Defects in timber may be broadly classified into three major categories :
Defects Due to Abnormal Growth of Trees
Defects due to abnormal growth of trees are natural in nature and they are knots, stakes, twisted fiber and rind galls.
Knots are too much frequent in many trees. They break the continuity of fiber and weaken the timber. These defects may of dead or live kind. below Fig. shows a knot defect. A dead knot will come out of the wooden piece leaving a knot hole. It is produced when a branch is broken off before the tree has finished growing. A live knot occurs when a branch separates from the tree after felling of the tree. Live knot will not become loose and fall out of its position but may tend to crack. This provides inlets for any fungi to attack the wood. If these knots are not too large and not too near the edge of the plank, they will not present a great problem however dead knots in timber makes the wood unsuitable for structural use.
Defects Due to Conversion and Seasoning
Defects due to conversion and seasoning of timber involve shakes, warping, bowing, twist, diamonding,casehardening and honey combing.
Warping is a kind of variation from a true or plain surface and may include a one or combination of cup, bow, crook and twist. Warping board which is tangentially sawn may invariably warp. This takes the form of a hollowing or cupping across the face of the board and when wide flat boards are required this will act as a serious drawback. Wind or twist defect occurs when thin boards are cut from a log having curved longitudinal grain. This tendency is for the board distort spirally
Diamonding in timber is the tendency of square cut pieces to become diamond shaped when cut from certain areas of the log. This happens when the piece has been cut with growth rings running diagonally, causing the unequal shrinkage between summer and spring growth to pull it out of shape.
Defects due to Fungi and Insects
Defects due to fungi and insects in timber involve fungi, dryrot, and wetrot
Fungi in timber use the wood as their food and destroy the same. It acts on the wood tissues and cells and cause decay. There are two types of defects in wood because of fungi and insects which are known as dry rot and wet rot
Dry rot is a type of fungus on dry wood which feeds on damp wood and breaks it down. Dry rot is so called because infected wood eventually becomes lighter in weight and takes on the appearance of being badly charred by fire, except that it is brown in color instead of black and crumbles under slight pressure. This fungus cannot develop in wood which has a sap content of less than 20% so that to prevent it, all that is really necessary is to use seasoned timber and to keep it dry.
Wet rot in timber is the decay due to dampness. The wood looks wet due to fungi attack on living trees. The effected positions of wood are reduced to a gray brown powder. Well seasoned wood, protected by painting etc., will avoid wet rot.