BUILDING MATERIALTopic 2: Timber
The Copyright of this document belongs to BCA. No part of this document may be used, disclosed, transferred, reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying recording or
otherwise to any person not authorized to use it without the express written permission of BCA
Topic 2: Timber
Introduction:Timber is a valuable material for a number of reasons:1.It is structurally useful because of its strength in relation to its density2.It is comparatively easy to work to a variety of shapes 2.It is comparatively easy to work to a variety of shapes either by hand or machine3.It is durable under appropriate conditions4.It gives a good finished appearanceHowever, as it is a product of nature which comes to use in countless varieties and qualities, it must be properly processed and selected to suit the work at hand
Topic 2: Timber
The advantages of timber as a building material:
* It is structurally useful because of its strength in relationto its density.
* It is comparatively easy to work to a variety of shapeseither by hand or machineeither by hand or machine
* It is durable under appropriate conditions
* It gives good finished appearance
Topic 2: Timber
Sapwood
Heartwood
Topic 2: Timber
Cross-section of a tree
Topic 2: Timber Outer bark:
Give protection to softer tissues inside the trunk
Inner bark:Helps in conducting food materials to active living layers and storagetissues
Cambium: Cambium:Responsible for the circumferential growth. Produces bark towards theoutside and wood towards the inside of the trunk
Sapwood:Responsible for food conduction and storage
Heartwood:Mainly concerned with providing mechanical stability and supporting theheavy top part of the tree.
Topic 2: Timber
CLASSIFICATION OF TIMBER
All commercial timbers can be classified into two groups: softwoods and hardwoods. The term softwood or hardwood does not necessary indicate the relative hardness or density. Sometimes hardwood is softer and hardness or density. Sometimes hardwood is softer and less dense than softwood. The true distinctive between the two groups of timber is botanical
Softwood produced from the coniferous trees such as pines and spruces which have characteristic needle like leave. The trees are generally evergreen. They are mainly found in temperate countries
Topic 2: Timber
Types of Coniferous Trees
Topic 2: Timber Hardwood is produced from dicothyledons group of trees which are broad leafed trees such as oak, beech and ash. They are found in most part of world. Hardwoods are generally denser, stronger more durable and more costly than softwoods
Dicothyledons trees
Topic 2: Timber
Timber in sawmill
Topic 2: Timber
CONVERSION OF TIMBER
Conversion is the term used to describe the processes in which the felled truck is sawn into marketable sizes of timber. Conversion of timber is usually done in fixed sawmills normally located close to the source of timber. The log is first sawn into manageable lengths either at The log is first sawn into manageable lengths either at stump in the forest or at the mill and is then de-barked giving a cleaner log for sawing
In large mills de-barking is done mechanically but in smaller mills it may still be done by hand. The logs are then swan lengthwise into the required sections using following method of sawing
Topic 2: Timber
Timber debarking
Topic 2: Timber
Tangential sawing:It is adopted when the timbers have distinct annual rings
Rift sawing:The log is first sawn into four pieces and each quarter is cut into boards. The rays appear irregularly on the surface to produce the silver grain which is so highly surface to produce the silver grain which is so highly valued for high class joinery work. It is an expensive form of conversion as much waste results
Slab sawing:The inner pieces are rift sawn and outer slabs approximate to tangential cuts. There is less waste by this method and it is therefore the cheapest
Topic 2: Timber
The advantages of various types of timber conversion methods:
Tangential sawn timber
This sawing method removes the maximum amount of material in clearer grades and leaves the knot defects mostly boxed into the central portion.
Rift sawn timber
The most economical method.
Slab sawn timber
Radially cut boards have greater dimensional stability across the width
Topic 2: Timber
Defects due to conversion:
Apart from the natural defects such as knots, common conversion defects arise from the cutting of timber from a round log. There are:
Waney Edge:The original surface of the tree which remains at the edge or edges of a piece of timber after conversion
Woolly Grain:The condition of a machined surface on which the fibres have been dragged up by the cutter or sawn instead of being cut
Topic 2: Timber
Waney Edge
Topic 2: Timber
Defects due to conversion - continue:
Machine Burn:Darkening or charring of wood due to excessive friction during conversion
SEASONING OF TIMBER:When just felled, timber contains a large amount of sap. Before the timber can be used, most of this moisture content must be removed otherwise the timber will shrink excessively causing defects in the works, a tendency to decay and insect attack
Topic 2: Timber
Sap
Topic 2: Timber
SEASONING OF TIMBER continue
Seasoning is the term used to describe the process of drying timber in a controlled manner to reduce its moisture content without introducing unwanted defects such as splits or distortion. Moisture is present in such as splits or distortion. Moisture is present in unseasoned timber both within the cell cavities and cell walls. When the timber is seasoned, the water within the cell cavities may be removed without any effect on the timber. This may reduce 73% of moisture content in timber. Drying below this point requires removal of water from cell walls which results in shrinkage. The art in seasoning is to remover this water with as little detrimental effect on the timber as possible
Topic 2: Timber
There are two methods of seasoning timber:
1.Natural Air Seasoning
In natural air seasoning, the timber is stacked in the open air or in open-sided shed in such a way as to promote drying without artificial assistance. The planks are drying without artificial assistance. The planks are stacked horizontally off the ground about 150 mm above the ground level, each plank being separately from the next by means of wedges. This system of open stacking allow free circulation of air and the stacks are protected form the sun and rain by a light roof. Softwoods are more porous than hardwood and therefore dry out more quickly.
