Timber is not only one of the oldest building materials, along with stone, earth and various vegetable materials, but has remained until today the most versatile and, in terms of indoor comfort and health aspects, most acceptable material.
However, timber is an extremely complex material, available in a great variety of species and forms, suitable for all kinds of applications. This diversity of timber products and applications requires a good knowledge of the respective properties and limitations as well as skill and experience in order to derive maximum benefits from timber usage.
Although only a small proportion of the timber harvested is used for building, the universal concern about the rapid depletion of forests, especially the excessive felling of large old trees (which take hundreds of years to replace) and the great environmental, climatic and economic disasters that follow deforestation, has led to a great deal of research into alternative materials and rationalized utilization. Since timber cannot be completely replaced by other materials, it shall long remain one of the most important building materials, and hence great efforts are required to maintain and renew timber resources with continuous, large scale re-afforestation programs.
• The cross-section of a tree trunk or branch reveals a number of concentric rings, with the innermost ring being the oldest. The trunk thickness increases by the addition of new rings, usually one ring each year, but because of the exceptions to this rule, they are called growth rings (instead of annual rings).
• The rings comprise minute tubular or fibrous cells (tracheids) which transport moisture and nutrients to all parts of the tree. The early wood (springwood) formed during the growth period has large cells, while in the dry season the late wood (summerwood) grows more slowly, has thicker cell walls and smaller apertures, forming a narrower, denser and darker ring, which gives the tree structural strength.
• As each new ring forms a new band of "active" sapwood, starch is extracted from an inner sapwood ring (sometimes substituted by natural toxins) adding a further ring to the "inactive" heartwood core. Mechanically there is hardly any difference between sapwood and heartwood, but sapwood is usually lighter in colour and contains substances (eg starch, sugar, water) which attract fungi and some insects.
• The slower the tree grows, the narrower are the growth rings, and the denser and stronger is the timber. Its resistance to biological hazards is also usually greater.
• Timbers are generally classified as hardwoods or softwoods. Hardwoods are from broad-leaved trees, in the tropics usually evergreen, in temperature zones usually deciduous (shedding their leaves annually). Softwoods are generally from coniferous (cone-bearing) trees, found mainly in temperate zones. The differentiation is only in botanical terms, not in mechanical properties, as some hardwoods (eg balsa) are much softer than most softwoods.
• In recent years, coconut timber has been found to be a good substitute for the common timber varieties. While cocowood is related to hardwood, there are some basic differences in growth characteristics: cocowood has no heartwood and sapwood, no annual rings and hence no increase in diameter; the age is determined by circumferential demarcations along the length of the bark; it has no branches and knots; the density decreases from the outer part to the centre, and from the lower part to the upper portion of the trunk. Coconut timber is commercially useful only after 50 years of age, when the copra yield begins to decrease rapidly.
Types and properties of timbers
• Timber for building construction is divided into two categories: primary and secondary timber species.
• Primary timbers are generally slow-grown, aesthetically appealing hardwoods which have considerable natural resistance to biological attack, moisture movement and distortion. As a result, they are expensive and in short supply.
• Secondary timbers are mainly fast-grown species with low natural durability, however, with appropriate seasoning and preservative treatment, their physical properties and durability can be greatly improved. With the rising costs and diminishing supplies of primary timbers, the importance of using secondary species is rapidly increasing.
• Research activities in several Asia-Pacific countries have shown cocowood to be a viable secondary timber, which is abundantly available in most tropical costar areas. However, special knowledge and equipment is required in processing cocowood, as each portion of the coconut trunk has a different density and strength, and its high silica content and hard outer portion causes rapid dulling of sawteeth (requiring special tungsten-carbide blades).
• Without considering the many exceptions, the main properties of timber are: relative low density compared with other standard building materials; high strength: weight ratio with the highest tensile and compressive strengths displayed parallel to the grain; elasticity; low thermal conductivity; growth irregularities; tendency to absorb and release moisture (hygroscopicity); combustibility; renewability.
• The shrinkage of wood is a common feature and varies according to the direction of shrinkage: radial shrinkage is about 8 % from the green to the dry state; the corresponding tangential shrinkage is about 14 to 16 %; in the longitudinal direction shrinkage is negligible (0.1 to 0.2%).
Seasoning and preservative treatment
• Seasoning is the process by which the moisture content of timber is reduced to its equilibrium moisture content (between 8 and 20 % by weight, depending on the timber species and climatic conditions). This process, which takes a few weeks to several months (depending on timber species and age, time of harvesting, climate, method of seasoning, etc.), makes the wood more resistant to biological decay, increases its strength, stiffness and dimensional stability, and reduces its weight (and consequently transportation costs).
• Air seasoning is done by stacking timber such that air can pass around every piece. Protection from rain and avoidance of contact with the ground are essential.
• Forced air drying is principally the same as air seasoning, but controls the rate of drying by stacking in an enclosed shed and using fans.
