1.TIMBER – 1

Degree of Bachelor of Science (Honours) in Built Environment StudiesYEAR 2 SEMESTER 1MATERIALS FOR CONSTRUCTION 1

Nomenclature

Timbers are botanically divisible into two classes: softwoods (gymnosperms), being produced by coniferous trees and hardwoods (angiosperms) by broad leaved trees. Note that some softwoods are harder than hardwoods e.g. balsa wood (Ochroma lagopus) is a hardwood while yew (Taxus baccata) is a softwood. Both classes contain timbers which vary in weight, strength, resistance to decay and colour.

Timber speciesSpecifications should state the characteristics required and, frequently, name the species. The common names used for timber species is often misleading or ambiguous. It is best to specify timber by some recognised standard (e.g. BS 881 and 589) or by the Latinised specific name. It is generally superfluous to name the origin of the timber.

In most developed countries about 80% of all timber used in the construction industry consists of softwoods. Typical softwoods and their region of origin include: pine (European redwood) and spruce (European whitewood) - North and Central Europe; hemlock, spruce, pine, fir, western red cedar, American redwood - North America; pitch pine - Central America; Parana pine - Brazil.

Over 100 hardwoods are used commercially, although over half the requirements consist of ash, beech, iroko, ramin, mahogany, meranti, oak and teak. Ash, beech and oak come from the temperate forest of North America and Europe, the rest from tropical forests.

Tree growthThe tree, a complex living organism, can be considered in three main sections: the branches with their leaves, the trunk or bole and the roots. The roots anchor the tree to the ground and absorb water with dissolved minerals from the soil. The leaves absorb carbon dioxide from the air adn in the presence of sunlight, together with chlorophyll as a catalyst, combine carbon dioxide with water to produce sugars. The sugars in aqueous solution are transported down the branches and the trunk to be subsequently converted, where required for growth, into the cellulose of the tree. The trunk gives structural strength to the tree, and acts as a store for minerals and food such as starch and also as a two-way transport medium.

The tree is protected from extremes of temperature and damage by the bark, inside which is the bast layer which transports downwards the sugars synthesised in the leaves. Radial rays then move the food into sapwood cells for storage. Inside the bast is the thin and delicate cambiumwhich is the growing layer for the bark and the sapwood. Growth takes place when the cambium is active, which in temperature climates is during spring and summer.

Growth ringsRings of timber added to the trunk or branches, usually one every year. The rings are wide if growth is rapid, and the wider the growth rings of a particular species, the less dense and strong is the timber.

Early wood or spring woodThe early growth of the ring; it is normally wide and soft.

Late wood or summer woodThe later growth of the ring; it is denser, darker and narrower than the early wood.

SapwoodThis forms the part of the trunk active in the life processes of the tree. It is lighter in colour than heartwood, and, as it contains sugars and starch, tends to be more attractive to insects.Heartwoodhis occurs at the centre of the tree and is made up of dead fibres.

GrainThis term refers to the general direction or arrangement of the fibres.

Slope of grainThe deviation in the direction of grain caused by branches or bends in the tree.

Natural defects in the tree

These are features which develop in the tree before or soon after it is felled, and which may detract from the usefulness of the timber. These include:

Natural inclusions

Minor defects which occur when peices of bark have become enclosed within the timber are common. Pitch pockets and resin streaks, containing fluid resin, are frequently seen along the grain of softwoods.

BrittleheartThe centre of the tree (pith) breaks with a brittle fracture

SapwoodSapwood requires better protection than heartwood and takes up moisture more readily.

Abnormal growth ringsCaused by rapid growth; hence lower density and strength. (The optimum number ranges around 5 rings per cm for softwoods and 3 rings per cm for hardwoods).

Two adjacent limber pine trees exhibit sinistral spiral grain (left) and straight grain.

Spiral grainDistorts structural timber in seasoning.

2. TIMBER - 2

Degree of Bachelor of Science (Honours) in Built Environment StudiesYEAR 2 SEMESTER 1MATERIALS FOR CONSTRUCTION 1

Reaction woodDenser and stronger timber which grows to counteract the effects of gravity, wind or other forces which tend to bend the trunk. In softwoods, compression wood is produced which is darker in colour; in hardwoods, tension wood is produced, which is lighter in colour. Reaction wood has an abnormally high longitudinal shrinkage, causing distortion during seasoning and tends to produce a rough surface when the timber is machined.

FissuresThese include checks(lengthwise separations of the wood normally occuring across the growth rings caused by seasoning, or flattening), splits (separation of wood due to tearing apart of wood cells), and shakes(lengthwise separations caused by slippage occuring between the growth rings, caused while the tree is growing.

Resin pockets

Fissures between growth rings which contain resin.

Rind galls

Surface wounds enclosed in growth.

Burrs

Swellings comprising highly contorted grain resulting from undeveloped buds which form over a wound.

Knots

Portions of branches over which the tree has grown. A knot may be decayed or sound, have a hole in it, be loose or tight, and may be intergrown with the growth rings of the tree or encased with no intergrowth.

Properties of timber

There are substantial differences in the general properties of timbers. Even within the same species there can be difference depending on the source of the timber. The main general properties are listed in the following paragraphs.

ColourMost timbers show variation in colour as well as changing colour in use and by the application of finishes. Timber exposed to light will change colour and unprotected timber exposed to the weather will eventually become grey in colour. Typical colours for common species are shown in the next slide:

COMMON NAME SCIENTIFIC NAME TYPE COLOUREuropean ash Fraxinus excelsior H white to light brown

Balsa Ochroma pyramidale H white

European beech Fagus sylvatica H whitish to pale brown

European birch Betula pubescens H white to light brown

European cherry Prunus avium H pinkish brown

Sweet chestnut Castanea sativa H yellowish brown

COMMON NAME SCIENTIFIC NAME TYPE COLOUR

Iroko Chrlorophora excelsa H yellow-brownLignum vitae Guaiacum spp. H dark green / brownAfrican mahogany Khaya spp. H reddish brownEuropean oak Quercus robur H yellowish brownParana pine Auracaria

angustifoliaS golden brown with red

streakPitch pine Pinus palustris

Pinus elliottiiS yellow brown to red

brown

COMMON NAME SCIENTIFIC NAME TYPE COLOUREuropean redwood Pinus sylvestris S pale yellowish brown

to red brownRamin Gonystylus spp. H white to pale yellowTeak Tectona grandis H golden brown with

dark markingsEuropean walnut Juglans regia H grey brown with dark

streaksEuropean whitewood

Picea abiesAbies alba

S white to pale yellow brown

Density

The density of timbers vary with the species. For most softwoods it is between 400 and 550 kg/m³.