Topic 2: Timber
Timber Natural Air Seasoning
Topic 2: Timber
When a number of such stacks may be constructed under the same sheet, there must be a space of not less than 450mm between stacks
The process of seasoning is checked by determining the moisture content of the wood when it is first piled and at moisture content of the wood when it is first piled and at definite intervals of time thereafter. The time taken for air seasoning depends on the temperature and humidity of the atmosphere, efficiency of stacking, thickness, moisture and the species of timber. Usually the seasoning period takes about 6 to 12 months
Topic 2: Timber
The advantage of air seasoning is that it is cheap method with very little loss in quality of timber if done properly. The advantages are that it is relatively slow process and very little control is possible
Kiln DryingKiln Drying
In Kiln Drying or artificial seasoning, temperature up to 80 degree celsius are used which cause the moisture in the wood to move more rapidly to the surface from where it is removed by the circulating air
A kiln is a closed chamber where circulation of air, humidity and temperature can be controlled so that drying can be regulated and damage due to shrinkage
Topic 2: Timber
Timber pieces to be seasoned are stacked inside the chamber such that spaces are left for free circulation of air. Warm and fully saturated air is forced inside the chamber. The air used in the kiln should be relatively in high humidity so as to keep the rate evaporation of moisture from the surface of the wood at a safe levelmoisture from the surface of the wood at a safe level
As seasoning proceeds the temperature can generally be raised and the humidity of the air lowered. Air circulation should be maintained fairly uniformly over the surface of the timber and from time to time some of the air should be replaced by a fresh supply for outside
Topic 2: Timber
The advantages of kiln seasoning are its rapidity and precision. A lower moisture content in timber can be achieved by kiln seasoning. In properly operated kiln, every piece of timber can be dried to uniform moisture content throughout. Moreover the temperature used are lethal to any insect or fungus that may be present in the lethal to any insect or fungus that may be present in the timber
Topic 2: Timber
Timber Kiln Seasoning
Topic 2: Timber
Results of seasoned timber:
1) Less danger of rot or stain developing2) Improved strength and resilience3) Resistance to fungal decay and insect attack
(furniture and powder post beetles) and dimensional (furniture and powder post beetles) and dimensional stability
4) Shipping weight of timber is reduced
Topic 2: Timber
Effects due to incorrect seasoning:
1) Case-hardeningThe condition existing in timber in which the outer layers have undergone some drying and become set without corresponding shrinkage, causing stress set without corresponding shrinkage, causing stress between the inner and outer layers
2) Warpdistortion in converted timber causing departure from its original plane
Topic 2: Timber
3) Split:A separation of the fibres along the grain forming a crack that entends through the piece from one side to another
4) Surface Stain:A brownish discolouration of timber casued either by A brownish discolouration of timber casued either by oxidation or by accumulation of the extractives during seasoning can be removed by planning
Topic 2: Timber
Defects due to improper seasoning
Topic 2: Timber
Defects (twist) due to improper seasoning - cupping
Topic 2: Timber
Defects due to improper seasoning
Topic 2: Timber
Twist
Defects (twist) due to improper seasoning
Topic 2: Timber
Spring
Defects due to improper seasoning
Topic 2: Timber
Knots are considered as defects because:
shrinkage is greater across the knot than in the surrounding wood. Encased knots may loosen and drop out.
although intergrown knots remain tight, they may develop radial cracks.
encased knots caused discolouration and the entraped bark associated with them.
disorientation of grain direction is troublesome when machining.
spike knot extending across a board may cause it to break in half under small loads.
