Timber – Classification, Types, Defects in Timber

Definition Of Timber

Timber or Lumber is nothing but wood used for building and other engineering purposes. It is obtained from the trunk of  trees. Trees give different types of timber which can be used for different kinds of works.

Classification of Trees :

Exogenous Trees :

These trees grow outward and have distinct consecutive annular rings in their horizontal section. They can be further divided into two types :

  • a) Soft wood – Chir, Deodar, fir, koil, pine, spruce, etc.
  • b) Hard Wood – Babul, Mahogany, oak, sat, teak, etc.

Lumber mostly used for engineering purposes comes from Exogenous Trees.

Endogenous Trees :

These trees grow inwards and fibrous mass can be seen in the longitudinal sections. Eg : Bamboo, Cane, Palm etc.  These trees have limited engineering applications.

Structure of Tree :

The horizontal section of a typical exogenous tree has following parts :

structure of timber

Pith :

This is the inner most part or core having soft tissues. It is found near the center of log or a Lumber. Its size varies from 1.5 mm to 10 mm in diameter.

Heart Wood :

The inner part of the tree surrounding the pith is called Heart Wood. It imparts rigidity to tree and provides strong and durable timber for various engineering purposes.

Sap Wood :

It is the area between the heart wood and cambium layer. Sap wood contains living cells and takes active pan in growth of tree. Usually it is light in colour and weight.

Cambium Layer :

If is the thin layer between sap wood and inner bark. It indicates sap which has yet to be converted into sap wood.

Inner Bark :

It is the inner skin or layer covering the cambium layer. It gives protection to the cambium layer.

Outer Bark :

It is the outer most protective layer of the tree. It consists of cells of wood fiber and it is known as cortex.

Medullary Rays :

These are thin radial fibers extending from pith to cambium layer. It holds together the annual rings of heart wood and sapwood. One ring is added every year which decides the age of the tree.

Defects in Timber

Defect occurring in timber are grouped into the following five categories :

  • Defects due to conversion.
  • Defects due to fungi.
  • Detects due to insects.
  • Defects due to natural forces.
  • Defects due to seasoning.

1. Defects due to Conversion :

a. Chip Mark : Marks or signs placed by chips on the finished surface of timber.

b. Diagonal Grain : Formed due to improper sawing.

c. Torn Grain : Small depression formed due to falling of a tree.

d. Wane : Pressure of original rounded surface on manufactured piece of Lumber.

2. Defects due to Fungi :

Fungi attacks the timber when moisture content is greater than 20% or there is presence of air and warmth in timber. Some of the important defects due to fungi are

a. Dry Rot : Certain types of fungi attack on wood and convert it into dry form.

b) Wet Rot : Certain types of fungi cause chemical decomposition of wood and convert timber into grayish brown powder.

3. Defects due to Insects :

The insects responsible for decay of timber are beetles, marine bores, termites.

4. Defects due to Natural Forces :

Important among these are Shakes : These are cracks which partly or completely separate the fibers of wood.

Types of Shakes :

a. Cup Shakes : 

  • Rupture of tissues occur in circular direction.
  • Separate one annual ring from the other.
  • Occur due to non-uniform growth or due to excessive bending during a cyclonic weather.

b. Heart Shakes :

heart shakes timber defect

  • Cracks which extend from pith to sap wood in the direction of medullary rays.
  • Occur due to shrinkage of interior part of tree.

c. Ring Shakes : 

ring shakes timber defect

These are cup shakes which cover the entire ring.

d. Star Shakes :

  • These are cracks extending from bark towards sap wood.
  • Formed due to extreme heat or severe frost during the growth of trees.

e. Radial Shakes :

radial shakes timber defects

  • These are cracks extending from heart wood towards sap wood.
  • Similar to star shakes but fine irregular and numerous.
  • occurs when tree is exposed to sun for seasoning after being fell down.

f. Rind Galls :

rind gall timber defect

  • These are peculiar curved swellings on the body of the tree.
  • Timber in this part is very weak and not durable.

g. Knots :

  • These are the bases of branches or limbs which are broken or cutoff from tree.
  • Continuity of wood fibers is broken by knots, they form a source of weakness.

h. Twisted Fibers :

twisted fibers timber defect

  • Fibers of wood are twisted in one direction.
  • Caused by twisting of young trees by fast blowing wind.

i. Upsets :

upsets timber defect

  • Wood fibers which are injure by crushing or compression.
  • Due to improper felling of tree or exposure of young trees to fast blowing wind.

