Timber is a synonym of wood, which is extracted from a tree. The word is used to describe the structural components of wood suitable for various formative purposes like the construction of doors, window, roofs, partitions, beams etc. The timber is also known as Lumber in North America.
Wide ranges of building materials are available for house construction. One of them is wood. Wood is widely used because of its valuable properties, i.e. ease of working, attractive appearance, reasonable cost, non-toxic, and recyclable nature and it offers prolonged service life if it get protected from moisture and insects properly.
Unfortunately, timber being natural possesses different types of defects. A defect is an irregularity or abnormality occurring inside or on the surface of the Wood or Timber which can affect its strength, appearance, durability and other properties.
Defects may be due to poor natural growth of the Wood, wood diseases, rapid seasoning and faulty processing. Here we are going to discuss various types of defects in timber.
Types of Defects in Timber
Timber defects are categorized into the following five divisions:
01. Defects caused because of insect manifestation
02. Defects in timber due to natural forces
03. Defects in timber due to Seasoning Impact
04. Defects Due to conversion of timber
05. Defects in timber due to fungi attackLet us discuss different types of timber defects in detail.
01. Defects Caused Because of Insect Manifestation
The insects usually found responsible for the decay of timber are termites, beetles, and marine borers.
a) Termites:
Termites are also known as white ants, and they are found in tropical and sub-tropical countries. These insects live in a colony and are truly fast in eating away the wood from the core of the cross-sections.
These insects make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover. Very few woods such as teak, sal etc. can resist the attack of termites. These timbers contain certain chemicals in their composition, and the smell of these chemicals is not palatable for termites.
b) Beetles:
Beetles are small insects, and they cause rapid decay of timber. They form holes of size about 2 mm diameter in wood and attack the sapwood of all species of hardwoods.
Tunnels are formed in all directions in sapwood by the larvae of beetles and wood gets converted into a fine flour-like powder. They usually do not disturb the outer layer of the timber. Hence the timber piece gets attacked by beetles may look sound externally till it completely gets withered inside.
c) Marine Borers:
Marine borers are generally found in salty water. Most of the varieties of marine borers do not feed on wood but they make holes or bore tunnels in wood for taking shelter.
The approximate diameter and length of holes or bores are between 25 mm and 60 mm.
The timber attacked by marine borers loses strength and colour. It is noted that no wood is completely immuned by the attack of marine borers.
02. Defects in Timber Due to Natural Forces
In many instances, the natural forces stand responsible for causing defects in timber. The defects are seen in the form of abnormal growth and harmful rupture of the wood-tissues. Natural forces cause the following defects:
a) Wind Cracks in Timber:
Wind cracks are the shakes or split on the side of the bark of timber due to the shrinkage of the exterior surface. It happens due to the exposure to atmospheric agencies like sun, wind etc.
b) Shakes in Timber:
Shakes are the abnormal cracks which wholly or partially get separated by the fibers of the wood. Different types of shakes are seen, some of the common shakes are cup shakes, heart shakes, ring shakes, star shakes, radial shakes etc.
i) Cup Shakes in Timber:Cup shakes are caused by the rupture of tissue in a circular direction. It is a curved crack, and it separates partly one annual ring from the other developing one due to non-uniform growth or due to excessive bending of a growing tree during a cyclonic weather. It may prove to be harmful if it covers only a portion of the ring.
ii) Ring Shake in Timber:When cup shake covers the entire ring of timber, they are known as ring shakes. According to ‘Roswell D. Carpenter, David L. Sonderman & Everette D. Rast’ (Author of book called Defects in Hardwood Timber), Ring shake in timber is a tangential separation of wood fibers along with the parts of the annual rings. It often looks so fine that is not found visible in greenwood and can be detected in dry wood later.
iii) Heart Shakes in Timber:Heart shakes occur in the center of a cross-section of timber due to the shrinkage of the interior part of a tree, which is approaching toward its maturity. It divides the tree in cross-sectional way into two to four pieces.
iv) Star Shake in Timber:Star shake cracks start from the bark and extend toward the sapwood. It is usually confined up to the plane of sapwood. These cracks are more extensive on the extreme ends and get narrower on the inside ends. Star shake cracks are typically formed during the growth of the tree.
v) Radial Shake in Timber:Radial shakes are same as star shakes, but as compared to star shakes, radial shakes are quite irregular, fine and numerous in shape. Generally, these cracks occur when timber is exposed to the sun for seasoning after falling.
