Timber is a natural resource used to construct buildings, bridges, and other structures. The numerous uses of wood can be attributed to its desirable qualities, which include its ease of use, aesthetic appeal, affordability, non-toxic nature, and ability to be recycled. Wood can have an extended lifespan if properly preserved against moisture and insects.
As wood is naturally occurring, it unfortunately has all kinds of defects. An irregularity or aberration that appears inside or on the surface of wood or timber and has the potential to impair the material’s strength, durability, appearance, and other qualities is called a defect.
Poor natural development of the wood, wood infections, quick seasoning, and improper processing can all result in defects. It is important to be aware of the defects that can occur in timbers so that they can be prevented during the manufacturing process. In this blog post, we will discuss the different types of defects that can occur in timbers and how they can be prevented.
Types of Wood Defects
6 main types of defects can affect timbers, these are defects caused by;
- Natural forces
- Insects
- Fungi
- Defective seasoning
- Defective conversion and,
- Artificial/ Secondary Defects.
Defects Caused By Natural Forces
Defects in timber are frequently the result of natural forces at work. Defects manifest as detrimental wood-tissue rupture and anomalous growth. Timbers can be affected by natural forces such as the wind, rain, and sun. The effects of these elements can cause defects in timbers. Some examples include:
Knots
This is the most common type of defect in timber, it occurs when branches of a tree grow into the trunk. Knots are the bases or roots of the tree’s little branches. These disrupt the fiber’s continuity, whether they remain alive or eventually die off and become part of the tree. The base section first feeds on the stem, but in the end, this leads to the production of the hard, dark ring called the knots. This can be prevented by cutting down trees at an early stage before they have time to develop any branches that may become knots in the future.
When these knots are small, feel solid, and have a circular appearance, they are not very dangerous. These timbers are primarily utilized for members other than tie bars and beans. Based on their diameters, knots are categorized under four categories: pin, small, medium, and big.
- Pin knots have a diameter of up to 6.50mm
- Small knot’s diameter ranges from 6.50mm to 20mm
- Medium knot’s diameter ranges from 20mm to 40mm
- Large knots have diameters greater than 40mm
Types of Knots
Live Knots: This type of knot is caused when a branch or shoot grows into the trunk and then dies, leaving behind dead timber. The knots are still attached to trees but they do not have any live tissue left inside them so they can be easily broken off by hand.
Dead Knots: This occurs when branches grow into the trunk and die, but the knot remains attached to the tree. The dead wood inside the knot can be a fire hazard as it is very dry.
Shakes
Shakes are irregular fissures in which the wood fibers either completely or partially split apart. This defect occurs when there is internal stress within the timber and it causes cracks along its wood grain direction; this defect cannot be seen on the outside surface. It occurs when timbers start to split along their grain due to changes in moisture content or temperature, this can also cause warping which makes it difficult for the timber to be used in construction.
Shakes come in various forms; some of the most popular ones include the star, radial, cup, heart, and ring shakes.
Types of Shakes
- Heart Shake: This is a type of shake that affects the middle of the timber, it is caused when there is stress on the outside of the tree and it causes cracks to form along the grain. When a tree gets closer to maturity, its interior begins to shrink, causing heart tremors in the middle of the cross-section. It splits the tree into two to four halves cross-sectionally.
- Star Shake: This kind of shake, which affects the middle of the wood, is brought on by stress on both sides of the tree, which results in the formation of cracks along the direction of the grain. Star shake cracks radiate outward from the bark into the sapwood. Usually, it is limited to the sapwood plane. The extreme edges of these fissures are wider than the interior ends, which are narrower. Star shake fractures usually occur when the tree is growing.
- Ring Shake: Cup shakes are referred to as ring shakes when they include the whole ring of timber. Wood fibres and the components of the annual rings separate tangentially in ring shaking in timber, according to “Roswell D. Carpenter, David L. Sonderman & Everette D. Rast.” It frequently appears so delicately that it is not noticeable in greenwood but is eventually discovered in dry wood.
