Common defects that occur in timber include knots, shakes, wind cracks, upsets, twisting, cupping, bowing, rot and insect damage. These defects are typically grouped into three categories: defects caused by natural forces, defects caused by defective seasoning and conversions and biological defects.
Knots, shakes, wind cracks and upsets are defects caused by natural forces. Knots are the hardened stumps of broken or sawn branches, which can weaken or disfigure the surface of timber. Shakes are cracks in the timber caused by excessive heat, frost or twisting due to wind. Wind cracks are the cracks that appear on the outside of a log due to the shrinkage of the exterior surface. Upsets are defects in the grain caused by excessive compression or crushing of the tree when it is young.
Twisting, cupping, bowing are defects caused by defective seasoning and conversion. Twisting occurs when constriction causes the edges of a piece of timber to move out of parallel. Cupping is when the timber dries out in a curved shape. Bowing is a warp along the length of the face of the wood.
Rot and insect damage are biological defects. Rot most often occurs due to the presence of fungi in the timber. Insects weaken the timber because they eat the wood.