Stability is one important aspect to consider when selecting the type of material to be used for flooring. Most of us consider stability
only in the sense of how much a certain measure of wood shrinks as it dries. There are other factors that are necessary to review when considering something as stable.
Wood is dimensionally stable when the moisture content is above the fiber saturation point. Wood changes dimensions as it gains or loses moisture below that point. It shrinks when losing moisture in the cell walls and swells when gaining moisture. This shrinking and swelling may result in
warping, either sideways developing a "crook," bow, or twist; or across the grain showing itself as a "cup."
Wood shrinks most in the direction of the annual growth rings (tangentially), about one-half as much as across the rings (radially), and only slightly along the grain (longitudinally). The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the
curvature of the growth rings.
The numbers in the chart reflect the dimensional change coefficient for the various species, measured as tangential shrinkage or swelling within normal moisture content limits of 6-14 percent. Tangential change values will normally reflect changes in plainsawn wood. Quartersawn wood will usually be more dimensionally stable than plainsawn.
The dimensional change coefficient can be used to
calculate expected shrinkage or swelling. Simply multiply the change in moisture content by the change coefficient, then multiply by the width of the board. Example: A mesquite board (change coefficient = .00129) 5 inches wide experiences a moisture content change from 6 to 9 percent a change of 3 percentage points
Calculation: 3 x .00129 = .00387 x 5 = .019 inches.
In actual practice, however, change would be diminished
in a complete floor, as the boards proximity to each other tends to restrain movement.
Although some tropical woods such as Australian Cypress, Brazilian Cherry, Merbau and Wenge appear in this chart to have excellent moisture stability compared to Northern Red Oak , actual installations of many of these woods have demonstrated significant movement in use. To avoid problems later, extra care should be taken to inform potential users of
these tendencies prior to purchase. This chart is best used for comparison.