Texture

Surface texture can be classified as fine (balsa, European beech, European birch, European cherry, lignum vitae, Parana pine), medium (European ash, sweet chestnut, iroko, African mahagony, European oak, pitch pine, European redwood, ramin, teak, European whitewood) and coarse (European ash, European oak, European walnut).

Moisture movement in service

This refers to dimensional changes that occur when dried timber is subjected to changes in atmospheric conditions. This movement is classed as small (balsa, iroko, African mahogany, teak), medium (European ash, European cherry, lignum vitae, European oak, Parana pine, pitch pine, European redwood, European

walnut, European whitewood) and large (European beech, European birch, sweet chestnut, ramin). For structural purposes the movement category of a timber is not normally important. For situations where varying humidities are likely to be encountered and the stability of a component is important, then a species exhibiting small movement should be specified.

Working qualities

This refers to the ease of working and is classified as good (European ash, balsa, European beech, European birch, European cherry, sweet chestnut, Parana pine, European walnut, European whitewood), medium (iroko, African mahogany, pitch pine, European redwood, ramin, teak) or difficult (iroko, lignum vitae, European oak). Particular care should be taken in machining timber with a difficult classification.

DurabilityThis relates to the resistance

to fungal decay of the species. Durability is expressed by one of five classes based upon average life of a 50 x 50 mm section of heartwood in ground contact. This is a particularly hazardous situation and timber used externally, but not in contact with the ground, will have a longer life than that indicated, even without treatment. These ratings refer to heartwood only; the sapwood of all species has been found to be non-durable or perishable. Consequently all sapwood should be given preservative treatment.

The classes used are: (a) very durable (more than 25 years: iroko, lignum vitae, teak), (b) durable (15 to 25 years: sweet chestnut, European oak), (c) moderately durable (10 to 15 years: European cherry, African mahogany, pitch pine, European walnut), (d) non-durable (5 to 10 years: Parana pine, European redwood, ramin, European whitewood), and (e) perishable (less than 5 years: European ash, balsa, European beech, European birch). Preservative treatment should be indicated where the timber is not sufficiently durable for the situation in which it is to be used.

PermeabilityThis refers to the ease with which timbers can be penetrated with preservatives applied by a standard pressure impregnation treatment.

Availability

Sawn timbers may be in regular or limited supply depending on the availability from the producers and levels of demand. Some timbers are available in veneer form (European ash, European beech, European birch, European cherry, sweet chestnut, iroko, African mahogany, European oak, Parana pine, European redwood, ramin, teak, European walnut, European whitewood).

Price

The price of timber depends on availability, and vary from high price (European ash, European cherry, lignum vitae, European oak, Parana pine, teak, European walnut), medium price (European ash, balsa, European cherry, sweet chestnut, iroko, African mahogany, European oak, Parana pine, ramin) and low price (European beech, European birch, African mahogany, pitch pine, European redwood, European whitewood).

UsesTimber can be used for a large variety of purposes in buildings. Some of them are listed here: interior joinery (European ash, European beech, sweet chestnut, iroko, African mahogany, European oak, Parana pine, pitch pine, European redwood, teak, European whitewood), heat, sound and vibration insulation (balsa), furniture (European beech, European birch, European cherry, African mahogany, European oak, European redwood, ramin, teak, European walnut), floorings (European beech, European oak, European whitewood), plywood (European beech, European birch, Parana pine), exterior joinery (sweet chestnut, iroko, African mahogany, European oak, pitch pine, European redwood, teak), fencing (sweet chestnut, European oak), bench tops (iroko, teak), construction (iroko, pitch pine, European whitewood).

SizesGenerally most hardwoods are available in thicknesses up to 150 mm, widths 150 mm and up, lengths normally from 1.8 m upwards. Most European softwoods are available in thicknesses up to 75 mm, widths 75 mm to 225 mm, lengths in 0.3 m increments up to about 5.7 m. North American softwoods are obtainable in thicknesses up to 100 mm and widths up to 300 mm, lengths from 1.8 m to 7.2 m. Specifiers should check on the availability of species and sizes with suppliers. The Tables on the next slides give the avialable sizes of softwoods and hardwoods respectively.

Thickness (mm) Width (mm)75 100 125 150 175 200 225 250 300

16 x x x x19 x x x x22 x x x x x x x25 x x x x x x x x x32 x x x x x x x x x36 x x x x38 x x x x x x x x x44 x x x x x x x x x

47 x x x x x x x x x50 x x x x x x x x x63 x x x x x x x75 x x x x x x x x100 x x x x x x150 x x x200 x250 x300 x

Standard lengths for softwoods are: 1.80, 2.10, 2.40, 2.70, 3.00, 3.30, 3.60, 3.90, 4.20, 4.50, 4.80, 5.10, 5.40, 5.70, 6.00. 6.30, 6.90 and 7.20m. Lengths over 6m may not be easily obtainable

Standard sizes of sawn softwood (20% moisture content) to BS 4471: 1996

Standard sizes of sawn hardwood (15% moisture content) to BS 5450: 1997

Thickness(mm)

Width(mm)

50 63 75 100 125 150 175 200 225 250 300

19 x x x x x

25 x x x x x x x x x x x

32 x x x x x x x x x

38 x x x x x x x x x

50 x x x x x x x x

63 x x x x x x

75 x x x x x x

100 x x x x x x

Hardwoods generally come in random lengths; certain structural hardwoodssuch as iroko are available in long lengths (6 to 8m) and large sections.

