Wood is a hygroscopic material, absorbing and releasing moisture as the relative humidity of the surrounding air rises and falls. And as the moisture content of a piece of wood changes, so do its dimensions and weight. When wood is assembled into a piece of furniture, the changes can produce problems—some great, some small. A cabinet door that shuts smoothly in December may not close at all in June; a perfectly square bookcase can literally pull itself apart at the joints as humidity changes throughout the year. Knowing how moisture affects wood will help you avoid these problems.
The water in wood is measured as a percentage of its oven-dry, or water – free weight. For example, if a 40-pound piece of wood drops to 30 pounds when oven-dried, the weight of the shed water—10 pounds—divided by the wood’s dry weight—30 pounds—is the moisture content of the original piece: in this case, 33 percent.
Wood holds water both as vapor-like moisture called free water in its cell cavities and as bound water in the cell walls. When wood is cut and exposed to the air, it sheds its free water first. When all free water is expelled, the wood is said to be at its fiber saturation point (FSP), typically between 23 and 30 percent moisture content. To this point there has been no change in the dimensions of the piece; it simply weighs less. As wood dries further, however, water is shed from the cell walls, causing them—and the board—to shrink.
Under normal circumstances, wood never regains its free water; a dried board’s cell cavities will always remain empty of moisture. But the amount of bound water contained in the cell walls changes with shifts in the humidity in the air. At 100 percent relative humidity, wood reaches its FSP. At 0 percent humidity, wood is drained of all water. The relative moisture in the atmosphere normally falls between these values, and the moisture content of most woods
ranges between 5 and 20 percent. Still, the fluctuation in relative humidity between typical North American winters and summers can cause substantial wood movement over the course of a year.
You can compensate for this in several ways. Use a humidifier in winter and a dehumidifier in summer to keep the indoor level of humidity as constant as
possible. Remember to make allowances for wood movement in the construction of your work. Using frame-and-panel joinery for example (page 32) will provide space for wood to expand and contract without affecting the overall condition of the piece. Some woods tend to move more than others; consult a lumber dealer to find the most dimensionally stable species for your projects.
Wood does not shrink uniformly; as shown by the dotted red lines in the illustration at right, tangential shrinkage—tangent to the growth rings—is about twice as great as radial shrinkage, which occurs across the rings. This difference causes boards and panels to warp as they shrink or swell with changes in relative humidity. It can also cause joints to loosen or tighten from excess pressure, as discussed below. Shrinkage along the length of a board is usually insignificant. A 2-by-10 plank that shrinks % inch across its width might lose less than Me inch along an 8-foot length.
The location of the tangential planes of mating boards will significantly affect a joint’s strength and stability. In the ideal situation—as shown in the illustration of a mortise-and-tenon joint at left—the tangential planes of joined pieces are parallel. This ensures that the boards will experience similar wood movement in the same direction as their moisture content changes. Orienting boards this way helps prevent a joint from coming loose; it also prevents the mating boards from splitting when they swell with higher levels of moisture.