timber

TIMBER

TIMBER

• Timber refers to wood used for construction works.

• The word timber is derived from an old English

word ‘Timbrian’ which means ‘to build’.

• A tree that yields good wood for construction is

called ‘Standing Timber.’

TIMBER

• After felling a tree, its branches are cut and its stem

is roughly converted into pieces of suitable length,

so that it can be transported to timber yard. This

form of timber is known as Rough Timber.

• By sawing, rough timber is converted into various

commercial sizes like planks, battens, posts, beams

etc. Such form of timber is known as Converted

Timber.

Seasoning of Timber

• This is a process by which moisture

content in a freshly cut tree is reduced to

a suitable level.

• By seasoning, durability of timber is

increased.

• Methods of seasoning:

– Natural seasoning

– Artificial seasoning.

Natural Seasoning

• It may be air or water seasoning.

• Air seasoning is carried out in a shed with a platform

• Timber balks are stacked on 300 mm high platform.

• Care is taken to see that there is proper air

circulation around each timber balk.

• Over a period, in a natural process moisture content

reduces.

• A well seasoned timber contains only 15% moisture.

Natural Seasoning

• Water seasoning is carried out on the banks of rivers.

• Thicker end of timber is kept pointing upstream side.

• After a period of 2 to 4 weeks, timber is taken out.

• During this period, sap contained in the timber is washed out to a great extent.

• Then timber is stalked in a shed with free air circulation.

Natural Seasoning

Artificial Seasoning

• Timber is seasoned in a chamber with regulated

heat, controlled humidity and proper air

circulation.

• Seasoning can be completed in 4 to 5 days only.

• Different methods of seasoning are:

– Boiling

– Kiln seasoning

– Chemical seasoning

– Electrical seasoning

Artificial Seasoning

• Boiling

– In this method timber is immersed in water and

then water is boiled for 3 to 4 hours.

– Then it is dried slowly.

– Instead of boiling water hot steam may be

circulated on timber.

– The process of seasoning is fast, but costly.

Artificial Seasoning

• Kiln Seasoning

– Kiln is an airtight chamber.

– Timber to be seasoned is placed inside it.

– Then fully saturated air with a temperature 35°C

to 38°C is forced in the kiln.

– The heat gradually reaches inside timber.

– Then relative humidity is gradually reduced and

temperature is increased, and maintained till

desired degree of moisture content is achieved.

Artificial Seasoning

• Kiln Seasoning

– Kiln used may be stationary or progressive.

– In progressive kiln, carriages carrying timber

travel from one end of kiln to other end gradually.

– The hot air is supplied from the discharging end

so that temperature increase is gradual from

charging end to discharging end.

– This method is used for seasoning on a larger

scale.

Artificial Seasoning

• Chemical Seasoning

– In this method, timber is immersed in a solution

of suitable salt.

– Then the timber is dried in a kiln.

– The preliminary treatment by chemical

seasoning ensures uniform seasoning of outer

and inner parts of timber.

Artificial Seasoning

• Electrical Seasoning

– In this method, high frequency AC is passed through

timber.

– Resistance to electric current is low when moisture

content in timber is high.

– As moisture content reduces, the resistance reduces.

– Measure of resistance can be used to stop seasoning at

appropriate level. However it is costly process.

– This technique has been tried in some plywood

industries but not in seasoning of timber on mass scale.

Defects in Timber

Defects in Timber

• Various defects which are likely to occur in

timber may be grouped into following 3:

– Due to natural forces

– Due to defective seasoning and conversions.

– Due to attack by fungi and insects.

Defects due to Natural Forces

• Knots

• Shakes

• Wind cracks

• Upsets

• Twisted Grain or Fiber

• Presence of Sapwood

• Sloping grains

Defects due to Natural Forces

• Knots

– When a tree grows, many of its branches fall and the

stump of these branches in the trunk is covered.

– In the sawn pieces of timber the stumps of fallen

branches appear as knots.

– Knots are dark and hard pieces.

– Grains are distorted in this portion.

– If the knot is intact with surrounding wood, it is called live

knot. If it is not held firmly it is dead knot.

Defects due to Natural Forces

Defects due to Natural Forces

• Shakes

– These are cracks in the timber which appear due

to excessive heat, frost or twisting due to wind

during the growth of a tree.

– Depending upon the shape and the positions

shakes can be classified as star shake, cup

shake, ring shakes and heart shakes.

Defects due to Natural Forces

• Wind Cracks: These are the cracks on the

outside of a log due to the shrinkage of the

exterior surface.

• Upsets: is due to excessive compression in

the tree when it was young. It is an injury by

crushing. This is also known as rupture.

• Twisted Grain or Fiber: is caused in the tree

itself due to action of wind.

Defects due to Natural Forces

• Presence of Sapwood: Sapwood is less durable than

heartwood & it should not be present in wood which is to be

sued for important places. Sapwood can be identified by the

colour which will be much lighter than that of heartwood. It

also does not take as good polish as heartwood.

• Sloping Grains: In living trees, cells do not always grow

perfectly vertical or straight & parallel to the length of the

trunk. They taper from bottom to top thus causing sloping of

the grain in timber.

Defects due to Defective Seasoning & Conversion

• If seasoning is not uniform, the converted timber may warp and twist in various directions.

• Sometimes honey combining & cracks appear.

• These types of defects are more susceptible in case of kiln seasoning.

• In the process of converting timber to commercial sizes and shapes the following types of defects are likely to arise: chip marks, torn grain etc.

Defects due to Fungi & Insects Attack

• Fungi are minute microscopic plant organism.