Moisture content of timber

The moisture content of green wood is very high (from 60 to 200%), but eventually equilibrium with the surrounding atmosphere is rapidly reached. After felling, the wood will lose the water held within the cell cavities without shrinkage, until the fibre saturation point is reached, when the cells are empty. Subsequently, water will be removed from the cell walls, and it is during this process that the timber becomes harder and shrinkage occurs.

Radical changes in the moisture content of the timber in a component can cause distortion, shrinkage and splitting. Moisture in wood is expressed as moisture content (m.c.) defined as the weight of water in wood expressed as a percentage of the weight of the oven-dried wood.

As cellulose is a hygroscopic material, in use, wood never ceases to change its moisture content with the seasons or other influencing factors. The strength of a given piece of wood increases as the moisture content decreases. Timber shrinks as it dries, but not uniformly. The

tangential movement is about twice as much as the radial movement, while longitudinal movement is negligible. Differential shrinkage results in distortion as the wood dries. This type of distortion is determined by the direction of the annual rings. Types of distortion include cupping of plain sawn boards and diamonding of squared timber.

Production of timber

Trees are felled, trimmed into logs and hauled to a saw mill for sawing into timber. The cutting of logs prior to seasoning is called conversion; subsequent cutting is called manufacture. The basic methods of cutting are sawing (for use as solid sections), peeling (thin layers used for the manufacture of plywood), slicing (to maximise the visual effect of colour and figure) and cleaving (for fencing). Finishing operations involving planing and sanding produce a visually smooth surface. Planed timber surfaces are said to be wrougbt.

The two main type of cuts, plain sawn and quarter sawn, refer to the angle between the timber face and the growth rings. If the angle is less than 45°, the timber is plain sawn, if more it is quarter sawn. Plain sawn timber is more decorative but more liable to cupping. Quarter sawn timber is harder wearing, weather-resistant and less likely to flake.

Conversion defects

Conversion defects include:

(a) Wane

This is the loss of the square edge of the cut timber owing to the incorporation of the bark or the curved surface of the tree. A degree of wane is acceptable in structural and floor timber and is a special feature in waney-edge fencing.

(b) Sloping grain

For maximum strength, timber should be approximately straight-grained. There is a loss of bending strength of 4% at a slope of 1 in 25 and 19% at a slope of 1 in 10. Sloping grain results from spiral growth or from conversion which is not parallel to the axis of the tree. Slope of grain should be limited to 1 in 11 in hardwoods (see BS 5756: 1996).

(c) Raised grain

An unevenness between springwood and summerwood on the surface of dressed timber.

Seasoning of timber

Seasoning is the process of reducing the moisture content until a suitable moisture content is reached compatible with the end uses. BS 5268 Part 2 and Eurocode 5 define three conditions or service classes: 1, 2 and 3, which relate to conditions which would give average timber moisture contents not exceeding 12% and 20%, and over 20% respectively. Any incipient fungal decay is arrested when the m.c. drops below 20%. Successful seasoning requires controlled rate of loss of moisture from the timber as too-rapid seasoning can result in cracking of the timber. Timber must be stacked, supported and sometimes restrained during seasoning.

Seasoning methods include:

(a) air seasoning: A relatively slow process which is satisfactory to reduce the moisture content to about 18%, which is too high for heated buildings. The timber ends have to be protected by a waterproof coating (bituminous paint) to prevent rapid moisture loss, which would cause splitting. The required moisture content can be achieved in a few months for softwoods, but may require a number of years for some hardwoods.

(b) kiln seasoning: This is carried out in specially designed chambers which control the temperature and the humidity of the air surrounding the timber. Moisture contents as low as 6 to 8% can be achieved. For economic reasons, timber is frequently air-seasoned to fibre saturation point, followed by kiln drying to the required moisture content. A softwood load would normally be dried from fibre saturation point in a few days, while hardwoods normally need two to three weeks.

Seasoning is a reversible process and hence it is important that the timber retains its moisture content and does not absorb additional moisture. Impregnation and surface coatings help to retain the moisture content. Seasoning is required to

• avoid subsequent moisture movement,

• allow the timber to be machined and finished,

• produce a suitable substrate for surface finishes, and

• avoid the incidence of decay.

Preservation of timberTimbers with low or moderate durability (including all sapwoods) can be upgraded by suitable preservative treatment to a point where they are satisfactory for external use. Wood preservatives are poisons designed to inhibit attack by fungi and insects. Preservative treatments should involve only materials currently approved by Regulations and a reputable standard. They have the effect of upgrading the natural durability of timber, but they cannot ensure its immunity in very severe circumstances. Good design is also essential. Timber treatments may be divided into the application of preservatives to new timber, and remedial treatments used to eradicate or reduce and existing problem.

3. TIMBER - 3

Degree of Bachelor of Science (Honours) in Built Environment StudiesYEAR 2 SEMESTER 1MATERIALS FOR CONSTRUCTION 1

Preservative treatment for new timberTreatment should be carried out after all machining is complete, as penetration is never deep. A good preservative has to penetrate well, protect the timber from attack by fungi and insects over a long period, be non-toxic to plants and animals, be odourless, be free from detrimental effects on adhesives, paints and polishes, and must not be washed out by rain. The two industrial processes involve the use of vacuum and pressure impregnation.

(a) Double vacuum impregnationThe timber (at less than 28% m.c.) is placed in a chamber, drawing a vacuum on the chamber which is then filled with an organic-solvent-borne preservative. A low positive pressure is applied for between several minutes and one hour, depending on the permeability of the timber. The vessel is then drained and evacuated to remove excess preservative from the timber surface. Treatment should be carried out on individual pieces of timber before assembly. Treated timber should be allowed to dry out before being handled, glued or any finish applied. This method is suitable for low- and medium-risk timber, such as external joinery.

(b) The pressure / vacuum process

A similar process using water-borne preservatives and the application of high pressure within a pressure vessel to ensure deep penetration. Timbers to be built into high-risk situations, such as industrial roofs, frames and floors, timbers embedded in masonry, etc should be treated by this process.

