Workshop

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.

Structure
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.

Production
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%.

Properties
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.