- Cup Shakes: A circular tissue rupture is the reason behind cup shakes. Due to uneven development or severe bending of a growing tree under cyclonic weather, it is a curving crack that partially divides one yearly ring from the other emerging one. If it just partially covers the ring, it can end up being dangerous.
Rind Galls
The meaning of the word rind is bark, and gall is a kind of abnormal growth. This leads to the discovery of an oddly healed sore on a tree branch. Rind galls occur when a branch of a tree is cut off improperly, causing abnormal bark growth in the area where it was cut off. Wood from this part of the timber is inadequate for use.
Twisted Fibers
Another name for twisted fibers is wandering hearts. The young trees’ tendency to twist makes them vulnerable to strong winds, which twist the wood’s fibers in one direction. This defect is caused by the fibers of the timber twisting around each other, it occurs when the tree is exposed to a high rate of strong wind at a young age. It makes the timber weak and unsuitable for construction.
Upsets
This is also caused by the fast-blowing wind. it affects the fiber of the tree. The tree looks crushed or compressed. Wrong felling/chopping of the tree could also cause these defects
Burls
Another name for burls is excrescences. These generally emerge when a tree is young and has been shocked or injured. Such damage can cause a tree’s growth to become completely unbalanced and for abnormal projections to start showing up on the wood’s body.
Defects Caused By Insects
Insects cause defects in timbers when they attack the timber. This results in holes, weakened structures, or reduced strength of the timber. Three main types of insects that can affect timbers; are Bettle, borers, and termites.
Borers
A borer is an insect that bores holes into wood, they don’t consume the wood rather they make wide holes in the wood to provide sheathe for themselves, Trees infested by these insects are usually discolored and very weak. Borers can infest any kind of tree.
Types of Borers
- Tent caterpillar borer: This is a type of borer that is found in the eastern part of the United States, it attacks the sapwood of the tree and leaves behind tunnels that are filled with sawdust.
- Marine borers: Salty water is typically home to marine borers. While the majority of marine borers do not consume wood, they do drill holes or tunnels through it to find cover. The holes or bores are roughly 25 mm to 60 mm in diameter and length. When marine borers attack wood, it becomes weaker and loses its color.
Termites
Termites are a type of insect that feeds on wood and other materials that contain cellulose. They have soft, white bodies with two pairs of wings that are about the same size as their body length; they also have chewing mouthparts at one end for feeding on food sources such as leaves from trees or grasses. When termites infect trees, they affect the inside of the tree which makes their presence unknown. Some trees however are termite resistant because of the chemicals in their cellulose e.g., teak, some oak species, and many others.
Types of Termite
- Subterranean: These are the most common type of termites, they live underground or in damp wood, and they create tunnels from their home to find a source of food such as cellulose which can be found on trees. They need moisture for survival and not all timber is suitable for them to infest.
- Drywood: These termites live in wood that is not wet, they don’t need moisture to survive and can infest any timber including furniture. They are found in tropical and subtropical areas.
- Formosan: Formosan termites are the most aggressive type of termite, they live in underground colonies and can damage wood from the inside out. They have a very high rate of reproduction which makes them more dangerous than other types of termites; they also need moisture like subterranean termites but unlike those, these don’t require it for survival or feeding purposes only the reproduction process needs moisture
Bettles
Tiny insects called beetles quickly deteriorate wood. They attack the sapwood of all kinds of hardwoods, leaving holes in the wood that are roughly 2 mm in diameter.
Beetle larvae tunnel into sapwood in all directions, turning the wood into a fine powder that resembles flour. Typically, they don’t agitate the timber’s exterior layer. As a result, the timber piece is attacked by beetles and may appear healthy on the outside until it entirely withers inside.
Types of Bettles
- Gypsy moth: This is one of the most common types of bettle and it is found all over North America, it feeds mostly on oak trees and leaves large holes in the leaves.
