Moisture and Wood Dimensional Stability
Moisture plays a large part in how wood behaves, both during the machining process and after installation.
Moisture content is defined as the weight of water in wood expressed as a percentage of the weight of oven-dried wood. Weight, shrinkage, strength, and other properties depend on the moisture content of the wood.
Wood is dimensionally stable when the moisture content is above the fiber saturation point (usually about 30 percent moisture content). Below that, wood changes dimension when it gains or loses moisture.
The ideal content for flooring installation can vary from an extreme of 4 to 18 percent, depending on the wood species, the geographic location of the end product, and the time of year. Most oak flooring, for example, is milled at 6-9 percent. Before installation, solid wood flooring should be acclimated to the area in which it is to be used, then tested with a moisture meter to ensure the proper moisture content. Note: Engineered flooring tends to be more dimensionally stable than solid flooring.
Different woods exhibit different moisture stability factors, but they generally shrink and swell the most in the direction of the annual growth rings, about half as much as across the rings, and only slightly along the grain. This means that plain-sawn flooring will tend to shrink more in width than quarter-sawn flooring and that most flooring will not shrink or swell much in length.