U. S. Department of Agriculture, Forest Service
FOREST PRODUCTS LABORATORY
In cooperation with the University of Wisconsin
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EFFECT OF STORAGE ON THE MOISTURE
CONTENT OF LUMBF.R _,o
-'"' CUMNIV or PI
-DOC uMEN T
By J. S. MATHEWSON
Senior Engineer . -
i US DEPOSITORY
August 1, 1935
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Digitized by the Internet Archive
EFFECT OF STORAGE ON TIHE MOISTURE CONTENT OF LU`1rER
J. S. Mathewson, Senior Engineer,
Forest Products Laboratory,1 Forest Service,
U. S. Department of Agriculture
The fullest efficiency and satisfaction to be derived from the
use of wood in arny construction are in large part dependent upon its
moisture content at the time of installation. If lumber manufacturers
and retail dealers sell only good and dry lumber, there is no apparent
reason why a larger demand for wood products will not be forthcoming.
It has been the practice at some mills to machine and ship
thoroughly seasoned lumber to wholesale and retail yards only to have it
there stored in piles unprotected from the weather. Such practice largely
offsets the benefits of proper seasoning because the wood may absorb moisture
to an undesirable extent before it is used. In order to obtain definite
information upon the effect of storage on the moisture content of lumber
the Forest Products Laboratory recently made a survey of storage methods
at sawmills, at wholesale and retail distributing yards, and at wood-using
plants in various parts of the United States.
Results of Survey
Illustrative of the results obtained is figure 1, relating to a
study at a Louisiana sawmill. Somewhat similar results were obtained at
a Florida savwmill and at several distributing yards in the vicinity of
Chicago. In the case of the Louisiana mill the stock was southern
yellow pine 1 by 4-inch by 12-foot flooring and 1 by 8-inch by 12-foot
boards surfaced four sides and solid piled, that is, piled without
stickers. A group of boards selected by means of a moisture meter for
uniformity of moisture content was piled in each of the following
Maintained at Madison, Wis., in cooperation with the University of
locations: (1) In a yard where the pile was well protected by a tight
roof, (2) in a partly open shed with metal roof, (3) in an open shed with
wood roof, and (4) in a closed shed with a wood roof. Each curve in
figure 1 represents the changes in average moisture content ef 0O boards.
It may be noted that the average moisture content values,
which were initially about 7.5 percent, ranged between 10 and 11 percent
after 10 months in the three sheds. In the yard the average moisture
content reached 13*5 percent during the same period. These are average
values; the ends and other surfaces of the boards exposed directly to the
air would have moisture content values higher than the average. The
relatively large absorption of moisture at the ends is objectionable
either in the rough lumber or in a finished product such as flooring.
In the rough lumber the boards will have a uniform width only at the
time of machining; subsequently the ends will shrink and will be narrower
than the remaining portions of the boards. In a finished product,
assuming that the boards are fairly uniform in moisture content when
machined, subsequent absorption of moisture mostly at the ends will
cause the ends to be wider than they should be.
Control of Moisture Content
To prevent such undesirable changes in moisture content as
just described, it is necessary to provide some means of controlling the
moisture conditions to which the lumber is exposed during storage. As
in a dry kiln, the control of moisture content within the shed is a
matter of temperature and relative humidity, and this is comparatively
simple. Control of the relative humidity in a storage shed can be
accomplished by controlling the temperature.
Referring to figure 2, it may be noted, for example, that at
the average indoor temperature of 70 F. and relative humidity of 42
percent the equilibrium moisture content of wood is 8 percent. This means
that for the most satisfactory service such products as flooring and
interior finishing woodwork in heated dwellings in most parts of the
United States should be installed at about S percent moisture content.
Lot us suppose that in an unheated shed the temperature and
relative humidity are respectively 300 F. and 75 percent. According to
figure 2 the corresponding equilibrium moisture content is 15 percent.
If the shed is tightly constructed so that no extraneous sources of
moisture are present, the equilibrium moisture content can be reduced to
6 percent by merely heating the air in the shed to about 45 F. (See
dotted line in fig. 2.) In this way steam sprays, water sprays, or
refrigeration are not required. Further, the heat loss under this
condition is less than if a common temperature of, say, 60 or 70 were
Throughout a considerable range of temperature and relative
humidity a fairly constant difference in temperature will give a desired
reduction in equilibr'iun moisture content. To apply this principle an
instrument called a differential thermostat was constructed with two
mercury-filled.coitrol bulbs. One bulb was placed outdoors and the other
inside a tightly-constracted shed about 10 feet wide, 20 feet long, and
10 feet high. The instrument could be so adjusted that if the outdoor
temperature dropped or rose, the inside temperature would drop or rise
about the same number of degrees; in other words, the difference between
outside and inside temperatures remained constant. The thermostat opened
and closed a valve on a 1/2-inch steam line carrying 75 pounds pressure.