• They grow in wood if moisture content is > 20°C and exposed to air.

• Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.

• Beetles, marine borers & termites (white ants) are the insects which eat wood & weaken the timber.

• Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.

Preservation of Timber

Preservation of Timber• Preservation of timber means protecting

timber from fungi and insects attack so that its life is increased.

• Timber is to be seasoned well before application of preservatives. The following are the widely used preservatives:– Tar– Paints– Chemical salt– Creosote– ASCO– Charring

Preservation of Timber

• Tar

– Hot coal tar is applied to timber with brush.

– The coating of tar protects the timber from the

attack of fungi and insects.

– It is a cheapest way of protecting timber.

– DISADVANTAGE: appearance is not good after

tar is applied it is not possible to apply other

attractive paints. Hence tarring is made only for

the unimportant structures like fence poles.

Preservation of Timber

• Paints

– 2-3 coats of oil paints are applied on clean

surface of wood.

– It protects the timber from moisture.

– It is to be applied from time to time.

– It improves the appearance of the timber.

– Solignum paint is a special paint which protects

the timber from the attack of termites.

Preservation of Timber

• Chemical salt

– These are the preservatives made by dissolving

salts in water.

– The salts used are copper sulphate, masonry

chloride, zinc chloride and sodium fluoride.

– After treating the timber with these chemical salt

paints and varnishes can be applied to get good

appearance.

Preservation of Timber

• Creosote– Creosote oil is obtained by distillation of coal

tar.

– The seasoned timber is kept in an air tight chamber and air is exhausted.

– Then creosote oil is pumped into the chamber at a pressure of 0.8 to 1.0 N/mm2 at a temperature of 50°C.

– After 1 to 2 hours timber is taken out of the chamber.

Preservation of Timber

• ASCO– This preservative is developed by Forest Research Institute,

Dehradun

– It consists of 1 part by weight of hydrated arsenic pentoxide (As2O5.2H2O), 3 parts by weight of copper sulphate (CuSO4 5H⋅ 2O)

and 4 parts by weight of potassium dichromate (K2Cr2O7) or sodium

dichromate (Na2Cr2O2.2H2O).

– This preservative is available in powder form.

– By mixing six parts of this powder with 100 parts of water, the solution is prepared.

– The solution is then sprayed over the surface of timber.

– This treatment prevents attack from termites.

– The surface may be painted to get desired appearance.

Preservation of Timber

• Charring– In this method the surface is burnt & the burnt

part acts as a protective coat.

Veneers & Veneer Plywood (IS 303-1989)

Plywood

• It is a type of manufactured timber made from thin sheets of oak trees.

• It is one of the most widely used wood products.

• It is flexible, inexpensive, workable, reusable, & can usually be locally manufactured.

• Plywood is used instead of plain wood because of its resistance to cracking, shrinkage, and twisting/warping, and its general high degree of strength.

Plywood

• Plywood layers (called veneers) are glued together

with adjacent plies having their grain at right angles

to each other for greater strength.

• There are usually an odd number of plies so that the

sheet is balanced—this reduces warping.

• Because of the way plywood is bonded (with grains

running against one another and with an odd

number of composite parts) it is very hard to bend it

perpendicular to the grain direction.

Plywood• Veneer plywood is commonly referred as plywood. • For making plywood, round logs are first cut into

logs of 1.5m to 2.5m lengths. • They are then steamed & veneers are cut from

these pieces with a wide sharp knife extending the full length of the log.

• These veneers are glued together to form plywood. Generally, 3 or more veneers are glued together with the grains crossing each other.

• The cross-graining increases the strength of plywood.

Classifications of Plywood

• Ordinary Grade: Plywood of ordinary grade is ordinary

plywood used for packing tea-chests, etc.

• Exterior grade: Exterior grade plywood is made of durable

wood bonded fully with waterproof glue. In marine grade

plywood, core ply & exterior veneers are of good quality &

close-fitting wood.

• Marine plywood: is specially treated to resist rotting in a

high-moisture environment. Its construction is such that it

can be used in environments where it is exposed to moisture

for long periods.

Plywood

• Plywood is used in many applications that need

high-quality, high-strength sheet material.

• Quality in this context means resistance to cracking,

breaking, and shrinkage, twisting and warping.

• Exterior glued plywood is suitable for outdoor use,

but because moisture affects the strength of wood,

optimal performance is achieved in end uses where

the wood's moisture content remains relatively low.

Fibreboards (IS 12406-1988)

• Fiberboard is known as low density fiberboard is

a type of engineered wood product that is made out

of wood fibers.

• Types of fiberboard (in order of increasing density)

include Particle Board, Medium-Density Fiberboard,

& Hardboard.

• Fiberboard is sometimes used as a synonym for

particle board, but particle board usually refers to

low-density fiberboard. 

Fibreboards (IS 12406-1988)

• Plywood is not a type of fiberboard, as it is made of thin

sheets of wood, not wood fibers or particles.

• Fiberboard, particularly medium-density fiberboard (MDF), is

heavily used in the furniture industry.

• Fiberboard is also used in the auto industry to create free-

form shapes such as dashboards, rear parcel shelves, and

inner door shells.

• These pieces are usually covered with a skin, foil, or fabric

such as cloth, suede, leather, or polyvinyl chloride.

Fibreboards (IS 12406-1988)

• Fiberboard has many benefits and is used in

residential and commercial construction.

• Different uses and applications include:

– sound proofing/deadening,

– structural sheathing,

– low-slope roofing,

– sound deadening flooring underlayment