Radial shakes cracks start from a short distance from the bark and gets extended toward the center, then follow the annual ring direction and then run toward the pith.
c) Twisted Fibers:
Twisted fibers are also known as the wandering hearts. The twisting of young trees causes these being impacted by fast blowing wind, and the fibers of timber gets twisted in one direction. The wood with twisted fiber is inapt for sawing.
d) Upsets:
Upsets are ruptures of a fiber of timber caused by some major impacts, injury, pressure, due to incompetent chopping and related potent effect.
e) Rind Galls:
The meaning of word rind is bark, and gall is a kind of abnormal growth. Therefore, an unusual cured swelling is found on the branch of a tree. This is known as the rind galls. Rind gall develops at the points from where branches are improperly cut off or removed. Rind galls are rarely found in a tree, and the timber in this part of tree becomes very weak and mostly fragile.
f) Burls:
Burls are also known as the excrescences. These are mainly formed when a tree has received shock or injury in its young age. Due to such damage, the growth of a tree may get completely upset, and irregular projections start appearing on the body of timber.
g) Water Stain:
When the timber comes into the contact with water, it loses its color. This is known as the water strain, and this defect is usually found in converted timber.
h) Chemical Stain:
The timber is discolored by the chemical action of some external reagents. This is known as the chemical stain.
i) Deadwood:
Wood obtained from a dead standing tree, i.e. after maturity, is known as deadwood. It is light in weight, it is found deficient in strength and it looks reddish in color.
j) Knots in Timber:
Knots are the roots or bases of small branches of the tree. These may stay alive or may become a dead part of the tree but they break the continuity of fibers. In the beginning, the base portion gets food from the stem, but finally it results in the formation of the dark and hard ring known as the knots.
These knots are not very harmful when these are small in size, feel hard, look round, and these timbers are mostly used for members other than tie bars and beans. Knots are classified being based on diameters such as pin knot, small knot, medium knot, and large knot. Timber containing large dead knots or many smaller ones should be rejected as these weaker points lack in strength and impact on the appearance negatively.
k) Coarse Grain:
If a tree overgrows, its annual rings are found widened. It is known as the coarse-grained timber, and this type of timber possesses less strength.
l) Foxiness:
Foxiness defect is indicated by the yellow tinge in the wood or it display reddish patches. They are known as stains or spots round the pith of tree creating the effect of discoloring of the timber. It is caused either due to poor ventilation during storage or by the onset of decay caused by maturity, or due to growth of a tree in a mushy soil.
m) Druxiness:
Druxiness is the sign of the early decay of a healthy wood indicated by its whitish spots. It happens due to fungal infection, and white spots are the indicator of underlying decay of the timber.
n) Callus:
Callus indicates soft tissue or skin which covers the wound of a tree.
03. Defects in Timber Due to Seasoning Impact
The following defects occur during the seasoning process of wood:
a) Bowing in Timber:
The bow is a timber defect indicated by the curvature formed in the direction of the length of timber.
b) Cup in Timber:
Cup is a defect of timber which is indicated by the curvature formed in the transverse direction of the wood.
c) Check in Timber:
Check is a crack which forms due to the timber seasoning process and it tends to separate the fiber layer of the wood.
d) Split in Timber:
When check extends from one end to the other in a wood log, it is known as a split. According to ‘Michael S. Mamlouk & John P. Zaniewski’ (Author of book called Material for Civil and Construction Engineers), Splits are lengthwise separation of the wood caused by either mishandling or seasoning.
e) Twist Wood Defect:
When a timber piece gets spirally distorted along its length, it is known as a twist.
f) Honey Combing in Timber:
Various circular and radial cracks get developed inside the timber due to stresses generated during the drying phase. Thus the wood internal layer assumes the honey-comb texture and the defect formed is known as the honey-combing.
g) Case Hardening:
In this case, the exposed surface of timber gets dried very rapidly. Therefore it shrinks and is comes under compression. As a result, the interior surface of the wood comes under the typical unwanted tension which has not entirely dried yet. This defect is known as case-hardening, which is caused due to the mismatch on timber texture. It usually occurs in timber, which is placed at the bottom during seasoning.
h) Collapse:
Due to uneven shrinkage, wood sometimes get flattened during the drying process. This is known as a collapse.
i) Timber Warping:
When a piece of timber has twisted out of its shape, it is called the warp.
