Installation Guide

4
days or longer, for unseasoned siding
or if acclimating in exceptionally
humid conditions.
Further precautions must be taken
if unseasoned or green materials,
with a moisture content of more than
19%, are to be used successfully:
1) Allow materials to acclimate, as
described, over a longer period
—at least 30 days and longer in
damp or humid conditions—
before installation.
2) Use patterns which allow for
some shrinkage, such as bevel,
channel or board-and-batten.
These patterns have a profile that
includes a gap that can more
easily accommodate dimensional
change.
3) Use as narrow a width as
possible. Dimensional change is
proportional; the wider the width,
the greater the change.
Priming and Prefinishing
Often, material which has been
properly seasoned, stored and han-
dled will pick up moisture after instal-
lation but prior to finishing. Later,
when the siding loses that moisture,
joints may open up.
Research from the U.S. Forest
Products Lab indicates that siding
exposed to the elements for two
stamped ‘‘S-DRY’’ or “KD”.
However, knotty grades can be
specified MC 15 or KD 15, and are
available through a buyer/seller
agreement, when a maximum
moisture content of 15% is desired.
Method of Drying — Lumber
may be air dried or kiln dried. These
terms do not necessarily refer to a
specific moisture content but refer
instead to the method used for dry-
ing. Air dried lumber has been
seasoned by exposure to the
atmosphere, without artificial heat.
Kiln dried lumber has been sea-
soned in a chamber with the use of
artificial heat. To assure compliance
with a moisture content level, a
moisture percentage should always
be referenced.
Siding Storage — All siding
may pick up or lose moisture in
transit or storage so it is important
to allow it to acclimate with the sur-
rounding air of its final site prior to
installation.
Stack the siding on evenly
spaced, vertically aligned stickers
(spacers between the layers) in an
area where there will be good air
flow through the stack. This should
be done in an open garage or other
area that is protected from the ele-
ments.
If stacked over concrete, use
2x4s or 2x6s on edge to elevate
the first course of siding at least 3¹⁄₂
inches above the surface of the
concrete. If the stack is over wet
ground or wet concrete, lay down a
vapor barrier so the wood doesn’t
pick up moisture from beneath the
stack.
Allow air to flow through and
around the stack for a week to 10
days for dry siding, prior to installa-
tion. Extend the time period to 30
For instance, if the climate in a
particular region causes wood to
maintain 9% to 14% moisture con-
tent, then the moisture content of
the siding should be within that
range when installed.
Siding can be dried at the time of
manufacturing to a variety of mois-
ture content levels or it can be
shipped from the mill unseasoned
or green.
Unseasoned or green wood
has a moisture content of more
than 19%.
Dry wood will be at a maximum
moisture content of 19% or 15%.
Dry siding will take less time to
acclimate to the air of its final
surroundings (refer to Table 2) and
it will have less dimensional
change, before and after instal-
lation, than unseasoned or green
siding.
When ordering siding, specify the
required moisture content level, but
keep in mind the reality of the mar-
ketplace: premium grades of siding
are readily available dry; some knot-
ty grades are available dry.
Remember too, dry has a differ-
ent meaning for the premium
grades than it does for the
knotty grades.
Dry for the premium grades
means that the siding has been
dried to a maximum of 15% mois-
ture content (MC 15). In addition,
under WWPA rules, MC 15 means
that at least 85% of the pieces in
the order will be at a moisture con-
tent of 12% or less.
When knotty grades are dried at
the mill, the siding is air- or kiln-
dried so that it will not exceed 19%
moisture content. Thus, dry knotty
siding will have a moisture content
of 19% or less and may be grade-
Checklist for a Siding
Material Specification
■ Select species suited to the
project.
■ List grade names, paragraph
numbers and rules-writing
agency. (Refer to Table 1.)
■ Specify surface texture for
exposed face.
■ Specify moisture content
suited to the project.
■ If gradestamped, specify
lumber be stamped on back
or ends. (WWPA’s Lumber
Specifying offers additional
information.)
■ Specify VG (vertical grain) if
appropriate and available.
(Refer to Table 1, footnote 5.)
■ Specify pattern and size.
(WWPA’s Standard Patterns
offers additional information.)
Table 2: MOISTURE CONTENT GUIDELINES
Recommended Moisture Content at Time of Installation
Use of Wood Most Areas Dry, Southwestern Damp, Warm South-
in Exterior of the U.S. States eastern Coastal Areas
Individual Individual Individual
Siding, Trim
Average
1
Pieces Average
1
Pieces Average
1
Pieces
and Sheathing
12% 9-14% 9% 7-12% 12% 9-14%
1
To obtain a realistic average, test at least 10% of each item, i.e. 10% of the siding pieces, 10% of
the trim pieces and random checks of the sheathing material. It is particularly important to check the
sheathing prior to the siding application if it has become wet after installation.
Source: Wood Handbook, 1999, from Table 12-2.