Ignorance of the nature of the materials you are using can cause a project to fail. Timber is a natural material and it is necessary to know something about the way it grows in order to understand how it can best be used and how it will behave in use.
Briefly, trees grow by adding new layers round the outside of the trunk, just underneath the bark. This makes the well-known annual rings. The sap which feeds these new layers also passes through to the centre or heartwood along the medullary rays which we call the figuring when the wood has been cut into planks. This means that the first few centimetres of the tree immediately under the bark are not fully grown cells and the mature timber is found towards the middle of the tree. The actual centre is known as the pith. These distinct areas behave differently in use.
Sapwood, that is the outer, immature wood, shrinks more than the inner heartwood. It is therefore more liable to twist. Shrinkage takes place round the direction of the annual rings from the outer part of the tree towards the central heart. If the tree is simply cut straight through into wide planks they will contain more sapwood on one side than the other and will warp away from the heart side, as the sapwood dries out and shrinks more than the heartwood. The pith should be avoided for all but the least important jobs because it is the most likely to twist as it dries or seasons.
When buying timber, as well as looking for the obvious defects such as large knots, shakes and cracks, and at general appearance, look also at the end grain, as it will tell you a lot about the future movement of the wood. The best pieces have the annual-ring marks fairly vertical across the narrowest width. Square timbers should not have the rings Funning diagonally as this will distort the shape.
Usage of timber
Timber not only dries and shrinks but also takes up moisture and swells. Its tendency to change shape affects the choice of wood and the way it is used. For example, you should always put the timber heart side up for table tops and the tops of wooden stools, and table tops should not be fixed firmly, only buttoned or attached with slotted plates to allow for this movement.
Further, if wide boards are to be made up using a number of narrow boards edge-jointed together, it is best to arrange the boards so that they are alternately heart side up and heart side down. This will cause them to pull against each other and in this way help to hold them flat. The battens underneath, which are used to hold the boards, must be screwed through slots not
through tight screw holes. In this way the whole top can adjust itself to the atmospheric conditions.
These comments apply to both hardwoods and softwoods, although the movement of softwoods is greater than that of hardwoods. However, the movement of the latter can often have a more damaging effect because these timbers are used for furniture show woods.
Hardwoods and softwoods
The terms hardwood and softwood are based on the structure of the wood and the characteristics of the trees. In general, softwoods are from the cone-bearing and evergreen trees; while hardwoods are from trees which shed their leaves in autumn. For practical purposes, softwoods are used for general joinery and structural work while hardwoods are generally used for furniture and show woods.
There are three main types of manufactured wooden boards — hardboard, chipboard and plywood (including blockboard). They are sold by the square metre, hardboard and plain, standard chipboard being cheaper than veneered chipboards, plywoods and blockboards of the same thickness.
Hardboard is shredded timber which has been pulped and remade as large sheets. It can be plain or patterned and there is an oil-tempered type which is for use externally. Hardboard is used for the base of boards finished with a woodgrain pattern or tiled finish. Fretted pattern boards are also made and form good decorative grilles.
Chipboard is made from timber chipped into flakes and then pressed, with a resin binder, to make a large sheet of material. It is available in stock sizes of length and width, and with a range of timber-veneer or plastic facings. Special grades are available for flooring. Chipboard is also available faced with different decorative surfaces. Most popular are wood-veneered chipboard and Melamine-faced chipboard which is ideal for use in kitchens and bathrooms.
Plywood is made in a very wide range of sizes and thicknesses. The number of plies varies according to the thickness required which can be from about 4mm to 25mm. All kinds of facing veneers are available from plain birch to mahogany and teak. For panelling there is a planked-pattern finish as well as plain wood.
Laminated boards consist of a core of wood strips glued or laminated into one solid piece, faced with veneer. They are obtainable in similar finishes to plywood. Pine-board also comes under the general heading of laminated boards. It is made from strips of pine edge-glued together and is available in a range of widths and lengths.
Usage of boards
Hardboards and softwood-faced plywoods can be used for similar purposes. When buying them take care that the sheets are completely flat, because once they have taken up a curved or distorted shape they are difficult to straighten.
Chipboards and blockboards also have similar uses and for the same reasons care must be taken that they are flat. Both boards are stable, but as shelving, blockboards will hold greater weights over larger spans. Blockboard has a surface which will accept a finish without further work, but chipboard needs to have a decorative surface applied to it.