Topic 2: Timber
Knots are considered as defects
Topic 2: Timber
Timber in sawmill
Topic 2: Timber
PRESERVATION OF TIMBER:
The sapwood of all timbers and heartwood of many timbers are liable to be attacked by wood destroying fungi, boring insects and termites. All untreated timbers placed in salt waters are liable to attack by a variety of organisms in salt waters are liable to attack by a variety of organisms collectively known as marine borers
The heartwood of some timber, such as Balau and Chengal are highly resistive to decay and insect attack and do not require little treatment if kept dry and out of ground contact. The sapwood of these species is not durable and the presence of sapwood is in many cases of initial attack in these species
Topic 2: Timber
Sapwood
Heartwood
Topic 2: Timber
Preservation of timber is the treatment of timber with toxic chemicals to protect it from attack by both fungi and insects
Types of preservations
The three types of preservative commonly in use are:The three types of preservative commonly in use are:1) Tar Oil2) Organic solvent3) Water-borne
Topic 2: Timber
1) Tar Oil Types:These preservatives are derivatives of coal tar and wood tar, for example coat tar are relatively inexpensive and effective but have some practical disadvantages such as odour, their liability to stain other materials.
2) Organic Solvent Types:It consist of various toxic chemicals in an oil solvent which is It consist of various toxic chemicals in an oil solvent which is volatile (e.g. white spirit and petroleum distillates). They are relatively costly but offer good penetration, dry quickly anc can be over painted.
3) Water-Borne Types:These are aqueous solutions of one or more toxic salts (e.g. copper sulphate or aluminum sulphate etc). They are convenience to use, offer good penetration and allow overpainting when dry. However, the re-drying of treated timber by kilning may be necessary.
Topic 2: Timber
The merits and demerits of three (3) common types of timber Preservatives method are as follows:
1) Tar oil types:
These preservatives are derivatives of coal tar and wood tar, for example coal tar creosote. They are relatively inexpensive and very effective, but have some disadvantages such as odour, their liability to effective, but have some disadvantages such as odour, their liability to stain other materials and the fact that they cannot be painted over directly with standard finishes.
2) Organic Solvent types
It consists of various toxic chemicals in an oil solvent which is usually volatile (e.g. white spirit and petroleum distillates). They are relatively costly but offer good preservation, dry quickly and can be painted over.
Topic 2: Timber
The merits and demerits of three (3) common types of timber Preservatives method are as follows:
3) Water-Borne types
These are aqueous solutions of one or more toxic salts (e.g. copper sulphate, zinc chloride and aluminium sulphate). (e.g. copper sulphate, zinc chloride and aluminium sulphate). They are convenient to use, offer fairly good penetration and allow over painting when dry. However, the re-drying of treated timber by kiln drying may be necessary. Some are unsuitable for external use owing to leaching tendencies.
Topic 2: Timber
The two principle types of pressure treatment processes used in timberpreservation are as follows:
a) Full-cell Process:
The, timber is placed in an enclosed pressure cylinder. The timber issubjected to low vacuum for a period of 30 minutes to I hour. While thevacuum is maintained, the preservatives, usually, water-born type is thenvacuum is maintained, the preservatives, usually, water-born type is thenintroduced to fill the cylinder. Pressure is then gradually increased andmaintained for periods up to several hours until the required amount ofpreservative has been introduced into the timber. The length of time isdepending on the type of timber and preservative.
Then the pressure is reduced and the cylinder is drained of preservative.A further vacuum is applied for a brief period just long enough to cleanthe surface of the timber.
Topic 2: Timber
b) Empty-cell Process:
This differs from the full-cell process in which no initial vacuum is used.As a result air remains in the cells and after treatment, the cell cavities oftimber are only partially filled with preservatives. This method is normallyConfined to treatment with creosote or tar-oil types.
Topic 2: Timber
The ten essential requirements of a good timber preservative:
A wood preservative should have the following properties:
a) Toxicity:High toxicity to wood destroying organisms is essential.
b) Penetration:b) Penetration:Ability to penetrate deeply into the wood is essential.
c) Permanency:Should remain permanently in the wood in service and should not leach or diffuse out of the wood in the presence of water or high humidity.
d) Chemical stability:Chemicals used should not break down nor lose their toxicity over a period of time.
Topic 2: Timber
The ten essential requirements of a good timber preservative - continue:
A wood preservative should have the following properties:
e) Safety:Should be safe to handle and use and non-poisonous to animals and humans when in the timber.
f) Interaction with other materials:The preservative should not react with other materials placed in contact with it to cause corrosion, staining, etc.
g) Cleanliness and odour:Preservatives used in buildings should be free from objectionable smells, clean to handle and use, and should not contaminate foodstuffs.
