Its green credentials are impeccable, it is infinitely renewable, providing the forests from which it comes are properly managed, it stores carbon dioxide and it can be recycled. Because of this, it has been used in the construction of many recent buildings.
The Millennium School at Greenwich, for example, is entirely clad in timber; Finnish timber firm Finn Forests' UK headquarters in Lincolnshire is built only from wood, and Velux has just completed a sales office and training centre in Northamptonshire that is also principally constructed from timber.
In this Workshop special, we take a close look at the way timber is produced, examine its properties and explore some common composites.
Timber is divided commercially into softwood and hardwood. This is not because hardwood is physically harder than softwood: it describes the nature of the tree and the configuration of the living cells forming the trunk. Softwood, or coniferous, trees have needle-like leaves that are usually retained all year round, and they carry their seeds in cones. Hardwood, or deciduous, species are broadleaved, lose their leaves in the winter and carry their seeds in an enclosed case.
Useful timber is derived from the trunk of the tree, which in life had to perform several functions. Water and nutrients from the roots pass up the tree and the food manufactured in the leaves passes downwards. The trunk has to support the weight of the "crown" of branches and withstand high winds.
Because of this, the cells of trees are nearly all aligned vertically. Some of the collections of cells are arranged horizontally – these are known as rays and give the wood a characteristic patterning, for example the flecks seen in beech. The arrangement of cells varies according to the specific function they perform and between species. In softwoods, the vertically arranged cells, known as trachieds, form an almost geometric pattern. In hardwoods, these vertical elements vary much more in size and arrangement. The general direction, or grain, of the fibres up the trunk can be described as straight, spiral, diagonal, interlocked or wavy.
After felling, the timber is sawn into planks. The easiest and cheapest method is "through-and-through" sawing where the log is simply cut up in parallel slices. The pricier alternative is quarter-sawing, where the timber is cut in a radial pattern. This makes the grain of the wood lie almost perpendicular to the surface, making it more stable during drying and producing attractive patterning on the surface.
Quarter-sawn oak is a good example of this.
Freshly felled, or "green air", timber is saturated with water. The timber is dried or seasoned until the moisture in the timber reaches equilibrium with the moisture in the air. In the UK, this is about 18% by weight. To reduce this, further kiln-drying becomes necessary. As the moisture content of wood will continue to vary according to the humidity it is subjected to, it is important to specify timber with a moisture content close to that of the final environment, as the timber will contract and expand across the grain. The susceptibility of timber to movement varies depending on the species, but for general construction the moisture content should be 18-24%.
The properties of timber vary considerably according to species, so the wood type is selected according to its end use, whether this is structural or decorative or a combination of both. Hardwoods tend to be stronger, stiffer, more durable and more aesthetically appealing than softwoods, offering much greater decorative variety. Unfortunately, they are also more expensive.
For structural purposes, timber is extremely strong in tension along the grain – some species equal aluminium and exceed steel in strength-to-weight ratio – but are weaker across the grain. Because it is a natural material, the quality of wood varies considerably from plank to plank, so it needs careful strength grading to take account of knots and defects caused by drying, such as splits. High-density timber tends to be stronger than the lower-density species. Timber used for structural purposes should be strength graded to the appropriate British or European Standard, and marked as such.
One disadvantage of the use of timber as a construction material is its vulnerability to fungal decay and insect attack. The susceptibility of timber to attack again varies considerably across different varieties. The inner part of the trunk – the heartwood – of some timbers will last unprotected for over 25 years, whereas others will decay in less than five. The outer part of a tree trunk, the sapwood, is less durable. The use of preservatives will enhance the durability of the wood, although only if the species in question is sufficiently permeable to absorb the treatment. If the timber is used in an environment where its moisture content will not exceed 20%, it won't suffer from fungal decay.
Structural timber can perform well in a fire. Wood is a good insulator, so it may char on the surface but be unaffected a few millimetres towards the centre. Timber has an advantage over other materials as it doesn't melt or collapse. The rate at which wood chars is known, depending on species, and is therefore predictable. This predictability can be factored into a design, and the fire performance of the timber enhanced with special treatments.
The ease of working and the finished texture also varies across species. Texture refers to how smooth the timber feels and how it looks after being cut. The timber can have an even or uneven texture.