Another two method which are of limited effect include:

(c) brush and spray:The least effective method and to be used either as a remedial measure or if some cutting is required after treatment.

(d) dipping: Immersion in a tank of organic solvent.

Remedial treatment of timber

Remedial treatments to existing buildings should be limited to those strictly necessary to deal with fungal or insect attack. Most timber decay occurs due to a building failure, such as dampness or the retention of water; the first remedial condition is therefore to ensure dry conditions. The method used is to remove severely decayed or affected timber, followed by appropriate preservative treatment to the remaining timber.

Organic-solvent fungicides and insecticides applied by brush or spray offer some protection, but application of pastes which deliver high quantities of the active ingredients are usually more effective. Where brushing or spraying is impracticable, then smoke treatment may be used, but this has to be repeated regularly, and is only effective against beetle attack.

Hazard Class 1 Above ground, covered and permanently dry - moisture content less than 20%.

Hazard Class 2 Above ground, covered but with risk of high humidity and occasional wetting - moisture content occasionally over 20%.

Hazard Class 3 Above ground, not covered and frequently wet -moisture content frequently over 20%

Hazard Class 4 Ground or fresh water contact, permanently wet -moisture content permanently over 20%

Hazard Class 5 In salt water, permanently wet - moisture content permanently over 20%.

Classification of timber preservative treatmentsUnder the European Standard EN 351-1: 1996 and BS EN 335-1, timber preservative treatment against wood-destroying organisms are categorised by performance standards and not to the individual chemical preservative treatment. The standards define wood preservatives according to their effectiveness in a range of environmental conditions. The level of treatment required to give the necessary performance is classified according to penetration into the wood and by retention or loading within the appropriate part of the wood.

CAUSE EFFECTFire Charring

Mechanical: excessive load, abrasion, erosion by rain, dust and sand

FractureLoss of surface

Water: flowing water

wetting and drying

Leaching of soluble colourErosion of surfaceExpansion and contraction causing

mechanical failure in the form of cracksSunlight Fading of colour

Embrittlement of surfaceChemicals Complete disintegration

DiscolorationBacteria Superficial discolorationFungi: in moist conditions

moulddry rot

in wet conditionsmicroscopic rotsvisible wet rots

Superficial discolorationComplete disintegration

Complete disintegrationComplete disintegration

Marine borers TunnelsInsects: termites

beetlesIrregular honeycombing or wide channelsTunnels and exit holes

DETERIORATION OF TIMBER

The specification of timber

The specification of timber for various uses may involve the particular hardwood or softwood, where particular visual properties are required. For many other situations, where strength and durability are the key factors, timber is specified by a strength class which combines timber species and strength grades. In addition to strength class, the specification of structural timber should include: lengths and cross-section target sizes, surface finish, moisture content and any preservative or special treatments.

Joinery timber

Joinery work requires timber that is dimensionally stable, appropriately durable, with acceptable gluing properties, and which can be machined well to good finish. Joinery-grade timber is categorised into four quality classes which are effectively appearance classes and make no reference as to durability, workability, etc. The four classes specified are: Class CSH and Classes 1, 2 and 3. Within each class there are variations in the allowability of specific defects depending on whether they occur (a) on an exposed surface, (b) a semi-concealed surface, (c) a concealed surface.

The criteria which are defined in the standard to classify a piece of timber are:Knots: Sound knots are not detrimental but it is desirable to limit their size and distribution. Unsound knots, dead knots and loose knots are unacceptable except on concealed surfaces; otherwise they should be cut out and the timber made good.

Splits, shakes and checks:End splits are permitted on exposed or semi-concealed surfaces if the length of the end split is not more than half the thickness of the piece. On concealed surfaces there is no limit on length. Ring shakes are not permitted on any exposed or semi-concealed surface.

Resin pockets: Not permitted on any exposed surface for Class CSH. Otherwise they should be cut out and filled.

Sapwood: Always permitted, but needs preservative treatment if used externally.

Wane: Not permitted on any edge which is exposed or semi-concealed

Straightness of grain: Slope of grain must not exceed 1 in 8 for hardwoods and 1 in 10 for softwoods.

Exposed pith: Permitted only on concealed or semi-concealed surfaces.

Rot: All timber for joinery must be free from rot and from insect attack.

Acceptability of plugs and fillers: Plugs and fillers may be used to replace a knot or other defect in many situations to enable a piece of timber to achieve a specific class, but these measures may be unacceptable when a non-opaque finish is specified. Plugs must be of the same species, have the same grain direction as surrounding timber, be well secured with a durable adhesive, be the full depth of the hole and be not more than 6 mm bigger than the maximum allowable knot size.

Joints: Laminated, finger-jointed and edge-jointed wood is considered acceptable when opaque finishes are to be used.

Rate of growth: The rate of growth should not be less than 6 rings per 25 mm for softwood to be used externally, and not less than 4 rings per 25 mm for softwood to be used internally. This specification is not applicable for hardwoods.

Moisture content: Components should be supplied and stored at the correct moisture content and installed in the building into conditions that will maintain it. It is the duty of the contractor to maintain all joinery at the right moisture content until handover.

Structural grading is the process by which timber is sorted into groups (Stress Grade) with ideally, similar structural properties in each group. Inevitably there is a very substantial range of properties within a group and significant overlap in properties between the groups. Structural grading can be performed in a number of ways including the following:Visual stress-grading and Machine stress-grading

Strength grading of timber

Visual grading: In a visual grading process, a trained grader examines each and every piece of timber produced. This visual inspection is undertaken in accordance with either the hardwood or softwood visual grading Standards, which define rules as to the types sizes and positions of physical characteristics that are allowed into each “group” or Structural Grade of material. The size and position of knots and other potential strength reducing characteristics in each piece is compared with the size and position of these characteristics allowed in the various grading classifications. The highest grades allow fewer and smaller characteristics in each piece of timber.