5. Defects due to Seasoning :

The different defects which occur due to improper seasoning are as follows :

  • Bow.
  • Case hardening.
  • Check.
  • Collapse.
  • Cup.
  • Honey coming.
  • Radial.
  • Split.
  • Twist.
  • Wrap

Requirements of Good Timber : 

  • It should have uniform colour and regular annual rings.
  • Good Timber should have bright appearance and silky luster.
  • It should give a clear sound when struck.
  • Timber should be well seasoned.
  • It should be free from natural defects.
  • Good Timber should not be affected by wood rotting, fungi etc.
  • It should be hard, durable and tough.
  • Timber should not split when nail is driven into it.
  • It should be sweet to smell when freshly out.

Preservation of Timber :

The art of treating the timber with some chemicals so as to :

  • Increase the life of timber structure.
  • To make the timber structure durable.
  • Protect the timber structures from the attack of destroying agencies such as fungi etc.

Types of Preservatives :

a. AsCu treatment :

• AsCu solution : 6 parts of AsCu powder + 100 parts water.

• AsCu in powdered form composed of – Hydrated Arsenic pentoxide : 1 part,  Blue Vitrol or Copper sulphate : 3 parts, Potassium dichromate : 4 parts.

AsCu solution is sprayed or applied on timber surface. Protects the timber against the attack of white ants.

b. Chemical Salts :

  • Water borne preservatives i.e salts dissolved in water.
  • Commonly employed salts are : Copper sulphate, Mercury Chloride, Sodium Fluoride and Zinc Chloride.
  • Solution is applied on the timber surface.
  • These preservatives have good penetration in timber and show immediate increase in weight.

c. Coal-tar :

  • Timber surface coated with hot Coal-tar with the help of a brush is called Tarring.
  • Adopted for doors and windows, rough timber work etc.
  • Provides effective resistance for parts embedded in ground.

d. Creosote Oil :

  • Timber surface coated with creosote or under high pressure and temperature a period of 2 hours is called as creosoting.
  • This oil is obtained from the distillation of tar.
  • Good resistance for fungi attack.
  • Doubles the life or timber and generally adopted for piles, poles, railway sleepers etc.
  • Should not be used for interior surfaces of dwelling houses.

e. Oil Paints :

  • Coated with 2 or 3 coats of oil paint.
  • Preserves timber from moisture and make it durable.

f. Solignum Paints : 

  • Highly toxic in nature hence preserves timber from white ants.
  • Applied in hot state with the help of brush.

g. Charring :

  • Lower ends of the posts and poles are to be embedded in ground are generally charred with a view of prevent dry rot and attack of worms.
  • It is done by quenching the ends of posts in water after that they are charred fire to a depth of 0.5 to 1 cm.

Fire Resistance of Timber :

With respect to fire resistance, timber is classified into two types :

  • Refractory Timber : These are non-resinous in nature and thus do not catch the fire easily. Example – sal, teak etc.
  • Non-Refractory Timber : These are resinous in nature and thus catches fire easily. Example – Chir, deodar, fir.

Different Methods to make Timber more Fire Resistant : 

a. Application of Special Chemicals : 

These chemicals are called ‘Fire Protection Compounds’ or ‘Antipyrines’. Example : Solution of borax or Sodium arsenate. When treated with antipyrines, timber does not inflame even at high temperature but burns slowly.

b. Sir Abel’s Process :

Timber surface is coated with a dilute solution of sodium Silicate. Cream like paste of slaked fat lime is then applied on it. Finally concentrated solution of silicate of soda is applied. This is a quite satisfactory process in making limber fire resistant.

Seasoning of Timber 

The process of drying timber or removing moisture of sap present in a freshly felled timbers under more or less controlled conditions is called as ‘Seasoning of Timber’. A well-seasoned piece of wood should have 10% to 12% of moisture equal to atmospheric humidity of a place.

Objectives of Seasoning of Timber : 

  • The main objective is minimize shrinkage, warping and splitting.
  • To increase strength, durability and electrical resisting power.
  • And to protect it from the attacks of insects and fungus.

Methods of Seasoning of Timber :

1. Natural Seasoning :

Timber pieces can either be stacked horizontally or vertically and the seasoning is carved out by natural air. Moisture content of wood can be brought down to about 10 – 20% by natural seasoning. Time required for natural seasoning of timber is around 120 to 180 days.