Solar timber seasoning kilns
• Kiln drying achieves accelerated seasoning in closed chambers by heating and controlling air circulation and humidity, thus reducing the time by 50 to 75 %, but incurring higher costs. An economic alternative is to use solar heated kilns.
• Seasoning time is greatly reduced if the timber is harvested in the dry or winter season, when the moisture content of the tree is low.
• Seasoning alone is not always sufficient to protect timbers (particularly secondary species) from fungal decay and insect attack. Protection from these biological hazards and fire is effectively achieved by preservative treatments with certain chemicals.
• The chemicals and methods of application are generally the same for timbers, as are described in the previous section on Bamboo. Hence the comments about the avoidance of highly poisonous preservatives are equally valid in the case of timber.
• When considering preservative treatment of timber, it should be remembered that timber is the healthiest of all building materials and it is paradoxical to "poison" it, especially when other methods can be implemented to protect it, for instance, with non-toxic preservatives and good building design (exclusion of moisture, good ventilation, accessibility for periodical checks and maintenance, avoidance of contact with soil, etc.).
• Pole timbers, generally from young trees (5 - 7 years) with the barks peeled off, seasoned and treated as required. The cost and wastage incurred by sawing is eliminated and 100 % of the timber's strength is used. A timber pole is stronger than sawn timber of equal cross-sectional area, because the fibres flow smoothly around natural defects and are not terminated as sloping grain at cut surfaces. Poles also have large tension growth stresses around their perimeters and this assists in increasing the strength of the compression face of a pole in bending.
• Sawn timber, mainly from older trees with large diameter trunks, cut in rectangular sections as beams or boards. The part of the trunk from which they are cut and the slope of grain have a great effect on the quality of the product (as shown in the diagrams). The cutting of logs before seasoning is called conversion; re-sawing and shaping after seasoning is called manufacture.
• Plywood, made of several plies ("peeled" off a pre-boiled log by rotating it against a knife) glued together such that the direction of grain of each ply runs at right angles to the ply on either side, producing extremely large panels of greater strength and lower moisture movement than sawn timber boards. As the outer sides must have uniform strength and moisture movement properties, there must always be an odd number of plies. Thicknesses range from 3 to 25 mm. A major problem is the use of formaldehyde-based glues, which are highly toxic.
• Blockboard, comprising a solid core of (usually secondary timber) blocks up to 25 mm wide, faced each side with veneers (of primary timbers), with their grain at right angles to that of the blocks.
• Glue-laminated wood, composed of layers of wood with the orientation of the grain of each layer usually in the same direction, or varied according to the intended use of the product. By this method, straight or curved structural members of very large (even varying) cross-sections and great lengths can be produced with low grade timbers of small sizes, achieving high strengths, dimensional stability and very pleasing appearance.
• Particle board (also called chipboard), principally made of wood chips (but also from other fibrous or small-sized ligno-cellulose materials), which are dried, blended with a synthetic resin and hot-pressed (requiring about 8 % binder) or extrusion-pressed (requiring only 5 % binder) to almost any desirable shape. Hot-pressed boards are stronger than extruded boards; and moisture movement acts at right angles to the plane of hot-pressed boards, and parallel to the plane of extruded boards. To improve their strengths, extruded boards are invariably veneered.
• Fibre board (ranging from "softboards" having good thermal insulation, to "hardboards" having properties similar to plywood) principally made of wood (or other vegetable) fibres, which interlock mechanically, requiring no adhesive as the lignin in the fibres acts as the bonding agent. The sheets are either hot-pressed (cardboards) or simply dried without pressing (softboards), and may contain additives such as water repellents, insecticides and fungicides.
• Wood-wool slabs, comprising long wood shavings saturated with an inorganic binder (such as portland cement or magnesium oxychloride) and compressed (for 24 hours, before demoulding and curing for 2 to 4 weeks). Various wood species can be used, except those that contain appreciable amounts of sugar, which retards the setting of cement. Wood-wool slabs are relatively light in weight, elastic, resistant to fire, fungal and insect attack, can be easily sawn like timber boards and plastered.
• Saw dust, and other finely chipped forestry or sawmill by-products, as additive in clay brick production. The wood particles are burnt out, producing porous, lightweight fired clay bricks.
• Tannin based adhesives, extracted from the bark of certain trees, used in particle board production.
• Wood tar, obtained from the dry distillation of timber, and used as a timber preservative.
• Complete or partial building and roof frame structures, using pole timber, sawn timber beams, or glue laminated elements.
• Structural or non-structural floors, walls and ceilings or roofs, made of pole timber (block construction), sawn timber boards, or large panels from plywood, particle board, fibre board or wood-wool slabs; in most cases, suitable for prefabricated building systems.
• Insulating layers or panels made of wood-wool slabs or softboard.