- Elm leaf beetle: This type of bottle is found mostly in the eastern United States, it feeds on elm trees and leaves skeletonized leaves behind.
- Cigar beetle: Cigar beetles are found mostly in the Eastern United States, they infest hardwoods such as oak
Defects Caused By Fungi
Fungi are a type of plant that can infest trees and cause defects in the wood. Some fungi are edible such as mushrooms but others cause diseases like black rot or brown rot which can kill trees if not treated early enough. When two of the following two circumstances are present at the same time, fungi attack wood:
- Wood has a moisture content of more than 20%.
- The air is present but not needed for the growth of fungus.
Fungi-related wood defects stop if any of the above conditions are controlled. Similarly, the lack of oxygen surrounding wood submerged in water prevents fungi from growing on it.
Types of Fungi
- White rot: This type of fungus affects the wood, it makes the wood soft and spongy and can make it crumble over time.
- Brown rot: Brown Rot is a type of fungi that affects the timber, it causes the timber to become brittle and mold quickly. It often starts at the ends of the timber.
- Black rot: This type of fungi affects both the sapwood and heartwood of trees, it causes the wood to become brittle and dark in color.
- Alternaria: Alternaria is a type of fungus that can cause defective timber, it makes black lesions on the surface of the wood which can spread over time and make it crumble.
- Fusarium: Fusarium is a type of fungus that can affect timber, it causes dark spots on the surface which can spread over time and make the wood crumble. This type of fungus requires moisture to grow so it’s most common in wet climates such as tropical areas where there’s high humidity.
Defects Due to Defective Seasoning
Seasoning is the process of drying and curing timber so that it can be used for construction purposes, if the timber is not dried or cured properly then defects can form in the wood. these defects can be from either too much or too little moisture.
Bow:
The bow is a defect that occurs when the wood has too much moisture in it, if not removed properly then this can cause problems such as twisting of timber which will make construction difficult.
Cupping
Cupping is caused by uneven drying of timber, it causes the edges of the wood to be at different levels than the center section causing distortion and warping.
Wedge
Wedge is a defect that is caused by the timber shrinking as it dries, this can cause the wood to split and crack.
Mold
Mold is a type of fungus that grows on timber when it’s wet, if not removed it can cause the wood to decay.
Stain
Stain is a discoloration of the wood that is caused by a chemical reaction between tannins in timber and iron, this can make it difficult to use.
Checking
Checking is the result of uneven drying, it causes cracks in timber which can cause problems during construction.
Burr
Burr is a defect that occurs due to improper seasoning or storage of timber, this can cause problems with construction as well as aesthetic value.
Defects Due to Defective Conversion
Conversion is the process of cutting timber into usable pieces for construction, if not done right then defects can form in the wood. these defects include:
Diagonal grain
Diagonal grain is when the wood is cut at an angle, this often happens when the timber is not straight. it can cause the wood to be unstable and prone to warping.
Cross-grain
Cross-grain is when the wood fibers are running in different directions, this can cause the wood to be unstable and prone to splitting.
Torn grain
Torn grain is when the fibers in the wood are ripped, this can often happen when the timber is being cut. it makes the wood weak and susceptible to breaking.
How to Prevent Defects in Wood
The best way to prevent defects in wood is by seasoning it properly, this means drying and curing the timber before use. you can also reduce the chances of defects occurring by storing the timber correctly and being careful when converting it into usable pieces.
For defects by insects, you can prevent them by treating the wood with insecticide before use. This helps to keep insects away from your timber and prevent damage caused by them.
Conclusion
Timber defects can arise at various stages, primarily in the growth phase, during the seasoning and conversion process, and also as a result of variations in grain characteristics. In addition to lowering strength and compromising the amount of usable wood, defects can degrade the wood’s look and accelerate its regular rate of deterioration. It is important for people who work with timber to be aware of these defects so they can take measures to prevent them.
