Why wood shrinks when it dries

Water in wood

Living wood holds water in the cell cavities and in the cell walls. When wood dries, the free water in the cavities leaves first and the wood does not change size. When the cell walls start to lose water, the wood shrinks. The moisture content at that turning point is the fibre saturation point, often taken as about 25–30 %.

Three directions

Shrinkage is not the same in every direction. Along the grain it is small. Across the grain it is larger tangentially (along the growth rings) than radially (across them). In the species data of this portal, tangential shrinkage is roughly 1.6 to 2.2 times the radial value, which is why flat-sawn boards cup and distort more than quarter-sawn ones.

A simple estimate

Between the fibre saturation point (30 %) and a target moisture content, shrinkage is often estimated as a straight line:

Change % of green size = S × (30 − target MC) / 30

where S is the total green-to-oven-dry shrinkage for the species and direction. With S = 7.5 % and a target of 12 %, a 100 mm wide board shrinks by about 4.5 %, to about 95.5 mm. Real boards also warp, so treat this as an estimate.

See the Wood species pages for sourced values and the Drying shrinkage calculator for the estimate.