Wood and Moisture: It’s All Biology
The hardwood lumber that you buy at the lumberyard began as a living tree. As a living tree, the trunk of the tree provided a way to transport water from the roots to the leaves. When the tree is cut down that water moving system is still present in that tree and much of that water is still in the lumber when it is first sawn.
Hardwood lumber just cut, referred to as green lumber, can have a moisture content of 40% or more! After hardwood lumber is sawn, it is typically air-dried to around 15% moisture content. Hardwood lumber will be continued to be dried in a kiln to a moisture content of 6-9%.
Wood is hygroscopic, which means just like a sponge the moisture content will change depending on the relative humidity of the surrounding air. When humidity increases, the wood absorbs moisture from the air causing the wood to expand. When the humidity decreases, the wood releases water into the air and the wood shrinks.
What Direction Does Wood Expand In?
When wood expands and contracts because of changes in moisture content, hardwood will move in a predictable way. Wood shrinks most in the direction of the annual growth rings (tangentially), and half as much across the rings (radially), and only slightly along the grain (longitudinally). or from the center of the tree to the outer edge).
Hardwood shrinks most in the direction of the annual growth rings (tangentially), about half as much across the growth rings (radially), and only slightly along the grain (longitudinally).
Flat-sawn boards will cup away from the heart of the tree. The shrinking will occur mostly in its width.
Rift-sawn boards will warp and shrink into a diamond or trapezoidal shape.
Quarter-sawn boards will shrink slightly in both length and width.
What Do I Do About Moisture and Wood Movement?
The solution to this problem is quite simple: don’t stop the wood from moving, but rather account for its movement. With a little bit of knowledge, you can predict the degree of wood movement, and take action to accommodate the movement.