04. Defects Due to Conversion of Timber
During the process of converting timber to commercial form, the following defect occurs:
a) Diagonal Grain in Timber:
The diagonal grain is a timber defect that is formed because of improper timber sawing indicated by a diagonal mark on the straight-grained surface of the wood.
b) Torn Grain in Timber:
The torn grain is also known as a form grain, and it is usually caused by a small depression formed on the finished surface of the timber by accidental falling of tool.
c) Chip Mark in Timber:
A chip mark is indicated by the marks or signs placed by chips on the finished surface of the timber. The parts of a planning machine may also stand responsible for forming these marks.
d) Wane Wood Defect:
Wane is a timber defect that is denoted by the presence of real rounded surface on the manufactured piece of timber.
05. Defects in Timber Due to Fungi Attack
Fungi attack wood when the following two conditions are found present simultaneously:
- The moisture content of wood is higher than 20%
- The unwanted presence of air for the growth of fungi.
If anyone of the above condition gets restricted, the defect in timber due to fungi gets stopped. Similarly, the wood submerged in water does not affect fungi because of the absence of air around. The following defect occurs in the timber due to the fungal attack:
a) Dry Rot in Timber:
Certain types of fungi feed on wood, and during feeding, they attack timber and convert it into dry powder form. This is known as the dry rot.
Following are the triggers of the dry rot issue:
- It occurs at the places where there is no free circulation of air is experienced: places like improperly ventilated basements, room etc. and humid spots like kitchens, toilets etc. are mostly vulnerable for timbers
- The unseasoned softwoods and sapwood easily get attacked by dry rot.
- The most favorable conditions for the rapid growth of fungus, which is responsible for dry rot, is the absence of sunlight, sogginess, presence of sap, stagnant air and warmth.
- It is also caused by painting and tarring the unseasoned timber.
- If timber is not properly stacked after cutting and shaped, it may get affected by an attack of dry rot.
b) Wet Wood Rot:
Some varieties of fungi cause chemical decomposition of timber. They convert wood into a greyish brown powder. This is known as wet rot.
The following points should be remembered in connection with the wet rot:
- The main reason for the development of wet rot is alternate dry and wet conditions.
- If unseasoned timbers are exposed to rain and wind, they become easily liable for attacks of wet rot.
- The well-seasoned timber should be used for exterior work to prevent wet rot problem.
- Timber should be covered by paint for ensuring protection against moisture. It is a preventive for wet rot issue.
c) Brown Rot in Timber:
Brown rot is used to indicate decay and underlying disease of timber. Specific Types of fungi remove cellulose compounds from wood, and hence the wood takes the brown color, which is known as brown rot.
d) White Rot in Timber:
White rot defect is just the opposite of brown rot defect. In the case of white rot defect, certain types of fungi attack lignin of wood, and the wood assumes the appearance of a white mass consisting of cellulose compounds.
e) Blue Timber Stain:
The timber stained to bluish color by the action of a particular type of fungi. This type of defect is known as a blue timber stain.
f) Heart Rot in Timber:
Heart rot defect is formed when branches have come out of a tree. In such a case, the heartwood is exposed to the attack of atmospheric agents. Ultimately the tree becomes feeble, and it gives out hollow sound when the hammer hits the log.
g) Timber Sap Stain:
The particular types of fungi do not cause the complete decay of timber, but they feed on cell contents of sapwood, and the sapwood loses its color and texture gradually. This is known as sap strain. It generally occurs when moisture content gets equal to higher than 25%.
Summing up, Defects in timber can occur at various stages, principally during the growing period, during the seasoning and conversion process, and also due to irregularities in the character of grains. Defects affect the quality, reduce the strength, compromise the quantity of useful wood, spoil the appearance, and call its decay faster than usual. Also, wood defects, in general, affect the structural safety, aesthetic aspects, and even in practical use hamper fire protection of the building.
Recognize more other Defects:
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Image Courtesy: Image 2, Image 6, Image 9, Image 14, Image 17 – blsurveyors, Image 18