Wood Likes to move it, move it

As a client told me the other day, wood is perpetual. It is an organic substance and is constantly moving from its form as a tree through its life in any wood product. The constant movement in wood is caused by wood fibers and cells absorbing and losing moisture in response to changes in the relative humidity of the surrounding environment. This loss of moisture causes the cells to contract and shrink, resulting in movement in the board; either bending, twisting, or cupping depending on the orientation of the fibers within. The fibers in wood can be pictured like a series of drinking straws running up the length of the tree. When a tree is felled, the open ends of these “straws” are immediately exposed to the atmosphere and will begin losing water, primarily out of the straw end, or cell lumen. Once milled into lumber, these cells will continue to lose moisture primarily through the ends, but also through the radial cells.

When a tree is first cut, the moisture content is anywhere from 30% to 200% of the weight of the wood. This water is held in the open space within the cell, the cell lumen, and is bonded to the wood fibers. As the wood dries, the water stored in the lumen, called free water, evaporates out first. The wood does not begin to move until all the free water has evaporated. Once the wood reaches 30% moisture content, the water in the fibers will begin to evaporate, causing the cells to shrink. The cells continue to shrink until the moisture content lowers to 4%-11%, or reaches equilibrium with the relative humidity of the surrounding environment. However, the wood will never actually stop moving. As the relative humidity changes surrounding the wood, the wood fibers will absorb moisture causing them to expand and the wood to move. Wood changes about 1% in moisture content for every 5% in relative humidity. The subsequent swelling and shrinking is dependent on the species and the cut pattern of the wood.

There are three types of movement within wood: tangential, radial, and longitudinal. The effect of this movement will vary depending on the cut pattern of the lumber, due to the orientation of the growth rings in the board, with quartersawn moving the least and plainsawn the most. The tangential movement is the movement along the growth rings and accounts for the greatest amount of movement with the wood. The radial movement is perpendicular to the growth rings, along the radial cells, and is about half as much as the tangential. The longitudinal movement is negligible. Within a species, it is the ratio between the tangential shrinkage and the radial shrinkage that will determine the stability of that particular species of wood. The smaller the ratio between the tangential and radial shrinkage, the more stable the wood will be.

There is a lot that can be done to mitigate the effects of wood movement, but nothing can truly stop it. Since wood will always move in response to changes in relative humidity, the most important thing is to dry the wood properly. Afterwards, let the wood acclimate if it has been moved from one part of the country to another. For example, if a slab is leaving the dry Colorado air on its way to southern Florida, it will be important to let the slab adjust to the higher humidity before working on it. Finishing techniques such as oiling and sealing slow the transfer of moisture, but this can never completely stop the wood from absorbing and losing moisture.

In the end, wood is like Jerome at the disco; you can do whatever you want to slow him down, but in the end nothing can stop him from showing how he likes to move it move it.

by Arron Hendricks
CSWoods Sales Staff