Although splits can occur in timber due to shear, breaking wood along the grain or at a 45 degree angle to the load, most splits in timber are due to imperfections and to tangential shrinkage of the wood as it is cured. Splits in wood occur due to checks, shakes, pitch pockets and knots.
Checks are gaps or separations along the length of a wood member, crossing the annual rings. These splits are caused by rapid, tangential shrinkage or uneven seasoning of the wood. Checks can be filled in with putty but the presence of these imperfections denotes a weaker structural capacity. Other imperfections in wood due to shrinkage and curing are cupping, bowing, crooking and twisting of wood planks and sheets. These imperfections depend on the direction in which the lumber was cut from the log.
Shakes are gaps or separations parallel to the annual rings. Unlike checks, shakes occur before or during felling of the timber, and these are the result of uneven stresses along the tree trunk. As with checks, shakes can be filled with wood putty, but the structural integrity of wood with shakes is lower than that without.
Pitch pockets are splits in wood between annual rings that occur while the tree is living. The pitch pockets, found in softwoods, are filled with tree sap, but as the wood is cured, the pitch pockets often empty or harden. Pitch pockets are imperfections that are difficult to hide and weaken the wood. Nevertheless, pitch pockets are somewhat rare in building grade lumber.
Knots occur at the intersection of trunks and branches. A knot is usually an ovoid cross-grained form and is very hard, relative to the rest of the wood plank or sheet. Often, the knot is removed from the wood, however the structural capacity of the timber cannot be improved when a knot is present, therefore wood structural grading limits the presence and location of knots in structural lumber.
Some splits are due to insects, such as termites, carpenter ants and pine borers, and other animals, such as woodpeckers. However, new, lumber grade wood almost never has signs of these animals.
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