Machine Stress Grading

Machine stress-grading uses a machine to bend each piece of timber (generally about its minor axis). The machine measures the stiffness of the piece and uses a loose correlation between stiffness and strength to assign a stress grade. A sorted group with a small range of E (stiffness) can produce a larger range of strength. The Evalue is also used to infer all of the other structural properties, including tension, compression and shear strength. A grade stamp is applied at the tail end of a machine stress grading process. The machine sorts the timber on the basis of its calibration and the measurements made on each length of timber.

Strength classes: The BS EN Eurocode 5, Design of timber structures defines strength classes on the basis of limit state design. The ultimate limit states are determined from the characteristic values of the loads or actions and the material properties, to which partial safety factors are applied.

4. TIMBER PRODUCTS - 1

Degree of Bachelor of Science (Honours) in Built Environment StudiesYEAR 2 SEMESTER 1MATERIALS FOR CONSTRUCTION 1

• Laminated timber

• Engineered timber

• Plywood

• Composite board including blockboard and laminboard

• Chipboard (or particle board)

• Fibre building board

• Flakeboard including waferboard and OSB

• Woodwool slabs

• Joinery components

• Compressed straw slabs

• Thatch

• Shingles.

Timber products

There are a large number of products derived from timber of use in the building industry. The physical properties of the material produced reflect a combination of the subdivision of the wood, the addition of any bonding material and the manufacturing process. The physical properties then determine the products' appropriate uses within the building industry.

Criteria for selection are similar to those for timber and include stability, durability and suitability to receive a surface finish.

Laminated timberThe size, shape and characteristics of the available trees limit the uses of ordinary timber. Large sections of timber frequently contain defects which lower the strength or quality of the timber, and it is also generally difficult to season large sections of timber. Timber cannot be bent into curves except for small cross-sections.

These problems can be solved if large timber sections are manufactured by gluing (or nailing or using pegs) together smaller sections of timber one over the other, wide face to wide face as laminations. No one lamination need be as long as the member, as they can be glued end-to-end to reach the full length. The individual laminations are placed so that weak spots are separated from each other to avoid a concentration of weakness. Wood with the best appearance is placed where it can be seen and disfiguring characteristics are hidden within the member. The best quality material can also be used at points of high stress.

Laminates are made to length by finger or scarf joints and then carefully planed or sanded to the required thickness, usually to 20 - 45 mm. Each laminate is numbered, coated with glue and placed in position in such a way that end joints do not lie adjacent, but are well staggered.. Pressure is then applied to clamp the laminates together.

Beams, portal frames, arches, grid frames, spiral stairs and ring beams can all be made like this. Sections can be manufactured to any transportable size, typically 30 m, although spans over 50 m are possible. Standard-size straight sections with sizes ranging from 180 mm x 65 mm to 1035 x 215 mm are available from stock. Sections can also be manufactured to order, to any uniform or non-uniform linear or curved form. The finished product is often clear vanished to expose the laminates, and special care is required during handling, transport and storage on site. The use of rough finished glue laminated timber is also popular. Laminated timber may be homogeneous, with all laminates of the same strength of timber, or combined, in which lower-strength-class laminates are used for the centre of the units

Strength classes to prEN 1194: 1995 for homogeneous and combined glulam.

Glulam strength class

GL18

GL20

GL22

GL24

GL26

GL28

GL30

GL32

GL34

GL36

GL38

Homogeneous glulam:Strength class of laminates

C14 C16 C18 C22 C24 C27 C30 C35 C40

Combined glulam:

Strength class of outer laminates

C16 C22 C22 C27 C30 C35 C40 C40

Strength class of inner laminates

C14 C16 C18 C22 C24 C27 C30 C35

Engineered timberEngineered timber is manufactured to three grades by laminating timber strands with polyurethane resin under heat and pressure. In one process logs are cut into flat timber strands 300 mm long; these are then treated with resin, aligned and hot pressed into billets of reconstituted wood. In the other process, 3 mm thick timber strands or sheets of veneer are coated with waterproof adhesive and bundled together with the grain parallel. The strands or veneers are pressed together and microwave cured to produce structural timber billets or sheets up to 20 m long. The uniform material is suitable for use in columns, beams, purlins and trusses and can be machined as solid timber.

Plywood

Plywood is a type of laminated timber consisting of thin laminations (called plies or veneers) glued together such that the grain of each one runs perpendicular to the veneers adjacent to it. A timber log is softened by water or steam treatment and rotated against a full-length knife to peel off a veneer or ply of constant thickness. The ply is then cut to size, dried and coated with adhesive prior to laying up to the required number of layers.

Not all plies are of the same thickness; often, thicker plies of lower grade materials are used in the core. Plies are normally built up with adjacent grain directions at right angles to give uniform strength and reduce overall moisture movement. The laminate of plies and glue is cured in a hot press, sanded and trimmed to standard dimensions for packaging. The standard size sheet is 2440 x 1220 mm with some manufacturers producing sheet sizes of up to 3100 x 1530 mm, with a sheet thickness which varies from 4 mm to 25 mm.

Plywood has many advantages over ordinary timber including (a) greater transverse strength which braces and strengthens the entire structure, (b) it has better resistance to bending and concentrated loads, (c) it can be worked closer to the edge without splitting, (d) it is less sensitive to moisture movement, (e) it is easier to bend to form curves, and (f) it requires less handling than the equivalent area of timber.

Types of plywood

The grade of plywood is determined partly by the adhesive used and partly by the durability of timber itself, the number of plies for a particular thickness and the surface condition of the outer plies. Durability can be upgraded by preservative treatment and by sealing the edges of the board and protecting the surface with a suitable paint or stain finish. Plywood is manufactured in a number of different types:

Interior Grade Face Back Inner

Plies Uses

A-A A A D Both faces are 'Finish Grade' Used for Cabinet Doors, Domestic Furniture etc.

A-B A B D As for A-A but only face side is finished grade, back face is smooth with no voids.

A-D A D D Finish grade face for panelling, built in furnitureB-D B D D Utility grade. Only one paintable side

C-D C D D Sheathing and structural uses such as sub floors andshuttering.

