Wood Shrinkage
A change in moisture content (MC) can alter the dimensions of wood. Wood swells when the MC increases, and shrinks when MC is decreased. However, these only happen when wood is not in a fully saturated state, i.e. MC is below fiber saturation point (FSP).
Radial and Tangential Directions in Wood
Flat-sawn (softwoods), and Plain-sawn (hardwoods)
Plain-sawn boards are the most common. The growth rings are typically 30 degrees or less to the face of the board. The changes in thickness of the board is in "radial" direction and width is in "tangential" direction.
Vertical-grain (softwoods), and Quarter-sawn (hardwoods)
Quartersawn wood is more expensive than plain-sawn. The grain is perpendicular to the face of the board showing the growth rings on edge. The changes in thickness of the board is in "tangential" direction and width is in "radial" direction.
Mixed Grain
Mixed grain boards display attributes of both quarter-sawn and plain-sawn boards. The grain appears to be at a 45 degree angle from the face of the board. The changes in thickness and width of the board is calculated as the average of changes in "radial" and "tangential" directions.
Wood does not absorb bound water at a MC above FSP, but only adds free water into void cell lumens. That is why the dimensions remain constant. Wood shrinkage/swellage can be measured using the following equations. FSP is assumed to be 30% for all species in the calculator. In the equation below S0 represents the shrinkage percent from green to oven-dry moisture content.
