As wood cells grow, they make long tube-like structures for sap to travel to and from the plant's leaves and roots. When the wood is made into lumber, the sap evaporates from it, first from the tubes, then from the cell walls. When the cell walls dry out, the wood slowly shrinks. However, when humidity is high, the cell walls reabsorb moisture and swell.

Logging. Felled Pine Logs Piled Firebreak
credit: Solovyova/iStock/Getty Images
The moisture in kiln-dried wood drops to 10 percent or less.

In and Out

In dry weather, wood shrinks as any remaining moisture in the cells walls evaporates. In a warm, humid atmosphere, the cells reabsorb moisture from the air and expand. A board cut across the grain, or perpendicular to the growth rings, may expand as much as 1/4 inch for every 12 inches of width, while a quarter-sawn board may expand 1/8 inch. The length of a board is nearly unaffected by the humidity in the air. An 8-foot-long green board may shrink up to 3/32 inch while being air or kiln dried.