Why wood moves
Wood swells as it takes on moisture and shrinks as it dries, almost entirely across the grain. Along the grain it barely changes, so a tabletop stays the same length all year while its width comes and goes. The change follows the moisture content, which follows the humidity: find yours for each season with the Wood Moisture Calculator, then enter the driest and dampest here.
The calculator scales each species' shrinkage from the USDA Wood Handbook (table 4-3) by the change in moisture content, the same way Wood Calc does. Movement stops above the fiber saturation point (28% for most species, 22% for western red cedar), so wetter numbers count as that. These are averages; individual boards can vary about 15%.
Flat sawn or quarter sawn
Wood moves more along the growth rings (tangentially) than across them (radially). On a flat sawn board, where the rings run across the face, the width moves tangentially; on a quarter sawn board, where they run through the thickness, it moves radially. The T/R ratio says how much more: red oak's 2.3 means a flat sawn board moves more than twice as much as a quarter sawn one. That's why quarter sawn stock is worth the price for wide panels and drawer fronts.
Wood shrinkage chart
Shrinkage from green to oven-dry for each species, from the USDA Wood Handbook (table 4-3), and what that means for a 12" wide board going from 6% to 12% moisture content, about the swing between winter and summer in a heated house. Worked out with the same math as the calculator.
| Species | Tangential shrinkage | Radial shrinkage | T/R ratio | 12" flat sawn moves | 12" quarter sawn moves |
|---|---|---|---|---|---|
| Ash | 7.8% | 4.9% | 1.6 | 0.201" | 0.126" |
| Beech | 11.9% | 5.5% | 2.2 | 0.306" | 0.141" |
| Cherry | 7.1% | 3.7% | 1.9 | 0.183" | 0.095" |
| Douglas Fir | 7.6% | 4.8% | 1.6 | 0.195" | 0.123" |
| Eastern White Pine | 6.1% | 2.1% | 2.9 | 0.157" | 0.054" |
| Hard Maple | 9.9% | 4.8% | 2.1 | 0.255" | 0.123" |
| Poplar | 8.2% | 4.6% | 1.8 | 0.211" | 0.118" |
| Red Oak | 10.3% | 4.4% | 2.3 | 0.265" | 0.113" |
| Sitka Spruce | 7.5% | 4.3% | 1.7 | 0.193" | 0.111" |
| Southern Yellow Pine | 7.4% | 4.8% | 1.5 | 0.190" | 0.123" |
| Walnut | 7.8% | 5.5% | 1.4 | 0.201" | 0.141" |
| Western Red Cedar | 5.0% | 2.4% | 2.1 | 0.164" | 0.079" |
| White Oak | 10.5% | 5.6% | 1.9 | 0.270" | 0.144" |
| Yellow Birch | 9.5% | 7.3% | 1.3 | 0.244" | 0.188" |
What it means for a tabletop or a panel
A tabletop fastened tight to its base splits or pulls the base apart. Attach it with buttons in a groove, figure-eight clips or elongated screw holes, so it can slide across its width.
A panel in a frame floats in the frame's grooves, unglued. Size it for the movement: built in dry weather, leave room for it to grow; built in humid weather, make the grooves deep enough that it doesn't shrink out of them.
A worked example
A 36" wide flat sawn red oak tabletop, in a house that goes from 6% in winter to 12% in summer, moves 0.795", a little over 3/4". Quarter sawn, it would move 0.339". Either way it needs fasteners that let it move.
A 10" wide flat sawn cherry door panel going from 6% to 11% moves 0.127", about 1/8". Built in winter, it needs about 1/16" of room at each side of the frame.