Hardwood Flooring Dimensional Stability 2017-12-23T17:23:52+00:00

DIMENSIONAL STABILITY

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.

WOOD SPECIES
COEFFICIENT
MESQUITE
.00129
MERBAU
.00158
CYPRESS
.00162
PADAUK
.00180
TEAK
.00186
WENGE
.00201
PURPLEHEART
.00212
MAHOGANY
.00238
CHERRY
.00248
HEART PINE
.00263
FIR
.00267
ASH
.00274
WALNUT
.00274
BRAZIL CHERRY
.00300
PECAN
.00315
BIRCH
.00338
MAPLE
.00353
WHITE OAK
.00365
RED OAK
.00369
JARRAH
.003396
HICKORY
.00411
BEECH
.00431

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