Underlay C Plugged D C, D Used as underlay for tile and carpets where sub floor

is uneven.

Interior type plywood which is bonded with a glue that is adequate for service indoors. In order of decreasing durability, these are CBR-grade (cyclic boil resistant), MR-grade (moisture resistant) and INT-grade (interior), using melamine-urea formaldehyde or urea formaldehyde resins.

Exterior Grade Face Back Inner

Plies Uses

A-A A A C Outside use where both sides can be finished for appearance purposes.

A-B A B C As for A-A but with only one face of 'Finish Grade'A-C A C C Soffits and Facias. Also used as a base for coatings.

B-C B C CUsed for general building and landscape purposes, such as shuttering and fences. Also used as a base for coatings

C-C Plugged

C Plugged C C Backing for tiles and wall coverings and high

performance coatings

C-C C C C Shuttering and rough building work. With great exposure to the weather. (Unsanded)

Exterior type plywood which includes WBP-grade (weather and boil proof, to BS 6566: 1985) which can be used externally without delamination, provided the timber itself is durable or suitably treated, and marine plywood (to BS 1088 / 4079: 1966) which is a combination of a moderately-durable timber with phenol formaldehyde resin.

The grading of plywoodThe grading of ordinary plywood refers only to the quality of the surface veneers. The European standard prEN 635 Parts 2 and 3 describes five classes of allowable defects (E, and I to IV, according to the decreasing quality of the surface appearance; Class E has almost no surface defects. The designations follow the following criteria: (a) veneer with minimal imperfections as peeled, (b) veneer with imperfections which have been repaired by plugging and filling, (c) veneer with imperfections which have not been repaired. Imperfections include beetle holes, splits, glue stains, poor jointing of veneers and poor matching of colours.

The European Standard prEn 636: 1996 gives performance specifications for plywood to be used in dry, humid or exterior conditions against criteria of bonding strength and durability with respect to biological decay, ranging from Class 1 for interior applications under dry conditions to Class 3 for exterior applications in unprotected situations.

The quality of the surface finish depends upon the species of timber used, surfaces with strongly figures surfaces being very popular. Stain or paint can produce a quality of finish adequate for most purposes on normal sanded grades of plywood.

In addition to the ordinary product plywood is made in the following forms:(a) with decorative veneer on both faces or on one face only, with the veneers being carefully matched and arranged in some pattern;

(b) plywood planks simulate natural wood planking in various ways;

(c) with etched and moulded surfaces prepared wither by sand blasting or by pressing;

(d) metal faced with galvanised steel, stainless steel, copper, aluminium and other metals, protecting the plywood from insect attack and increasing its strength to weight ratio.

(e) plastics faced plywood with decorative facings or for concrete formwork. Some interior grade plywoods are available with a special resin / paper laminate face to provide a high quality surface for paint finishing.

(f) with tongued and grooved edgesused mostly for floor construction;(g) lead cored for radioactive shielding or for sound insulation.

Uses of plywood

Plywood is used extensively in the building industry because of its strength, versatility and visual properties. Box and I-sections are popular because of the shear strength of the material. Box beams can be manufactured to create flat, pitched and arched roof forms. Plywood of 8 to 10 mm thickness is frequently used as a sheeting material in timber-frame construction and for complex roof forms such as domes. Plywood is used extensively for concrete formwork

Plywood used for structural purposes require quality assurance. As a structural material, plywood has its own peculiar properties, in particular as the veneers from which it is formed are strong in one direction and weak in the other. Plywoods for structural use are always built up from an odd number of veneers, so that plywood sheets are stiffer in the direction of its face veneers than at right angles to them, this effect being most marked in the thinner sheets (e.g. 3-ply sheets).

Composite board

Composite board or core plywood is made by applying one or two veneers to a solid core of timber strips, or to a core of suitable particle board.

Blockboard and laminboardwere the initial types of composite board, with core strips up to 7 mm wide in the case of laminboard, and between 7 mm and 30 mm wide in the case of blockboard. These strips are then faced on both sides with either one or two veneers from 1.2 mm to 3.7 mm thick. As with plywood, the grain directions are perpendicular from layer to layer. Composite board is generally bonded with urea formaldehyde adhesives appropriate for interior applications only.

The standard sheet size is 2440 x 1220 mm, with a thickness range from 12 to 25 mm, although larger sheets up to 45 mm thick are available. Blockboard can be finished with a wide range of decorative wood, paper or plastic veneers for use in fitted furniture. Composite boards are mostly used for large flat surfaces where rigid panels are required, in particular for the manufacture of interior fittings and furniture.

Boards with a core of particle board, phenolic foam or polystyrene are also available. Battenboard, which has core blocks wider than 30 mm, is available, but no longer in common use.

5. TIMBER PRODUCTS - 2

Degree of Bachelor of Science (Honours) in Built Environment StudiesYEAR 2 SEMESTER 1MATERIALS FOR CONSTRUCTION 1

Particle board

Particle boards (or chipboards) are made from wood chips or shavings bound together with an adhesive to approximately 8% by weight under the action of pressure and heat (200°C). The properties of the chipboard are largely determined by the size and shape of the chips and their distribution in the board. The wood chips are either formed randomly into boards giving a uniform cross-section, or distributed with the coarse material in the centre and the finer chips at the surface to give a smoother finish.

Boards are finally sanded, trimmed and packed. In another method, the mixture of wood chip and resin is extruded through a die into a continuous board; however, in this method, the wood chips are predominantly orientated at right angles to the board face, giving a weaker material. The standard size of particle board are 2440 x 1220 mm, 3050 x 1220 mm and 3600 x 1220 mm with the most common thicknesses ranging from 12 to 40 mm, although much larger sheets and a wider range of thicknesses are available.

Types of particle board

Average densities of particle boards vary from 600 to 800 kg/m³. The durability of particle board is dependent upon the resin adhesive. The common binders are synthetic resins, cured rapidly and irreversibly by the application of heat. The properties of the finished board can be improved by additives including anti-swelling agents, fire retardants, insecticides and fungicides.

