The hundreds of different timber species, which vary widely in their appearance and properties, are usually classified into two broad groups hardwoods and softwoods. Hardwoods come from broad-leaved trees such as ash and oak. Softwoods come from coniferous trees which mostly have needle-like leaves – Norway spruce, for instance. This is a botanical classification and the terms are only a general guide to the hardness of the woods – balsa, for example, is probably the softest wood available yet it is classified as hardwood, whereas yew, a hard wood, is classified as a softwood.
Traditionally, softwoods are cheap, easily-worked timbers used in house construction for flooring, rafters, joists, windows and doors. They are often painted. Hardwoods, on the other hand, are generally more expensive and considered to be more durable, and more difficult to work. They are used for making furniture and sometimes finished with stain and then polished or varnished to bring out their natural decorative features of grain and texture.
In recent years, many of the traditional British and European hardwoods such as ash, elm and lime – have been replaced by varieties from regions of Africa, the Far East and North and South America. The most commonly available hardwoods are the so-called ‘red’ hardwoods (mainly mahogany) and yellow-coloured ramin, but there is a wide range available through timber merchants.
Because of the high cost of hardwood, it has largely been supplanted in do-it-yourself work by man-made boards covered with a thin veneer of hardwood (teak or mahogany, for example) widely used in furniture making and shelving.
There is a good selection of man-made boards available as a substitute for using plain boards of timber: there is also a wide choice of both hardwood and softwood mouldings for use in the home. The more common sizes of softwood and some types of hardwood are available in the large do-it-yourself superstores as well as some do-it-yourself shops; for the full range, go to a timber merchant.
Conversion and seasoning
A tree which has just been felled contains a lot of moisture. To make it into timber which can be used, it has to be sawn and dried. These processes are known as conversion and seasoning. Some softwood may be preservative treated.
Timber is generally sawn:
o through-and-through (sometimes called plain-sawn or slash-sawn). In this method the log is cut into planks by simply slicing through the tree. Most of the growth rings make an angle of less than 45 degrees with the surface. The moisture content of a tree is greater around the outside than at the centre, so when through-and-through sawn wood is dried all the planks, apart from those which are cut directly through the centre of the tree, have a tendency to warp. The planks have to be planned flat after drying
o quarter-sawn. Timber has growth rings at an angle of 45 degrees or more to the surface of the board. It is often more expensive than through-and-through sawn timber but much less likely to warp.
Timber is dried by either stacking it in the open air or by drying it in a kiln. Drying makes timber stronger, more resistant to fungi and better for painting and varnishing.
Once timber has been dried it can still gain and lose moisture – it shrinks as it loses moisture and expands as it gains moisture. Timber which has been stored in the open will shrink when brought indoors, particularly into a centrally heated room. The effect this has on the timber depends on how the timber was sawn – through-and-through sawn timber tends to warp whereas quarter-sawn timber tends to shrink evenly. Whenever timber is moved between environments which are likely to have very different humidities, it is important to allow time for the timber to come to equilibrium with its new surroundings. This may be as little as a few days for small sizes but well over a week for larger ones.
Some timber is available pretreated with preservative. This has had the preservative pressure impregnated and will lengthen the life of timber used outside and also in structural work. Cut ends should be additionally treated.