The steam line extended along one side of the shed about 14 inches above
the floor. A wallboard guard was fixed in position about 2-1/2 inches
from the pipe and parallel to it, for the dual purpose of increasing the
natural upward air movement over the pipe and of protecting lumber near
it from excessive heat. Six thin -ioces of wood were placed in the shed
to serve as indicators of the variation in moisture content in different
It may be noted in figure 3 that during the period October 2,
1933, to August 9, 1934, the average moisture content values inside the
shed varied from about 7.8 to 9.1 percent whereas the moisture content
values outside varied from 7.8 to 20.6 percent. At any one time the
moisture content near the ceiling was about 1-1/2 percent less than that
near the floor in the shed. Io steam was required during MIlay and June
because of the low outdoor relative humidity.
Stock seasoned to low moisture content values should be stored
in heated sheds. The average moisture content of seasoned stock stored
within tightly constructed sheds can be satisfactorily maintained through
the use of relatively inexpensive equipment.
Whether an unheated shed is closed or nearly closed makes little
difference in its effect on the average moisture content changes of the
stock stored within. Storage in an unheated insulated or lined compartment
is but little better than in an uninsulated compartment provided the
stock is protected from the ground moisture by means of a floor or by an
ample ventilating space beneath the pile.
The interior of a solid pile of dry lumber in storage for 7
months in either an open or closed unheated shed absorbed very little
moisture, but the ends change in moisture content quite readily with
changes in air conditions. This end pick-up may cause an appreciable
increase in width at the ends. Overhanging ends are objectionable in this
respect. On account of t.h'.' hent of the _;uL and the cooling effect of the
:ro'.-Lid, stori'1-# in tnh upper part of a shed io bettor tian near the Lrouxid
for Ci aimiii:., c_-os in the moisture cent' 'V. of upper-- rade stock.
Solil-pilei floorin; in oheds, durin i; a period of l-l/2 years,
picked up from 1-1/2 to 2 percer-t more ncisture ti'n did stock surfaced
on four L. ir.luz. In tht yard where the wind had a chnnco to act this
diff'e'.-Lce w.',s 3-1/2 perce-t.. ThAe c -Lt end2, of oak flooring boards .hic.
jere tted to.- cthLr within a ,ile 12 feet lonec: 1ick:ed up no more moisture
ti, t .--.ij:-cent uncut sections. In storin& 1tock of low moisture
co:-t.' t7 a-jh as hardviwood flooring, the pile should be as big and solid
as practicable because the interior of a pile ch,'-os in moisture content
vE -r slowly.
Southern yellow pine shiplm-p kicked up moisture more rapidly
thi. did Doula;. fir flooring. This is apparently a species effect, but
n1a, be d'.v partly to tw'z fact that the southern yellow pimi was about
50 picreCent sap-ooli whereas the Do"-.4l."z fir vias practically all heartwood.
The difference in moisture content of the two species was approximately
1 percent during: ; Iust of the store period.
90 120 150
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Partly op-en chEd I.etal roof j
I ------ -_
"> . .- q l-< B- = - .- --i - l^ -- I
t ~*~ ^ lose,.i slieu Qo r0o __
0 \ O.e- she.. 'To o-. roof f
Figure 1.-Average moisture content changes in southern yellow pine 1 by 4 inch flooring and 1 by S inch boards
surfaced four sides, stored in a solid pile in each of four different locations.
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-^ -- -v^ -^ ir r -4---
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Te.-per'ature (Degrees s ."hrenheit)
Figure 2.--Relation of the equilibrium moisture content of wood to the temperature and relative humidity
of the u-arrouniing atmosphere. (Dotted line illustrates thz- example cited.)
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. October vz-vjmber,December Jnuaary Febaar,- 'arc'. uril
C ?C 60 90 120 15,' 1.:7.
I ADgas t
-i6u.re 3.---,oitu'"re content cnani'i-es inside and outside t:e storage- sed.
UNIVERSITY OF FLORIDA
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