M. Kraffl, deuxiinie parties, plunche 70 et 34.

80. Ibid.. plate

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page 70


. .R7v,7-n'- So*.

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PrJk. 4- art, 98.






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