Timber is one of the oldest and most widely used building materials in construction due to its strength, durability, workability and aesthetic appeal. However, being a natural material, it is prone to various defects that may develop during the growth of the tree, during conversion from logs to timber, or during seasoning and storage. These defects reduce the strength, durability, appearance and service life of timber, and therefore a clear understanding of them is essential in building construction and maintenance. Defects in timber may be broadly classified into natural defects, defects due to seasoning, defects due to conversion, and defects due to biological agencies such as fungi and insects. Natural defects arise during the growth of the tree. Knots are among the most common natural defects. They are formed when branches of the tree are embedded in the trunk. Knots may be live or dead. Live knots are firmly held in the wood and are generally sound, whereas dead knots are loose and may fall out, leaving holes. Knots disturb the continuity of wood fibers, thereby reducing strength and making the timber unsuitable for structural members subjected to heavy loads. Shakes are another important natural defect. These are cracks or splits that occur in the wood due to stresses developed during growth. Heart shakes originate at the pith and extend outward towards the sapwood. Star shakes start from the outer surface and extend inward along the medullary rays. Ring shakes occur along the annual growth rings, separating one ring from another. Shakes weaken the timber and permit the entry of moisture, which further accelerates decay. Wind cracks are splits on the outer surface of standing trees caused by the action of wind and weather. Twisted fibers, also known as spiral grain, occur when the fibers of wood are not parallel to the axis of the tree. This defect