Topic 2: Timber
The ten essential requirements of a good timber preservative - continue:
A wood preservative should have the following properties:
h) Painting:It must be possible to paint, stain, varnish or polish treated timber in the same manner as untreated timber.
i) Fire hazard:No treatment should increase the flammability of timber, and should preferably impart fire retardant properties to timber.
j) Moisture repellent:The treatment should preferably impart moisture repelling characteristics to wood.
In practice, a series of treatments are in use to meet these specialised requirements.
Topic 2: Timber
Method of Preservation:
The main methods of preservation include:
1) Brushing and Spraying
This is the earliest and least effective method which can be used with all types of preservatives. Suitable for treatment of timber in situ. all types of preservatives. Suitable for treatment of timber in situ. The penetration obtained seldom exceeds 1.5 mm. Tar oil should be preferably applied hot.
2) Immersion
This method may be specified for a variety of preservatives, although the organic solvent type is most frequently used in this method. The period of immersion may vary only a few seconds, to a few minutes or up to one hour. Deeper penetration is obtained and hence this method is more effective than the brushing and spraying
Topic 2: Timber 3) Pressure Process:
Unlike the previous methods which are non-pressure processes, here the preservatives are impregnated by application of pressure. This gives deepest penetration and is the most effective method of treatment for most timbers.
Topic 2: Timber
Group A Group B Group C Group D
Externally Strong Very Strong Strong Weak
Chengal (HH)
Balu (HH)
Tembusu (HH)
Kapur (MH)
Teak (MH)
Ramin (LH)
Durain (LH)
Jelutong (LH)
Gian (HH)
Bakau (M)
Merbau (HH)
Kempas (MH)
Meranti (LH)
Nyatoh (LH)
Damar Minyak (S)
HH Heavy Harwood S SoftwoodMH Medium Hardwood M - MangroveLH Light Hardwood
Topic 2 - Timber
PROPERTIES OF TIMBER:1. Density:The density of solid cell wall material in wood is effectively constant, irrespective of species. The value is 1500 kg/m3. though the density of commercial timber pieces differs for though the density of commercial timber pieces differs for softwood and hardwood. Variations in density from tree to tree are often the results of differences in growth rate under the influence of latitude, climate and soil conditions. The major practical importance of density in timber is its role as an indicator of strength
Topic 2 - Timber2. Strength Properties:Density and a wide range of naturally occurring defects affect the strength of timber, its strength properties differ with respect to the direction of loading relative to the grain direction. Timber strength is also affected by moisture content, rate of application and duration of loading and content, rate of application and duration of loading and temperature. For example, the wet strength in bending are up to 50% less than the dry strengths. The effect of temperature is a linear decrease in strength with increase in temperature.
Topic 2 - Timber
3. Deformation:Timber experience deformation due to applied loads, change in moisture content or changes in temperature. Like all materials, timber expands and contracts on heating and cooling. The deformation response of timber to load is quite complex. Under low levels of load applied to load is quite complex. Under low levels of load applied for short duration, timber deforms elastically. If load applied is increased beyond a certain point, deformation increase more rapidly and the stress-stain relationship becomes more curvilinear before strength failure finally occurs.
Topic 2 - Timber
4. Durability:Timber is bio-degradable. Different species of timber will deteriorate at different rates. Timbers are generally classified in five broad categories ranging from perishable (heartwood stakes survive less than 5 years) to very durable (survival for more than 25 years). The very durable (survival for more than 25 years). The very durable timbers are all hardwoods and include teak.
Topic 2 - Timber
USES OF TIMBER1. Structural Appliances:1) Marine Work:
Much of the marine works traditionally require timber for wharves, piers, sheet piling and cofferdam. The wharves, piers, sheet piling and cofferdam. The requirements for the timber in marine work are high density, close grain structure and natural durability and wear resistance. All these qualities are found only in hardwoods.
Topic 2 - Timber2) Heavy Construction Work:
Timber is used in construction work mainly for piling.Other uses include gantries, bridges and shoring etc.
The requirements of timber for this use arehigh density, closeness of grain, resistance to impactand chemical attack.and chemical attack.
3) Medium/light Construction Work:Roof trusses, partitions, screens, floors and wall panels are produced using timber. The requirements in the class of work are resistance to insect and fungal attack and minimum dimensional change due to temperature and humidity variations. The timber suitable for this use is softwoods such as whitewood and redwood.