SoftwoodEuropean redwood and European whitewood
Most of the softwood used in construction is either European redwood, which is the same species as Scots pine, or European whitewood. European redwood is a pale yellow to red-brown colour and European whitewood is white to yellow-brown. They originate from Scandinavia and Russia, and are readily available and inexpensive. These species are used for both joinery and structural purposes, although European redwood has a tendency to bleed resin around the knots. Both species are not very durable, although redwood can be treated with preservatives. If European redwood is pressure-treated, it can be used for decking.
Western red cedar
Western red cedar is reddish-brown and originates from North America. Its principal use is as a cladding material as it has the same durability as oak, yet is comparatively inexpensive. It is a relatively defect-free straight-grained timber that weathers to an attractive silvery-grey, or it can be stained for a more durable colour. Special fixings such as stainless steel have to be used as it causes some metals to corrode.
The UK was once covered in oak forests and it was used widely for timber-framed buildings and other purposes. It is extremely durable – many of the buildings constructed hundreds of years ago still stand today. The white oak family consists of two species, European oak and American white oak, both of which are a yellow-brown colour. American red oak is not as durable as the white oak family. Oak is a widely used hardwood, being suitable for joinery, cladding and flooring. It is fairly widely available, and is in the middle price range for timber.
Maple is an attractive creamy-white colour with a fine textured surface. Its high resistance to abrasion makes it ideal for flooring subjected to heavy wear, such as sports halls. It is used for some joinery, including kitchens and bedroom furniture, but is not durable enough for external use. It is readily available and is in the middle price bracket.
Ash is used for internal joinery and trim detailing including skirting boards and architraves. It is also used for handrails, and some stock is suitable for bending. Two varieties are available: European ash is white to light brown and American ash is a grey to brown colour. It is not durable so cannot be used outside, but is readily available and is in the middle price bracket.
The ubiquitous medium-density fibreboard is manufactured by steaming wood then reducing it to fibrous pulp. The fibre is sprayed with an adhesive and pressed into sheets that vary in thickness from 3 mm to 25 mm. It is available with a range of different properties that includes high density where greater strength is required, and moisture proof, fire-resistant and formaldehyde-free variants. Its high strength, super-smooth surface and excellent machining characteristics combined with low cost mean it is extensively used for joinery where a panel product is required. Changing Rooms wouldn’t be the same without it.
Wood chips are glued together under pressure to make sheets of chipboard. A range of grades can be specified, depending on the end use. These include the flooring grade, a very popular application, where the material has to be able to span joists without bending, and furniture grades suitable for applications including kitchens. Moisture-resistant chipboard should always be used in environments where there are high levels of humidity otherwise it could disintegrate into a heap. Chipboard is inexpensive, although the surface finish is a bit rough.
Chipboard and mdf have largely replaced plywood as a flooring and furniture product, but laminates are still used to build flat roof decks. Shuttering plywood is used as a former for concrete. Probably the most interesting laminated product is glue-lam beams, which are made from thin sheets of timber glued together in a sandwich. The advantages they offer over a solid timber beam are greater strength and the availability of lengths limited only by transportation methods. Bespoke beams that can be curved in two planes are also available offering greater creative potential for architects – and they look good, too.
Timber suppliersMost of the companies listed below supply hardwood, softwood and panel products:
A & W Cushion (Norwich)
AJ Scott (Northumberland)
AW Champion (branches throughout the South-east) www.championtimber.com
Channel Woodcraft (Folkstone, Kent)
DW Archer (branches throughout Central England) www.dwarcher.co.uk
FH Thompson and Son (Gateshead)
James Latham (outlets throughout England) www.lathamtimber.co.uk
John Fleming & Co (Aberdeen)
MH Southern & Co (Jarrow)
Mid-Sussex Timber Co (branches throughout Sussex) www.mid-sussex-timber.co.uk
Panel Agency (Longfield, Kent) www.panelagency.com
R & D Aiken (Coleraine, N. Ireland)
RK Timber (Branches throughout the UK) www.rktimber.co.uk
Rowan Timber Supplies (Scotland) (Lanarkshire)
Timbmet (Oxford, Glasgow and Rochdale) www.timbmet.com
VA Luck (Birmingham)
Vickers Timber Co (Brentwood, Essex)
WJ Odds (Sittingbourne, Kent)
Information for this article was kindly supplied by TRADA, which offers publications on timber, consultancy and a telephone advisory service – see www.trada.co.uk.