What Are the Different Types of Lumber Defects?

Lumber may include defects that affect either its appearance, its mechanical properties, or both. These defects can have many causes, such as natural growth of the wood, wood diseases, animal parasites, too rapid seasoning, or faulty processing. Common defect types are shown in Figure 10.11.

Knots are branch bases that have become incorporated into the wood of the tree trunk or another limb. Knots degrade the mechanical properties of lumber, affecting the tensile and flexural strengths.

Shakes are lengthwise separations in the wood occurring between annual rings. They develop prior to cutting the lumber and could be due to heavy winds.

Wane is bark or other soft material left on the edge of the board or absence of material.

Sap Streak is a heavy accumulation of sap in the fibers of the wood, which produces a distinctive streak in color.

Reaction Wood is abnormally woody tissue that forms in crooked stems or limbs. Reaction wood causes the pith to be off center from the neutral axis of the tree. It creates internal stresses which can cause warping and longitudinal cracking.

Pitch Pockets are well-defined openings between annual rings that contain free resin. Normally, only Douglas fir, pines, spruces, and western larches have pitch pockets.

Bark Pockets are small patches of bark embedded in the wood. These pockets form as a result of an injury to the tree, causing death to a small area of the cambium. The surrounding tree continues to grow, eventually covering the dead area with a new cambium layer.

Checks are ruptures in wood along the grain that develop during seasoning. They can occur on the surface or end of a board. Surface checking results from the differential shrinkage between radial and tangential directions and is confined mostly to planer surfaces. Cracks due to end checking normally follow the grain and result in end splitting.

Splits are lengthwise separations of the wood caused by either mishandling or seasoning.

Warp is a distortion of wood from the desired true plane (see Figure 10.10). The four major types of warp are bow, crook, cup, and twist. Bow is a longitudinal curvature from end to end. Crook is the longitudinal curvature side to side.  Both of these defects result from differential longitudinal shrinkage.

Cup is the rolling of both edges up or down. Twist is the lifting of one corner out of the plane of the other three. Warp results from differential shrinkage, differential drying due to the production environment, or from the release of internal tree stress.

Raised, Loosened, or Fuzzy Grain may occur during cutting and dressing
of lumber.

Chipped or Torn Grain occurs when pieces of wood are scooped out of the
board surface or chipped away by the action of the cutting and planing tools.
Machine Burn is an area that has been darkened by overheating during cutting.

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