BRE - DRYING DISTORTION OF TIMBER Guidance on selection, conditioning and handling

Organization: BRE
Publication Date: 30 April 2010
Page Count: 6


Timber is a naturally variable, hydroscopic and anisotropic material, subject to changes in dimension on drying. Inherent in its structure are features which affect dimensional stability (ie distortion on drying). These include juvenile wood, reaction wood (tension wood and compression wood), and spiral grain. Distortion on drying can cause construction problems such as poor fit at connections, creaking floors, and gaps opening up between boards. Timber will also distort on uptake of moisture, resulting in problems such as cumulative expansion. These defects can be very expensive to rectify but may be avoided by relatively simple measures.

Distortion on drying (as distinct from deformation under load) takes a number of forms (Figure 1). Bow and spring are essentially caused by differences in longitudinal shrinkage from one face or edge of the board to the other. Twist is driven by the presence of spiral grain, but is also a function of ring curvature. Cup is the result of differences in radial and tangential shrinkage within the growth ring structure.