2. Artificial Seasoning :

a. Boiling :

In this method, the Timber is immersed in boiling water or exposed to the action of steam. This method is a quick process of seasoning the timber. The timber seasoned by this method becomes brittle and easy to break.

b. Chemical seasoning :

An aqueous solution of chemical salts like common salt or urea have low pressure. If the outer layers of timber are treated with such chemicals the vapour pressure will reduce and a vapour pressure gradient is created.

The interior of timber, containing no salts, retains its original vapour pressure and therefore tends to dry rapidly. Corrosive effect of common salt is a drawback of this method.

c. Electrical seasoning :

The logs of wood are placed in such a way that their two ends touch the electrodes. Electricity is passed through this setup. Wood being a bad conductor of electricity resists the flow of electricity. In this process heat is generated which results in drying of timber. The drawback is that the wood may split due to overheating.

d. Kiln Seasoning :

Carried out in an air tight chamber under controlled conditions of circulating air, relative humidity and temperature. Desired degree of moisture content in attained. Time required for kiln seasoning is 12-20 days.

e. Water Seasoning :

The timbers like bamboo are placed in the flowing water. The flow of water takes away the sap wood. After one week the bamboo poles are removed from water and then dried out under shade. Timber loses its elasticity end becomes brittle as it is losing sap wood completely.

Testing of Timber : 

Specific gravity : 

The timber specimen of 50*50*150 mm is taken and its weight is measured. The specific gravity of the timber is

testing of timber


W = weight of the specimen, V = volume of the specimen, M = moisture content.
The specific gravity of good timber should be approximately 1.54.

Volumetric Shrinkage : 

The specimen is weighted initially and the volume is determined. The specimen is kept in an oven at 130 ± 2°C until approximately constant weight is reached. Volumetric shrinkage of the timber = (Vi-Vf) / Vi. Moisture content in percentage = (Wi-Wf)/ Wi. Oven dry specific gravity, G =( Wf/Vf ). Where V and W are initial volume and weights respectively and Vf and Wf are final volume and weights respectively.

Compressive Strength :

The timber specimen 50*50*200 mm is used with grains generally perpendicular to the direction of the application of load. A load of 2.5 kN is initially applied to set the specimen. Deformation under compression is then measured over a central guage length of 150 mm.

The final reading at maximum load is taken. Compressive strength of good timber at 15% moisture content is 30 to 80 MPa.

Tension Test :

The test specimen with cross section 7* 7 mm is used for this test, with the grains along the length of the specimen. The specimen is gripped in UTM and the load is applied at constant rate of 2.5 mm per minute till maximum load is applied.

The tensile strength of the specimen is the ratio of maximum load to the cross sectional area of the specimen. The tensile strength of timber parallel to the fibres is of the order of 80 to 190 MPa.

Bending Test :

A test specimen in the form of a beam of 50*50*100 mm is used for this test with the grains parallel to the length of the specimen. Two point loads are applied at middle third points. The load is applied at a constant rate of 3 mm per minute till crack in tension zone is seen.

The bending strength of the specimen is calculated based on simple bending equation. Bending strength of various timbers varies from 40 to 100 MPa.

Different Types of Industrial Timber

Veneers :

  • These are thin sheets of 0.4 mm to 6 mm thick.
  • Timbers suitable for veneers are Mahogany, Oak, Rosewood, Sissoo, Teak etc.
  • It is used to produce plywoods, batten boards and lamin boards.

Plywood :

  • Three or more Veneers in odd numbers are placed one above the other with direction of grains of successive layer at right angles to each other.
  • Placing of veneers normal to each other increases the longitudinal and transverse strengths of plywood.
  • Plywood has greater impact resistance to blows than ordinary wood.
  • Used for various purposes such as ceilings, doors, furniture, partitions, paneling, walls, packing cases, railway coaches, form work for concrete etc.
  • The percentage of moisture of plywood should be between 8 to 16%.

Fibre Boards :

  • The pieces of wood, cane or other vegetable fibres and chippings are heated and pressed under pressure between steel plates to obtain the fibre boards.
  • Thickness varies from 3 mm to 12 mm.
  • Density varies from 350 to 800 kg/cm³.
  • Used for preparing flush doors, table tops, insulating materials of heat and sound etc.

Impreg Timber : 

  • Timber fully or partly covered with resin like phenol formaldehyde.
  • Possess more electrical insulation.

Compreg Timber :

  • Preparing of compreg timber is same as impreg timber except curing is carried out under pressure.

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