Drying Defects
Jim Wiley

Shrinkage of wood
Shrinkage of wood is the basic cause of many problems that occur in wood during drying and also in service. When water begins to leave the cell walls at 25 to 30 percent moisture content, the walls begin to shrink.
Even after drying, wood will shrink and swell in service as relative humidity varies (table 1-6). Drying stresses develop because wood shrinks by different amounts in the radial, tangential, and longitudinal directions and because during drying, shrinkage starts in the outer fibers before it starts in the inner fibers. These stresses can cause cracks and warp to develop. When wood is dried to 15 percent moisture content, about one-half of the total possible shrinkage has occurred; when dried to 8 percent, nearly three-fourths of the possible shrinkage has occurred. Figure 1-9 illustrates how Douglas-fir shrinks with loss of moisture. While these curves are not straight, the relationship between moisture content and shrinkage is generally approximated a straight-line relationship

Moisture content (percent)

Typical relation of moisture content toshrinkage of Douglas-fir. Although the curves are not
straight lines, they may be considered as such for practical shrinkage calculations.