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Home Builder Magazine, 7, 6, p. 19, 1994-11
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Dry framing lumber makes big difference in performance
Archer, J. W.
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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
N R C C - 3 8 7 4 1
A r c h e r , J . W .
N o v e m b e r 1 9 9 4
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
fohnW.Archer
II.
"The drier the lumber
wben installed, the less
impact shrinkage and
warping it will have."
•
John WArcher Is a senior adViser In the Industry liaison Branch, Institute for Research tn Construction, National Research Council ofCanada.
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 becausedry 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 25to30 per cent (the fibre saturation point), moisture in the ceii walls begins to be lost. During this second stage of
dry-ing, 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 sea-soning 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
mois-ture 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 mois-ture content.
Why
19"10
Moisture Content?
The 19 per cent moisture content figure reflects sev-eral 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 of wood's total potential for shrinkage has occurred. Therefore, 19 per cent is the practicai upper limit for most
situa-tions.
• The drier the lumber when installed, the less impact
shrinkage and warping it will have. During the winter,
wood candry 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 thatoccurs
bythe time wood reaches five to six per cent can lead
to improper seating of floor joists on siil plates,
pro-ducing more apparent deflection and vibration, as well as squeaking. Shrinkage can also increase the air-leak-iness ofwallslparticularly around doors and windows.
• The drier the lumber the less prone it is to decay.
MostwQodMdecayingfungi require a moisture content above 20 per cent to grow. In modern construction, wet lumber can take several years todryin 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 againif
tbe wood is re-wetted. This can happen when
temper-ature 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,｢ｯｷｾ
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