BS EN 120:1992, BS EN 309:1992, BS EN 311:1992, Particleboard. Methods of sampling, conditioning and test

BS EN 312-5:1997, BS EN 312-7:1997Particleboard. Specification for wood chipboard

BS EN 300:1997Particleboard. Specification for oriented strand board (OSB)

BS EN 1328:1997, BS EN 634-2:1997Particleboard. Specification for cement bonded particleboard

BS 5669:Part 5:1993Particleboard. Code of practice for the selection and application of particleboards for specific purposes

BS 7916:1998Particleboard. Code of practice for the selection and application of particleboards for specific purposes

When exposed to relative humidities in excess of 65%, or to excessive wetting, chipboard will absorb moisture and swell. Types C1, C1A and C2 should not be exposed to moisture, even during construction. Types C3, C4 and C5 are tolerant to occasional wetting and

relative humidities over 85%. All grades of wood chipboard are permeable to water vapour, and hence are not suitable for external use.

Particle boards have Class 3 spread of flame, but they can be treated to the requirements of Class 1 or even Class 0.

The surface of Type I standard boards is comparatively rough and requires sanding to receive paint. Chipboard is available prefinished, including melamine-faced, veneered or hardboard faced. Domestic flooring-grade particle board, usually 18 mm or 22 mm, may be square-edged or tongued and grooved.

Uses of particle board

Large quantities of Types C1 and C1A chipboard are used in the furniture industry. Other applications include flooring, roof decking, wall and ceiling linings, etc. Some of the material available is of very poor quality; boards which conform to some recognised standard (e.g. BS EN 312) should be clearly colour-coded for identification along two opposite edges.

Cement-bonded particle board

Cement-bonded particle board is manufactured from a mixture of wood particles and Portland or accelerated magnesite cement. The sheets, which are light grey in colour, have a smooth cementitious finish to both faces and can be painted, papered or veneered. The material is approximately 65% wood by volume, producing a material with a density of 1000 to 1250 kg/m³.

Types and uses of cement-bonded particle board

BS 5669 Part 4: 1989 describes two types of this material: Type T1 based on magnesite cement, which is not suitable for damp conditions, is not frost resistant but can be used as a lining board, and Type T2 based on Portland cement, and has good resistance to fire, water, fungal attack and frost and may be used internally or externally. The material has Class 0 surface spread of flame.

The material is often used for soffits, external sheathing and roofing on both modular and timber-frame buildings. The heavier grades, generally tongued and grooved, are suitable for flooring owing to the high impact resistance of the material.

Fibre building board

Fibre building boards form the largest single category of building boards and include: hardboard, medium board and softboard. Hardboard and softboard are produced by a wet process: formed by pressing the wood fibres using heat and pressure only. Medium density fibre board is produced by a dry process and also includes a resin in its composition.

Fibre boards are made by reducing natural wood to its constituent fibres and then reforming these fibres into sheets which are compressed to varying degrees. Fibre boards have a homogeneous structure throughout their thickness, the surfaces are free from grain or blemish and have good resistance to fungal and insect attack.

Hardboards

These are relatively thin and dense (> 900 kg/m³), with one smooth face and a fine mesh pattern on the reverse (boards with two smooth faces are also available). Standard sheet sizes are 1220 x 2440 to 3600 mm, and 1700 x 4880 mm, and also door sizes. Standard thicknesses vary from 3.2 to 6.4 mm although a wider range is available.

The European Standard BS EN 622 Part 2: 1996 specifies six grades of hardboard according to loadbearing requirements and environmental conditions: HB for general purpose boards for use in dry conditions; HB.LA for loadbearing boards for use in dry conditions; HB.H for general purpose boards for use in humid conditions; HB.HLA1 for loadbearing boards for use in humid conditions; HB.HLA2 for loadbearing boards for use in humid conditions; and HB.E for general purpose boards for exterior use.

Standard hardboard is suitable for internal use, typically panelling, wall and ceiling linings, floor underlays, facings to doors, furniture, etc. A range of perforated, embossed and textured surfaces is available. Applied coatings include primed or painted and various printed wood grain, PVC or melamine foil.

Special hardboards

Tempered hardboard, which is impregnated with oils during manufacture, is primarily intended for structural purposes where high strength and moisture resistance is needed, e.g. floorings. This hardboard is denser (> 960 kg/m³) and has good abrasion resistance. Two types are available: THN and THE (normal and extra), the latter having better performance.

Pre-decorated hardboard describes a large variety of surfaced and embossed hardboard (e.g. enamelled, plastics faced, veneered, simulated tile or wood plank). Patterned surfaces include simulated brick, stone and rough plaster. High resistance melamines are available for bathrooms, kitchens, etc and flame retardants can be incorporated during manufacture.

Medium board and soft boardThese are made in a similar way to hardboard, except that

in softboard the final pressing is omitted. Two types of medium board exist: low density (400 to 560 kg/m³) and high density(560 to 900 kg/m³). Soft board is a low density board (210 to 400 kg/m³). High density medium board has a dark-brown shiny surface, low density medium board has a light-brown soft finish, while softboard has a fibrous, slightly textured surface.

High density medium board extra grade (HME) can be used for exterior cladding. The normal grade (HMN) and the extra grade are used for wall linings, sheathing, partitions, ceilings and floor underlays. These boards are also widely used in joinery because they can be machined very accurately, and high quality finishes can easily be obtained. Low density medium board (LME - extra, and LMN - normal) is used for wall linings, panelling, ceilings, and notice boards. Softboard (SBN) is used for its acoustic and thermal insulating properties.

TYPE CONDITIONS OF SERVICEMBL, MBH and SB General purpose boards for use in dry conditions

MBH.LA1, MBH.LA2 and SB.LS

Loadbearing boards for use in dry conditions (For softboards instantaneous load duration class only)

MBL.H, MBH.H and SB.H

General purpose boards for use in humid conditions

MBH.HLS1, MBH.LHS2 and SB.HLS

Loadbearing boards for use in humid conditions (For medium boards - short term load duration class only; for softboards - instantaneous load duration class only)

MBL.E, MBH.E and SB.E

General purpose boards for exterior use

The European Standard BS EN 622 Part 3: 1996 specifies 10 grades of low- (L) and high- (H) density medium board and 5 grades of softboard (S) according to Loadbearing requirements and environmental conditions.