• Facing of inferior qualify timber elements with timber ply or veneer, to obtain smooth and appealing surfaces, or facing of other materials (brickwork, concrete, etc.) with boards and shingles.
• Door and window frames, door leaves, shutters, blinds, sun-screens, window sills, stairs and similar building elements, mainly from sawn timber and all kinds of boards and slabs.
• Roof constructions, including trusses, rafters, purling, lathing and wood shingles, mainly from pole or sawn timber.
• Shuttering for concrete or rammed earth constructions and scaffolding for general construction work, from low grade pole and sawn timber.
• Furniture, using any or combinations of the timber products described above.
• Timber is suitable for construction in all climatic zones, and is unmatched by any other natural or manufactured building material in terms of versatility, thermal performance and provision of comfortable and healthy living conditions.
• Timber is renewable and at least secondary species are available in all but the most arid regions, provided that re-afforestation is well planned and implemented.
• Most species have very high strength: weight ratios, making them ideal for most constructional purposes, particularly with a view to earthquake and hurricane resistance.
• Timber is compatible with traditional skills and rarely requires sophisticated equipment.
• The production and processing of timber requires less energy than most other building materials.
• Timber provides good thermal insulation and sound absorption, and thicker members perform far better than steel in fire: the charred surface protects the unburnt timber, which retains its strength.
• The use of fast growing species helps to conserve the slow growing primary species, thus reducing the serious environmental problems caused by excessive timber harvesting.
• Using pole timber saves the cost and wastage of sawing and retains its full strength, which is greater than sawn timber of the same cross-sectional area.
• Since cocowood was previously considered a waste material with immense disposal problems, its utilization as a building material not only solves a waste problem but provides more people with a cheap, good quality material and conserves a great deal of other expensive and scarce timber resources.
• All the timber-based sheets, boards and slabs provide thin components of sizes that can never be achieved by sawn timber. Apart from requiring less material by volume (which generally consists of lower grade timber or even wastes), larger, lighter and sufficiently strong constructions are possible.
• Demolished timber structures can often be recycled as building material, or burnt as fuelwood, the ash being a useful fertilizer, or processed to produce potash (a timber preservative).
• High costs and diminishing supplies of naturally resistant timber species, due to uncontrolled cueing and exports, coupled with serious environmental problems.
• Extreme hardness of some dried timbers (eg cocowood) making sawing difficult and requiring special saws.
• Thermal and moisture movement (perpendicular to the grain) causing distortions, shrinkage and splitting.
• Susceptibility of cheaper, more abundantly available timber species to fungal decay (by moulds and rot) and insect attack (by beetles, termites, etc.).
• Fire risk of timber members and timber products with smaller dimensions.
• High toxicity of the most effective and widely recommended chemical preservatives, which represent serious health hazards over long periods.
• Failure of joints between timber members due to shrinkage or corrosion of metal connectors.
• Discoloration and embrittlement or erosion of surface due to exposure to sunlight, wind-borne abrasives or chemicals.
• Conservation of forest resources by comprehensive long-term re-afforestation programs, and use of fast growing timber varieties and forestry by-products, thus also reducing costs.
• Harvesting timber in the dry or winter season, when the moisture and starch content, which attracts wood-destroying insects, is lowest.
• Sawing of hard timber species (eg cocowood) when still green, since the moisture in the fresh logs lubricates the saw.
• Reduction of moisture content to less than 20 % by seasoning, in order to prevent fungal growth. Care should be taken to control and slow the rate of drying to avoid cracking, splitting or other defects.
• Temperatures below 0° C and above 40° C also prevent fungal growth, as well as complete submersion in water.
• Chemical treatment of timber against fungi, insects and fire should only tee done with full knowledge of the constituent substances, their toxicity (especially the long-term environmental and health hazards associated with their production and use), the correct method of application and the requisite precautionary measures. Opinions from different experts should be sought, in order to determine the least hazardous option. Proposals, such as facing of particle board with wood veneer or plastic laminate, are not always acceptable, as the emission of formaldehyde fumes is not reduced but takes place over a longer period.
• Indoor and outdoor uses of timber should be differentiated according to durability and degree of toxicity: under ideal (dry, well-ventilated, clean) conditions, even low-durability timbers can be used indoors; treated timbers that could represent a health hazard should only be used externally, but well protected from rain, if leaching out of toxic chemicals is expected.
• Good building design using well seasoned wood, good workmanship and regular maintenance can considerably reduce the need for chemically treated timbers.
• Good design of timber constructions includes: avoidance of ground contact: protection against dampness by means of moisture barriers, flashing and ventilation; avoidance of cavities, which can act as flues spreading fire rapidly; accessibility to all critical parts for regular maintenance; provision of joints designed to accomodate thermal and moisture movement; avoidance of metal connectors in places exposed to moisture, protection of exterior components from rain, sunlight, and wind by means of wide roofs and vegetation.
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