As wood loses or gains moisture, it will shrink or swell until it reaches equilibrium with the level of moisture in the air of its immediate surroundings. Because of its cell structure, wood shrinks primarily in thickness and width and very little in length.
Wood siding is no exception. It will shrink and swell regardless of pattern or material quality. Problems can occur after installation if the siding shrinks or swells unevenly or very rapidly, particularly if it has been improperly nailed and its natural movement has been restricted. However, problems such as twist, cup, warp, splits and checks can be minimized.
To avoid potential problems and to minimize dimensional change after installation, the moisture content of the siding should match the local climate as closely as possible at the time of installation.
For instance, if the climate in a particular region causes wood to maintain 9% to 14% moisture content, 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 moisture 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 and it will have less dimensional change, before and after installation, than unseasoned or green siding.
When ordering siding, specify the required moisture content level, but keep in mind the reality of the marketplace: premium grades of siding are readily available dry; some knotty grades are available dry.Remember too, dry has a different 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% moisture 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 content 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-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 drying. Air dried lumber has been seasoned by exposure to the atmosphere, without artificial heat. Kiln dried lumber has been seasoned 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 surrounding 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 elements.
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 installation. Extend the time period to 30 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.