Predicting Wood Movement in Service

From shrinkage tables to everyday movement

The green-to-oven-dry shrinkage numbers describe drying. Once wood is dry and in use, the question changes: how much will this board move between a dry winter and a humid summer? For that, the Wood Handbook provides dimensional change coefficients, the fraction of a dimension that changes per one percent of moisture content, valid between 6 and 14 percent ¹.

The equation

ΔD = D × C × (M_final − M_initial)

D is the starting dimension, C is the coefficient for the direction that matters (C_T for tangential, C_R for radial), and the moisture contents are in percent ¹. A negative result is shrinkage.

Coefficients for common Wisconsin woods:

Worked examples

A cabinet door panel. A 12-inch-wide flatsawn red oak panel goes from 11 percent in August to 6 percent in a heated January:

12 × 0.00304 × (6 − 11) = −0.18 inch

That’s about 3/16 inch. Quartersawn, the same panel moves 12 × 0.00137 × 5 = 0.08 inch, under 3/32.

A dining table. A 40-inch-wide flatsawn white oak top built at 7 percent that reaches 11 percent in a humid summer:

40 × 0.00376 × (11 − 7) = +0.60 inch

Over half an inch. If that top were screwed tight to the aprons it would split or bow the base.

A pine board. A 10-inch eastern white pine shelf over the same 5-point swing moves 10 × 0.00212 × 5 = 0.11 inch. White pine’s reputation for stability is earned.

Try your own numbers:

Designing for a Wisconsin year

With indoor moisture swinging from about 5 to 7 percent in winter to 10 to 12 percent in summer, plan for a five-point swing and these traditional solutions:

• Frame-and-panel construction. The panel floats in grooves, unglued. Size the panel by the season you build in. Built in dry winter, the panel is near its narrowest: leave room in the grooves for it to swell. Built in humid summer, it’s near its widest: fit it close, but make the grooves deep enough that it stays captured when it shrinks. A dab of glue or a pin at the center of the top and bottom rails keeps it centered.
• Tabletops. Attach with buttons in slots, figure-eight fasteners, or screws in elongated holes, never with rigid screws across the width. Fix the center and let the edges move.
• Breadboard ends. Glue or pin only at the center; pin the outer tenons through elongated holes so the top can move under the breadboard.
• Cross-grain glue joints. Gluing a board across the grain of another (a cleat glued across a top) fights the movement and fails over time. Keep cross-grain glue joints narrow.
• Drawers. Fit drawers to the season: snug in winter, a little loose in summer.

These rules, and the coefficient arithmetic behind them, are the practical payoff of drying wood to the right moisture content: the closer the wood is to the middle of its service range when you build, the smaller the movement in either direction.

What to take from this

Dimensional change coefficients turn a moisture swing into inches: ΔD = D × C × ΔM. Use the tangential coefficient for flatsawn width and the radial one for quartersawn. A five-point Wisconsin swing moves a foot of flatsawn oak about 3/16 inch and a 40-inch tabletop over half an inch, so every wide panel needs room to move.