SCMIE IFIICTS CF STUIAG[ ON
Information Icviucwcd and rcIaffirmnd
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconain
Digitized by the Internet Archive
,00" EFFECTS OF STT.AM-g ON SEASO1LD LD7.7.7
J. S. 1'iA :T7'S?', Engineer
E. F. PAS.USSL.., 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 any construction are in a large part dependent upon its moisture
content at the time of installation. If users are supplied with lumber suit-
ably dried for the job, there is no apparent reason why they should be con-
cerned about its serviceability so far as shrinking and swelling are
Some mills machine and ship thoroughly seasoned lumber to wholesale
and retail yards, only to find that it is stored in piles unprotected from
the weather. Such storage largely offsets the benefits of proper seasoning,
because the wood may absorb moisture to an undesirable extent before it is
used. This moisture remain followed by a moisture loss during_ fabrication
or use may result in warpin; and end checking. Correct storage practice is
orneesary fresl pro vap, C..
necessary for properl.- kiln-dried lun'oer, therefore, to maintain the lumber
at the desired moisture content. By so doing, defects as well as changes in
moisture content occurri!i, during storage will be minimized, if not entirely
eliminated, thereby reduciig loss of material during fa rication and sub-
Effect of Storage on Moisture Content
In order to obtain definite information upon the effect of storage on
the moisture content of lumber, the Forest Products Laboratory surveyed storaF-e
methods at sau.wmills, wholesale and retail distributing yards, end wood-using
plants in various parts of the United States.
Illustrative of the results obtained is figure 1, bcsed on a study at
a Louisiana sawmill. Somewhat similar results v'er3 obtained at a Florida
sawmill and at several distributinti' yards in the vicinity of Chicago. The
stock at the Louisi'na mill ,,;as southern yellow pine 1- 4-inch by 12-foot
flooring and 1- by 8-inch "j, 12-foot boards surfaced four sides and solid
piled -- that is, piled without stickers. A group of boards selected for
uniformity of moisture content by moans of a moisture meter was piled in each
lMaintained at }1adison ii, xis., in cooperation with the University of ,isconsin.
Report R1071, revised.
of the following 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 of 80 boards.
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. Then the rough lumber is surfaced, 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 and subsequently absorb moisture during storage, the ends will
become wider and thicker than they should be.
The effect of temperature and relative humidity on equilibrium moisture
content is shwvn in figure 2. For example, at the average indoor temperature
of 70 F. and relative humidity of 42 percent, the equilibrium moisture
content of wood is 8 percent. It has been found that, for the most satis-
factory service, such products as flooring and interior finishing woodwork
in conventionally heated dwellings in most parts of the United States should
be installed at about 8 percent moisture content.
Assume that in an unheated shed the temperature and relative humidity
are respectively 30 F. and 75 percent. According to figure 2, the corre-
sponding 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 8 percent by merely heating
the air in the shed to about 45 F. (See dotted line in figure 2.) In this
way steam sprays, water sprays, desiccants, or refrigeration are not required.
Further, the heat loss under this condition is less than if a common temper-
ature of, say, 60 or 700 F. were maintained.
Undesirable changes in the moisture content of lumber within closed
storage sheds can be largely preventod.-
During storage, all the lumber tends to come to the same moisture
content. In other words, the range in moisture content between individual
pieces becomes less with storage (fig. 3). This is advan-e...9ous if storage
conditions are properly maintained, particularly if the moisture content of
the lumber varijs considerably after kiln drying.
Report Nb. R1071, revised.
-Forest Products Laboratory Report No. R1140, ';Moisture Fluctuations in Lumber
WJithin Closed Storage Sheds Controlled WVith Electrical EqiipmnL-."
Effect of Storar-e on Moisture Gradient
and Drying Stresses
A further advantage of storage is that it gives time for the moisture
content to equalize throughout the thickness of each piece. With some woods,
especially softwoods, the storage period and equalization of moisture content
may be sufficient for the relief of drying stresses. Some species, partic-
ularly hardwoods, on the other hand, may show severe drying stresses even
after extended periods of storage.
Under poor storage conditions, stresses may be induced in the lumber.
Outside storage of kiln-dried lumber, for example, may cause reverse case-
hardening in boards that are exposed to snow or rain. Further, the absorption
and subsequent drying may cause old checks to open up and new ones to devclol.
