Home Remodeling Guide >>  Designing With Materials>>  Wood >>

When Water and Wood Combine: Wood Flooring Shrinkage and Swelling

Moisture content in solid wood is defined as the weight of water in wood expressed as a percentage of the weight of oven-dry wood. Weight, shrinkage, strength and other properties depend on the moisture content of wood. In trees, moisture content may be as much as 200 percent of the weight of wood substance. After harvesting and miring, the wood will be dried to the proper moisture content for its end use.

Wood fibers are 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. Here are some quick points about shrinking and swelling:

• Shrinkage usually begins at 25 to 30 percent moisture content, the fiber saturation point. Shrinkage continues to zero percent moisture content, an oven-dry state.
• Swelling occurs as wood gains moisture, when it moves from zero to 25 to 30 percent moisture content, the fiber saturation point. Different woods exhibit different moisture stability factors, but they always shrink and swell the most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially) and only in minuscule amounts along the grain longitudinally). This means that plainsawn flooring will tend to shrink and swell more in width than quartersawn flooring, and that most flooring will not shrink or swell measurably in length.
• Generally, flooring is expected to shrink in dry environments and expand in wetter environments.
• Between the fiber saturation point and the ovendry state, wood will only change by about .1 percent of its dimension along the grain (lengthwise in a flatsawn board). It will change by 2 to 8 percent across the grain and across the annular rings (top to bottom), if quartersawn; and 5 to 15 percent across the grain and parallel to the annular rings (side to side), if plainsawn.
• Wider boards tend to move more than narrower boards. Movement in a 5-inch-wide plank is more dramatic than in a 2 1/4-inch strip.

The ideal moisture 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 time of year. Most oak flooring, for example, is milled at 6 to 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: Laminated wood flooring tends to be more dimensionally stable than solid flooring, and may not require as much acclimation as solid flooring prior to installation.)

A wood's weight and moisture content

Wood is hygroscopic--meaning, when exposed to air, wood will lose or gain moisture until it is in equilibrium with the humidity and temperature of the air. Moisture content (MC) from 5 to 25 percent may be determined using various moisture meters developed for this purpose. The most accurate method in all cases, and for any moisture content, is to follow the laboratory procedure of weighing the piece with moisture, removing the moisture by fully drying it in an oven (105 degrees C) and reweighing. The equation for determining moisture content is MC% = weight of wood with water - oven-dry weight / divided by oven-dry weight X 100.