Oriented Strand Board (OSB)

OSB is manufactured from softwood timber flakes, usually pine, tangentially cut and measuring approximately 75 x 35 mm. These are dried and coated with wax and 2.5% of either phenol formaldehyde or melamine-urea formaldehyde resin. The mix is laid up in three (occasionally five) layers, with the grains running parallel to the sheet on the outer faces and across or randomly within the middle layer. The boards are then cured under heat and pressure, sanded and packaged.

Flakeboard has two advantages over chipboard:

(a) less adhesive is required to achieve the same strength, and

(b) the strength and stiffness properties of the boards are enhanced by the use of large-flake particles.

Grades and uses of OSBBS 5669 Part 3: 1992 specifies two grades of OSB, both being moisture resistant. Grade F1 is suitable for formwork, while Grade F2, which has enhanced properties in terms of strength, moisture resistance and fungal attack, is suitable for roof sarking, flooring and flat roof decking. OSB is manufactured to a thickness range of 6 to 25 mm, although 10 to 18 mm sheets predominate.The European Standard BS EN 300: 1996 specifies four grades of OSB according to loadbearing requirements and environmental conditions similar to fibre board.

OSB CHIPBOARD PLYWOODGrade Grade 0 - 2 Type II / III CSP +

Property andtest method

Standard CAN 3-0437M BS 5669Thickness < 12.7 mm 6 - 19 mm 9.5 mm

Modulus of rapture, tested

dry (N/mm²)

Tested in long* direction

Tested in short* direction

29.0

12.4

19.0

19.0

(50 - 65)

(16 - 30)

Modulus ofrapture, after 2h

boil, tested wet (N/mm²)

Tested in long*direction

Tested in short*direction

14.5

6.2

-

-

(24 - 32)

-

Modulus of elasticitytested dry(N/mm²)

Tested in long*direction

Tested in short*direction

5500

1500

2750

2750

(6500 - 7700)

(500 - 1500)

Tensile strength Perpendicular

(N/mm²)

DryAfter V313 cyclic

test

0.345(0.120)

0.50.25

(0.72 - 0.86)(0.58)

Thickness swellafter 24h cold water soak (%)

Mean, 4 central points

Mean, 3 edge points

25

(35)

(6)

8

(6)

(7)

Waferboard, OSB, chipboard and plywood; property levels specified in standards or attained in laboratory tests

Woodwool slabs

Woodwool slabs consist of long wood fibres which are chemically treated with cement and compressed in moulds. The slabs are rectangular in shape and present a flat surface with an appearance of randomly dispersed fibres. Woodwool slabs are used for partitions, wall cladding, permanent shuttering, linings, acoustic and thermal insulation and roof construction.

Woodwool slabs provide good fire protection and have good standards of thermal insulation. The grey product has an open texture which may be left exposed, spray-painted or as an effective substrate for plastering. It is also a suitable material as permanent shuttering for concrete. Slabs are available in a range of thicknesses, from 25 to 125 mm, typically 600 mm wide and up to 4000 mm long.

TYPE DESCRIPTIONA Non-loadbearing slabs manufactured to 25, 38 and 50 mm

thicknesses, suitable for ceilings, partition wall linings and permanent concrete shuttering

B Loadbearing slabs manufactured to 50, 75, 100 and 125 mm thicknesses suitable for roof decking.

SB Loadbearing slabs with greater impact resistance than Type B owing to the incorporation into the centre of the slab of a high-strength nylon mesh, suitable for roof decking. These generally incorporate a steel channel as well.

Woodwool slabs are available either plain-edged or with interlocking galvanised-steel channels to the longitudinal edges. Three grades are specified in BS 1105: 1981 as shown.

Woodwool slabs are rated Class 0 with respect to fire spread. The material is resistant to fungal attack and is unaffected by wetting. They also offer good sound absorption qualities. The material has good thermal insulation properties with a typical thermal conductivity of 0.77 W/mK at 8% moisture content.

Compressed straw slabsCompressed straw slabs are manufactured by forming straw under

heat and pressure, followed by encapsulation in a fibreglass mesh and plastering-grade paper. The slabs are 58 mm thick by 1200 mm wide, in a range of standard lengths from 2270 to 2400 mm. The product cannot be used where it will be subjected to moisture. They are typically used for internal partitioning and they are normally finished with a 3 mm skim of board plaster. When skim plastered they have a 30 minute fire resistance rating, a Class 0 spread of flame, and a sound reduction of typically 35 dB over the range 100 to 3200 Hz.

Thatch

There has been a resurgence of interest in the use of thatch as a roofing material in countries where it had been traditionally used. Thatch is made from a suitable weed or straw using long, undamaged stems. Thatched roofs have a life of between 20 and 50 years but need re-ridging every 10 years. A minimum roof pitch of 45° is required, although 50 to 60° is more common as the steeper pitches are more durable. Thatched roofs are susceptible to fires but fire retardants can be used. Thatch gives good thermal insulation (a U-value of 0.35 W/m²K can be achieved with a roof 300 mm thick).

ShinglesA durable timber such as Western red cedar (Thuja plicata) has to

be used for making shingles, to be used as a roofing material or for external cladding. Shingles may be tapered or straight. Shingles are typically 400, 450 or 600 mm long and between 75 and 355 mm wide; they may be treated with copper / chrome / arsenate (CCA) wood preservative to improve durability. A flame retardant may also be used. Shingles should be fixed with corrosion-resistant nails, leaving a spacing of 5 to 6 mm between adjacent shingles. Three layers are normally necessary.

Joinery components

Many joinery components are available as proprietary components from specialist manufacturers, either as standard products from catalogues or as made-to-order items for specific projects. They are fabricated in a standard form, with the finished dimensions being modified within predetermined criteria.