Topic 2 - Timber
2. Falsework Carpentrythe uses include shuttering for in situ or precast concrete work, support formwork for brick or stone arch. Timber is also used for scaffolding purposes. The principle requirements for the timber are dimensional stability, ease of working, weight for dimensional stability, ease of working, weight for handling and transportation and resistance to impact and abrasion
Topic 2 - Timber
3. Finishing joineryThis category includes all of the non-structural timber used in buildings including window, doors, stair, floorboards, facings and paneling. Any softwood or hardwood timber can be element within this class of work. The requirements for timbers for joinery uses work. The requirements for timbers for joinery uses are ease of working and finishing, good gain pattern and appearance when clear-finished, dimensional stability, Resistance to infestation and fungal attack, availability and cost. The most importance is often appearance.
Topic 2 - TimberUsage Timber Recommended
Foundation piling on land driven below water table
Bakau and all timber in strength group A and B
Trusses, purlins, rafters, post, beams, joists etc
All timber in strength group A and B especially suitable are Balau, joists etc especially suitable are Balau, Chengal and Kapur
Heavy flooring including paraquetry Balau, Chengal, Gian, Tembusu and Merbau
Topic 2 - Timber
SHEET MATERIALS:Sheet materials are processed timber products include:1) Plywood2) Blockboard3) Laminboard3) Laminboard4) Hardboard5) Chipboard
Topic 2 - Timber
Plywood
Topic 2 - Timber
Blockboard
Topic 2 - Timber
Laminated Boards
Topic 2 - Timber
Hardboard
Topic 2 - Timber
Chipboard
Topic 2 - TimberSHEET MATERIALS - continue:Plywood:Plywood is a compound wood made up of several thin layers of wood, glued together underpressure and usually arranged so that the grain of alternate layers are generally at right angle to each other. In this way, greater strength and lightness together with high dimensional stability can be obtaineddimensional stability can be obtaineda sheet of plywood usually consists of odd numbers of piles (3-ply, 5-ply or 7-ply etc), depending on the thickness. Those which have more than three plies are known as multi-ply boards.
Topic 2: Timber
Plywood is generally produced in standard size of 2440 x 1220 mm and in varying thickness of 3 to 25 m. Plywood is obtained in many kinds of wood finishes. Timber for manufacture of plywood include red meranti, kapur and teak etc
Plywood UsesPlywood panels are used throughout house construction. Some of Plywood panels are used throughout house construction. Some of the common uses are exterior siding, wall and roof sheathing, interior wall paneling, flush door and concrete formwork etc
Advantages of Plywood:The balanced construction of plywood offer utmost in strength, lightness and practicability. The following factors influence its usefulness.
Topic 2: Timber
Advantages of Plywood - continue:
1) Plywood is comparatively light in weight2) Screws and nails can be driven close to the edge without danger
of splitting the wood3) Plywood can be bent more readily than solid wood4) Plywood panels can be built to any size, while wood is limited to
the width of the treethe width of the tree5) Plywood minimises the tendency of wood to buckle or twist due to
changes in moisture content7) Under normal use plywood will not shrink or swell appreciably
Plywood has a greater strength across the grain than solid wood of the same thickness and checking and splitting is negligible
Topic 2: Timber 2) Blockboard
Blockboard consists of a core of wood made from strips up to 25 mm wide placed together with or without glue between each strip to form a slab with their grain direction at right angles to the grain of the core
3) Laminboard3) Laminboard
It is similar to blockboard, except the core strips are narrower. The core is built up from strips of wood glued together face to form a slab.
It is heavier than blockboard, since more glue and in some cases denser timbers are used in its construction and its greater cost
Topic 2: Timber 4) Hardboard
Hardboard panels are made of wood fibres. Wood chips are broken down into individual fibres then the fibres are formed into mat and compressed by hot presses into a dry board. It is denser, more durable and resistant to water absorption than other wood composition boards. Hardboard is available in a variety of finishes and thickness (3 to 12 mm)and thickness (3 to 12 mm)
5) Chipboard
It is made from dry wood chips which are bonded together by pressure and heat with a resin bond. The size and type of the chips used determines the textures and properties of the board. The manufacture of chipboard is similar to hardboard except that the binder is some type of resin that is resistant to decay and to termite attack. Hardness and acid resistance are also increased. At the same time, the wood becomes more brittle. Uses: Flooring, wall, ceiling linings and furniture
Topic 2: Timber
6) PlasteredboardIt consists of an aerated gypsum core sandwiched between and bonded to strong paper liners. Most boards have one ivory-coloured surface for direct decoration and one grey-coloured surface which has better adhesion properties for plastering.
