Amid all the predictability and uniformity of our manufactured, well-planned society, we’re enjoying a renewed appreciation for the natural beauty and other qualities of organic materials like wood. At the same time, though, we can grow frustrated when those natural resources aren’t as uniform or predictable as their engineered counterparts. For those working with wood, understanding wood movement is important in order to avoid unnecessary problems.
Understanding Wood Movement
The most predictable aspect of wood’s movement is simply that it constantly moves. Age, fastening systems, and sealing may lessen movement, but nothing will curb it altogether. Almost as reliable as movement in general, though, is the type and amount of movement a particular species and cut of wood will perform. This kind of predictability makes figuring safe tolerances possible.
In order to understand wood movement, envision a tree as a bundle of straws. During periods of growth, the straws are filled with water; these straws transport growth-producing nutrients throughout the tree. When that tree is harvested and milled into lumber, those water-filled straws begin shedding the water from the cut ends, causing the straws to narrow. Surrounding humidity will, in turn, produce growth in the straws. The constant process of expansion and contraction can be slowed by kiln drying, sealing, and various finishing techniques, but movement will never cease entirely.
Predicting Wood Movement
The distinctive characteristics of each wood species can go a long way toward predicting movement in various conditions. Two basic types of movement are tangential shrinkage and radial shrinkage. The first type of shrinkage, tangential shrinkage, describes movement along growth rings, or swelling across the diameters of the straws. Second to tangential shrinkage, radial shrinkage also affects lumber. This movement perpendicular to the growth rings certainly bears consideration. The ratio of tangential vs. radial movement is referred to as the T/R ratio, and it can help designate the stability of a given species: the closer the numbers are to one another, the lower the chances of warping or cupping.
Responding to Wood Movement
Once you understand the basics of wood movement, you can avoid major calamities throughout seasonal shifts in humidity and temperature. However, since wood does not expand or contract in a uniform way, time is our ally in avoiding problematic movement after installation. To understand why movement is not uniform, think of the moisture being lost from the ends of the straws; the ends will become dry before the middle has a chance to empty out. The results include cupping, bowing, twisting, and warping. To avoid such problems, we must allow for acclimatization periods after lumber has been transported to a new environment. The goal is to allow warpage to occur and then an equilibrium to be achieved, causing the movement to slow prior to installation.
The lumber you receive from J. Gibson McIlvain will have already been kiln dried, lessening the wide swings of moisture content. At the same time, be aware that lumber will always be a natural, organic product that produces movement with shifts in moisture levels.