The longitudinal shrinkage of redwood


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The longitudinal shrinkage of redwood
Physical Description:
Koehler, Arthur
Luxford, R. F ( Ronald Floyd ), 1889-1984
Forest Products Laboratory (U.S.)
University of Wisconsin
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
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aleph - 29508662
oclc - 757529015
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Full Text


January 1931

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Madison, Wisconsin
In Cooperation with the University of Wisconsin

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Department of Silvicultural Relations
R. F. LUXFORD, Associate Engineer

Redwood and one or two other native species of wood have the reputation of
shrinkage excessively along the grain. Seemni.ily extravyant statements of
butt joints in redwood structures opening up to an unusual degree have been
made, yet measurements made at the Forest Products Laboratory on several
hundred pieces of redwood from different trees, indicate that the great bulk
of redwood does not shrink excessively along the eraln. On the other hand,
an examination of redwood siding on a large number of houses in Los Angeles
revealed frequent instances of abnormally wide openings in butt joints. In
some instaices the openings had been filled in with putty, and in others
sheet metal had been nailed over them. Only some of the butt joints had
opened upD, however; others were as tight as could be desired. Small slivers
for microscopic examination were taken from 50 pairs of pieces of siding
adjoining as many butt joints that had opened from 1/8 to 9/16 inch, to see
if the wood were abnormal in any way.

A microscopic examination of 100 pieces of redwood siding formirng open butt
joints, already mentioned, showed that all but four of the 50 pairs had com-
pression wood (Fig. 1) on either one or both sides of the joints. Of the
four pairs that did not have compression wood on either side of the open joint,
one member of a pair was wavy grained, one member of another pair was found
to have comnoression wood some distance from the joint, and the other two
pairs showed indications of having been put on in the first place with open
joints, since the nails at the end showed no indication of pulling toward the
end as those at practically all of the other open butt joints did. Compres-
sion wood, therefore, was almost the universal cause of open butt joints in
the siding examined. It should be mentioned that the smiles were taken
almost entirely from joints in which the gap was especially large.

Recently a large number of measurements of the longitudinal shrinkaLe of
redwood have been made at the Forest Products Laboratory on both virgin and

1ubished in The Timbrmn, January 191
-Published in The Timbturman, January 1931.


second growth. The longitudinal shrinkage of some SOO strips of virgin and
second-growth redwood was plotted against their specific gravity (Fig. 2).
Some of the spIecimens had abnormally large shrinka-:e. A careful examination
of the 59 specimens having a shrinkage of threo-tenths of 1 percent and over
showed that 4l consisted of compression wood, and of the remainder two were
cross grained and the others had either very little or no sur-erwood. Ex.
clusive of the points above 0.300 percent shrinkage, which for the most part
represent abnormal wood, as just stated, there is a general downward trend in
shrinkage with increasing specific gravity. Pieces of the soft, light spring-
wood whittled out by themselves showed the most longitudinal shrinkage and
pieces consisting entirely of the hard, dense summerwood showed the least
shrinkage from the green to the oven-dry condition, in fact, some of the
summerwood specimens elongated during the tests in the laboratory.

Measurements on second-growth redwood indicate that it has a larger percentage
of specimens with relatively high longitudinal shrinkage than virgin growth.
This is especially true of the more openly :-rown material. Of the specimens
of wood from trees that grew in close proximity to one another, 64.5 percent
shrank less than two-tenths of 1 percent, -n. of those that grew more openly
56.0 percent shrank less th:u' this limit. This difference between virgin and
second-growth wood w,.as largely due to the presence, in the' second-growth
specimens measured, of a certain amount of the wide-rieged wood with non-dense
summerwoo-l having different structure already referred to.

The question as to whether redwood contains appreciably more compression wood,
wood light for the species, and spiral, wavy or curly ::rain, cannot be defin-
itely answered at the present time, since it would require the t.xa;i.,tUion of
a large number of representative trees of each species to give a reasonable
answer, and that has not been done so far. Of 56 old growth trees examined
in connection with a specific gravity survey of redwood by the Forest Products
Laboratory, 62 percent contained compression wood. This may be a larger per-
centage of trees than in most other species, but that fact does not necessar-
ily mean that a larger percentage of pieces of redwood lumber contain com-
pression wood than occurs in other commercial species. Compression wood is
peculiar in that frequently it is formed only at irregularly occurring times
in a tree's life. A large tree like redwood, from 500 to over 1000 years old,
would, therefore, have a greater chance of developing compression wood during
some time in its life than a tree of a smaller size and lower life limit.

Redwood may possibly contain a greater percentage of wood light in weight for
the species than is found in several other trees, since the very narrow
annual growth rings found in many of the old trees and 'the very wide rings
in some of the second-growth trees contain a high percentage of springwood,
which on account of its different cellular structure has a greater potential
longitudinal shrinkage than the summerwood.

There is no reason to believe that redwood has more spiral grain than other
conifers. On the other hand, less diagonal grain is produced artificially in
sawing redwood than in cutting up some of the smaller, more crooked trees of


other species. Hence spiral and diagonal grain do not seem to be any more
responsible for abnormal shortening of pieces of redwood than of other species.

Redwood may perhaps have a little more wavy grain than many other conifers,
since wavy-grained boards are found rather frequently in redwood lumber.

The long length in which redwood lumber is usually cut and frequently used
does cause greater absolute shortening of the members during drying than
would occur in shorter lengths of the same or other species, other things
being equal.

Since the bulk of redwood does not shrink a great deal along the grain,
where did redwood get its reputation for excessive longitudinal shrinkage?

The opening of butt joints, no matter what the species or type of wood,
shows that excessive shrinkage occurred after the wood was put in place,
since butt joints, are usually made tight when constructed. The fact that
the wood at the ends of siding next to the open butt joints previously re-
ferred to frequently had sheared along the grain at the nails, causing
noticeable displacement, is further evidence that shrinkage took place after
the siding had been nailed on and that the open joints in general were not
due to faulty carpentry. Widely open butt joints, therefore, mean in general
that the wood dried in place, which in turn means that it was not dry enough
when fastened in position. Even if it is dried, some openings in butt joints
may result from using lumber with a high potential shrinkage. Such lumber
would expand in damp weather, crushing its ends or displacing abutting pieces,
and when it subsequently shrank in dry weather it would leave open joints.
Much redwood lumber has been marketed and used in an insufficiently dry con-
dition; this is known definitely.. If the timber had been properly dried
before use much direct end shrinkage could have been avoided and many of the
pieces with abnormal shrinkage would have been thrown out on account of
crookedness and would not have been used for the better class of work, and
thereby trouble in use caused by end shrinkage would have been further min-

Siding of other species, if inadequately dried before use, would also show
occasional open butt joints caused by compression wood and other abnormal
types. Most other species of wood, however, could not be marketed in a wet
condition without quickly deteriorating from stain and decay. Redwood, on
account of its high decay resistance and natural dark color, which does not
show stain readily, can be shipped and stored in a wet condition for longer
periods of times

If all virgin redwood had been properly dried before it was put into use and
if the crooked pieces had been thrown out, most of the stock that has given
redwood its reputation for excessive longitudinal shrinkage would have been
eliminated and the size of the open joints caused by the abnormal pieces
that were not eliminated would have been greatly reduced.



Figure l.--Cross section of an 18-inch second-growth redwood log
showing compression wood on the lower side. The wide rings
with wide summerwood constitute the compression wood.

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Figure 2.--Longitudinal shrirLkae of redwood in relation to its
z v 42136 F specific gravity.


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