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Dry framing lumber makes big difference in performance
Archer, J. W.
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Home Builder Magazine, 7, 6, p. 19, 1994-11
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Dry fra m ing lum be r m a k e s big diffe re nc e in pe rform a nc e
NRCC-38741
Archer, J.W.
November 1994
A version of this document is published in / Une version de ce document se trouve dans:
Home Builder Magazine, 7, (6), pp. 19, November, 1994
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Dry'raming lumber Malee. Big DiNerence
in Performance
fohn W. Archer
II.
"The drier the lumber
wben installed, the less
impact shrinkage and
warping it will have."
•
The National BuildingCode of Canada states that
the framing lumber in a house must have a moisture
content no greater than 19 per cent when installed,
For a long time, builders in many parts of Canada
ignored this requirement because dry lumber just was
not availabie.
Recently, inspectors have begun to enforce the
requirement, which raises the question: "What is the
significance of 19 per cent?"
More than half the weight of a tree can be water.
The tree begins to lose moisture as soon as it is cut. If
the log is allowed to dry, it cracks and splits because
the outside shrinks faster than the inside. This is why
lumber manufacturers saw the logs while they are still
llwet."
The sawn lumber dries in two stages. First, the wood
cells give up free water; this moisture loss does not
cause the wood to shrink or distort. Second, when the
moisture content of the wood has dropped to 25 to 30
per cent (the fibre saturation point), moisture in the ceii
walls begins to be lost. During this second stage of drying, shrinkage or warping occurs,
The extent of shrinkage and warping depends on
the angle of the grain in the board, how dry it gets and
how it is stored while drying. With proper protection,
stacking and restraint to resist warping, most spruce,
pine and fir species can be air-dried flat and straight in
about one summer. Adrying kiln can do the same seasoning in days, depending on the species and size of
the pieces of wood.
How to Read a Grading Stomp
The grade stamp on lumber indicates the moisture
content at the time the rough-sawn lumber is planed
(called surfacing.) S-Grn means over 19 per cent moisture content (unseasoned.); S·Dry means no more than
15 per cent of the batch exceeds 19 per cent moisture
content; andMC 15 means 15 per cent maximum moisture content.
Why 19"10 Moisture Content?
John WArcher Is a senior
adViser In the Industry liaison
Branch, Institute for Research
tn Construction, National
Research Council ofCanada.
The 19 per cent moisture content figure reflects several considerations. They include:
• Equilibrium moisture contents for wood stored under
cover during the summer in most inland areas is 11 to
12 per cent, while in the coastal areas it is 14 to 16 per
cent. At these levds, about half to two-thirds ofwood's
total potential for shrinkage has occurred. Therefore,
19 per cent is the practicai upper limit for most situations.
• The drier the lumber when installed, the less impact
shrinkage and warping it will have. During the winter,
wood can dry to moisture level$ as low as five to six
per cent. If the wood starts off with a high moisture
content (over 19 per cent), the shrinkage that occurs
by the time wood reaches five to six per cent can lead
to improper seating of floor joists on siil plates, producing more apparent deflection and vibration, as well
as squeaking. Shrinkage can also increase the air-leakiness ofwalls l particularly around doors and windows.
• The drier the lumber the less prone it is to decay.
Most wQodMdecaying fungi require a moisture content
above 20 per cent to grow. In modern construction, wet
lumber can take several years to dry in an enclosed
wail. Even if there should be insufficient moisture after
growth ,has started, the fungi do not die, they merely
become dormant and active growth can start again if
tbe wood is re-wetted. This can happen when temperature changes during the heating and cooling seasons
create moisture gradients.
• Shrinkage of wood along the embedded depth of
nails leaves their heads above the wood surface. This
is the cause of "nail-popping" in drywall. Wood dried
to 19 per cent doesn't shrink as much as wet wood and
reduces nail popping. Wood that is below 15 per cent
is even better and is recommended.
• When construction proceeds rapidly, wet framing
lumber will be enclosed before the effects of shrinkage
have occurred. This shrinkage commonly is noticed
around windows and doors where the lintels shrink
away from the supporting jack studs and where metal
hangers support unseasoned wood joists around floor
openings. After the framing is enclosed, corrective action
is impossible.
• Using unseasoned lqmber to build roof trusses may
lead to truss uplift problems. If the ceiling is installed
before the moisture level of the trusses stabilizes, ｢ ｯ ｷ
ing stresses and deformation can be increased.
Keep it Dry
Increasingly, suppliers are providing builders with
dry lumber, but it is important to keep it dry once it
reaches the building site. You should not pile lumber
at low points where water may pool; always keep it
covered from rain or snow; don't store on bare earth,
on concrete floors or other moisture sources; keep the
original seal wrappings on S-Dry grade lumber until it
is used; and store the lumber flat and well supported
so it doesn't deform.
HB
Home BUIIDER
November/December '94
19