10 Timber Defects you did Not Know!

Edited by Namakhonje

Timber planks.

Timber is processed wood, ready to be used for construction. It is one of the most important construction materials. Its relevance spans a wide variety of functions. When constructing Inclined roofs timber is used to construct trusses which are the frames of roofs. It consists rafters, purlins, ties, struts, wall plates and the ridge that form the triangular frame of the roof. Timber can also be used to construct some interior fittings like cupboards, wardrobes, tables and chairs. Some framed structures even have their frames made entirely of timber. The relevance of timber is unquestionable.

Timber framed structure.

For that reason, timber has to go through a number of processes to improve its quality, durability and make it ready for use. Trees first undergo felling where their stems are cut down into large chunks of wood called logs. These logs are then chopped up into smaller pieces called planks in a process called sawing. In this case the wood is sliced along its length to produce long planks. Wood from a freshly cut tree contains a lot of excess unnecessary moisture which ought to be removed before the wood can be fit for use. This is done through a process called seasoning where excess moisture is removed using methods like, natural seasoning by open air, water seasoning where wood is placed in running water to drive out sap in wood tissue. Other methods used also include use of chemicals like sodium chloride, urea among others. After seasoning, wood can be used, however for better quality timber additional processes like painting or vanishing when carried out increase its durability. However, in some cases, the resultant timber is of low quality and is unfit for any form of construction. This arises because of a number of reasons, either by natural defects in the crude wood, or an inefficiency during the production process. Owing to this, the final timber product develops defects. Some defects are conspicuous and can be spotted immediately, however others are subtle and can’t easily be spotted. If they go undetected and used for construction, they may have serious repercussions on the structure. These defects lower the qualities of standard timber like, tensile and compressive strength, elasticity, rigidity, workability among others. Bad timber is not durable hence may easily rot within a short time thus it’s imperative that you’re able to spot defective timber and reject its use in construction. Here’s a look at ten timber defects you can easily spot.



These appear as dark parches on the surface of timber. They mark the point of detachment of a branch previously attached to that point of a stem from which the timber was obtained. It is when this branch breaks off or dies off that this parch forms. This defect may appear harmless at first sight but make no mistake, that small parch will become a weak spot on that timber piece. It can only be witnessed when the timber is used under conditions that apply a lot of stress on it. That’s even without considering the fact that they wouldn’t look good on a piece of furniture.

Cross grain

Normal grain
Cross grain

Originally, timber should consist layers of grain that are as a result of the nature of wood of growing in form of successive rings. When the wood is cut into timber, it is in form of grains that are supposed to be parallel and run along the length of the timber. In most cases, the rings of a tree may be slightly inclined towards each other but never actually meet. However, on rare occasions, these rings meet and the resultant timber develops a defect called cross grain. In such a case, the grains overlap. This greatly lowers the tensile and compressive strength of the timber.

Wood rot.

window frame with rotting wood.

This occurs when the timber begins to decay due to action of fungi and bacteria under moist, damp and poorly aerated conditions. There are two types of timber rot, dry rot and wet rot. For dry rot, the timber slowly begins to decay into a white powder. It is caused as a result storing imperfectly seasoned timber in a damp warm confinement with limited access to fresh air. For wet rot, timber takes up a greyish coloration. This type of rot is caused by placing timber in alternating dry and moist conditions. Rot has adverse effects on properties of the timber like tensile and compressive strength.


Fully processed timber usually comes in form of straight flat planks. However, in some situations, that is not always the case, sometimes it comes deformed in a number of irregular undesirable shapes. These irregular shapes come in various forms including bowing, cupping, crooking and twisting.
Bowing takes place when timber is deformed in a longitudinal direction. The surface gets curved forming the shape of a bow.


Cupping is a distortion along the breadth of the timber plank. The length remains undistorted. This gives the timber a u-shape along its breadth forming the shape of a trough.


Twisting is an extreme kind of distortion in which one side of the timber plank is so distorted it overlaps the other. The distortion takes place along the diagonal of the plank.


Crook is a bend that takes place at the side and along the length of the plank.


These deformations are caused either naturally on the tree before it is cut, or during the manufacture process of timber. Natural causes may be due to strong wind that may deform a tree hence deformed resultant timber. Artificial causes occur during the conversion process of wood to timber for example, uneven seasoning may make some parts expand or contract more than others causing such deformations.



These are cracks that develop longitudinally along the length of the timber planks and cuts across the grains of the timber. This results into a very vulnerable line of weakness on the timber plank and can snap anytime a large amount of stress is applied. The main cause of checks is uneven seasoning, one side is exposed to air but the other isn’t, not equally anyway. So one side contracts faster than the other and this stress causes this slight tear which causes these cracks. If unattended, the crack may develop and spread from one edge to the other causing a total separation of wood pieces and this is called a split.

Case hardening.

This occurs when the surface of the timber hardens but the inside remains soft. This is also caused as a result of uneven seasoning where the top surface is more exposed to the seasoning compared to the rest of the wood and thus contracts and becomes harder than the inside. In that case, the timber develops uneven tensile and compressive strengths making it unreliable. This defect may not be immediately apparent at first sight, but can be identified by examining texture of timber by hand.



FThese occur when there is a longitudinal separation, due to cracks that develop and run across the length of the log. This defect develops even before felling when the wood is still on a tree. It develops due to old age of the tree. As the tree grows older, the inner most parts begin to shrink hence causing cracks the run radially from the center outwardly to the bark. This defect greatly reduces the shear strength of a log of timber.

Reaction wood.

reaction wood.

This occurs when timber develops dark brown parches. These denote regions of increased tensile and compressive strengths. This occurs entirely due to natural causes. When a tree is consistently subjected to wind forces in one direction, it tends to bend in that direction. The tree’s resistance to that constant bending force develops that point of thickened growth, the point of reaction to the bending force. This isn’t good for construction since it produces timber of uneven strength.



This is growth of a thickened cover on the surface of a tree at a point of injury. When a tree is irregularly cut or scratched, new sap is generated to cover the wounded area and this develops into an irregular thick cover. This ought to be removed before the log is further manufactured into timber otherwise it produces timber of undesirable qualities.

Burr and Curl.

Burr and curl

This is a thickened overgrowth that develops at the point a branch meets the stem or the branch meets another branch. A swelling develops at these junctions most times because of over twisting perhaps due to swaying by wind. These overgrowths are irregular and ought to be removed during the manufacture process of timber.

Knowledge of the various timber defects can help you discern good timber from defective timber. As a home owner or contractor, this can save you a whole lot of expenses in poor quality work or potential hazards that may be caused due to bad timber. As a timber manufacturer or supplier, this can be an eye opener into the importance of care and precision during manufacture since most defects develop during the manufacture process especially during seasons.

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