Effect of Storage on Splitting of Lumber
Frequently, rather heavy losses from splitting occur during storage
of lunfmber or partially made products, especially in workshops heated during
the winter. It is not uncommon, during winter months, for the equilibrium
moisture content in heated shops to be as low as 4 percent while being as
high as 18 percent or more in unheated storage sheds. :he ends of lumber
stored in these sheds conseq-ently may have a very high moisture content, and
upon exposure to the dry conditions of the workshop will end dry rapidly,
which may cause end checks and splits. Exposure of freshly cut ends of lumber
that have a comparatively high moisture content to dry conditions may also
cause end checks and end splits.
The faces of partially fabricated pieces may split if the pieces are
stacked so that they are only partially exposed to the low equilibrium
conditions. Shrinkage of the e.r-osed face may also be sufficient to cause
opening of glue joints. Splits and openings of this nature are more corimon:
in stock with a high core moisture content, one face of which has been
partially cut away, thereby exposing the zone of high moisture content. A
good example of this is a chair saddle seat.
Storage of Green, Air-Dried, and Kiln-Dried Lumber
It is difficult to store green lumber for an appreciable period of
time without loss from stain, decay, or insect attack. It can be done,
however, by under-water storage or storage at lowi temperatures, that is below
35 F. The draner of stain and decay can also be minimized by treating the
green lumber with some of the toxic chemicals used to prevent blue stain.)
J. S. Forest Products Laboratory. "Cause and Prevention of Blue Stain in Wood.
Technical Note No. 225. (In cooperation with Div. of For. Path., Bur. Plant
Ind., Soils, and Agr. Er.ir.)
Report No. R1071, revised.
lie best practice, however, is to start/drying as soon as possible after
cutti.:v; the log into lumber.
If the season of the year is advantageous, air-dried lumber may be
stored by leaving it stickered in the air-drying pile. The best method of
storage, however, would be to pile it in open, partially closed, or unheated
closed sheds. It could be sticker piled or, if the average moisture content
is below 20 percent, solid piled. Storage in this manner will best maintain
the final air-dried moisture content without causing losses or decay from
exposure to the elements.
Kiln-dried lumber should be given the best of care in storage. There
is no advantage in kiln drying lumber to a low moisture content if during
storage it eventually attains the moisture content of air-dried stock.
Preferably, kiln-dried lumber should be stored in closed heated sheds in
which the equilibrium moisture content is controlled. If such sheds are not
available, storage in the upper part of a closed or partially closed shed in
which the temperature is slightly higher than the outside temperature is
RIport ;*. B1071,revised
/3 1___ -Y 1 __ _
/-/ Y_.Y __________ PARTLY OPEN SHED-METAL ROOF _
S10-- -- CLOSED SHED- WOOD ROOF------^"e-
--k-----. f^^^^.... OPEN SHED-WOOD ROOF-
AP. AYJUEJUY UG SP. CT NV.DC.JA/FB.MR. APR. MA JUNE JULY AUG. SEPT.
0 30 60 90 120 150 /80 210 240 270 300 330 360 390 420 450 480 510 540
Figure l.--Average moisture content changes in southern yellow pine I by I inch floorinlo and 1 by 8
inch boards surfaced four sides, stored in a solid pile in each of' four different locations.
ZM 20453 F
\o \'o \o \<- \.-V------
\<40 \0 \" Y X^\
TEMPERTURCEN \0 /5 \
I^AA40^ - - - -
io .......------- --- ------ ------ 7=
0 /0 20 30 40 50 60 70 80 90
Figure 2.--Relation of thile equilibrium moisture content of wood to the temperature and relative
humidity of tne surrounding atmosphere. (Dotted line illustrates the example cited.)
ZM 44797 F
'0 --- COIV0 --- ----
,n ' ;'
Fi ur 3. --R n e o o tnif a p c a e o -b
1 year wi hi a -~sc --'- e sh d In t a av r g
moistur cotn 1_ o- Falvegemis........
1 3 ;i i ...
|~ 3___ L|- ------
0 40. 2_ 0Zo /6.6 20.6 246 28.0
>'0 YI-FE (ON'TENT (f^P CE/V9
Figure 3.--Range of 11,-i, ._-.. content of a package of 1- by
6-inch Douglas-fir Doords before and after storage for
1 year within a clos. -i L. ited4 shed. Initial average
moisture content ',-' _, .5 :.er-':ent. Final average moisture
content was 12.1 percent.
UNIVERSITY OF FLORIDA
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