Timber is a natural fibre-reinforced polymer that has been used as a construction material for many centuries.
All timbers are composed of a large number of close-packed tubular filaments of cellulose bonded together with an organic resin. In this sense they are composite materials with two different elements (with quite different properties) forming the material.
Trees grow by adding an annual ring below the protective outer layer (bark).
This "ring" is in fact a layer of cells which are large and plentiful during good growth conditions (spring, summer), but smaller and fewer in number during less favourable times (autumn and winter). As the tree grows, the inner cells age and die becoming harder and more compact. It then becomes heartwood which gives the tree its strength and support. This is the part of the tree that we use as a construction material.

Timber, once sawn into lengths, needs to be seasoned to lower the moisture content to around 10-20% prior to use. This can be performed in specially constructed huts, air seasoning, and takes a number of years. Kiln drying has now reduced the time taken for this process to about 10 weeks.

In its natural state, timber falls into two categories: Softwood and Hardwood. Softwood comes from coniferous trees which are distinguished by being evergreen with needles and/or cones. These trees are relatively fast growing and tend to be produced in countries which have a temperate climate ~ Northern Europe, Canada, USA. Their heartwood is porous and therefore less dense with the result that softwoods tend to be easier to cut.
Examples of softwoods, properties and their uses are given below:

Redwood (Scots Pine) Quite strong and hard with a straight grain. Easily worked and durable when preserved. General woodwork, doors, shelves, building construction.
Whitewood (Spruce) Tough but easily worked. Straight grain. Indoor work only. Low cost furniture, steps, flooring.
Parana Pine Tough with a fine grain. Not very durable. General indoor woodwork, fitted furniture.
Cedar Light, soft and weak. Very durable (contains natural oils). Decorative and protective. Garden furniture, fences.
Douglas Fir Fairly strong, durable and water resistant. Window and door frames.

 

Hardwoods come from deciduous trees which shed their leaves annually. They tend to grow more slowly than softwood and can be found in both temperate and tropical climates ~ Europe, USA, Japan, Africa, Central and South America. As a general rule, hardwoods are denser than softwoods and so tend to be more difficult to shape and cut although there are some notable exceptions such as balsa which is produced from a deciduous tree.
Examples of hardwoods, properties and their uses are given below:

Beech Strong and hard with a close grain that works well. Not durable outdoors. Furniture, childrens' toys, tool handles.
Mahogany Hard and strong but not easy to finish. Furniture and panelling.
Oak Very strong, dense and durable. Good quality furniture, gates, fencing.
Teak Durable and resists decay (due to the presence of natural oils). Very hard (will blunt tools quickly). Quality furniture, especially outdoor, boat construction.
Maple Fairly hard-wearing. Attractive. Musical instruments, butchers' blocks.

Additional information on timbers can be found on this link: http://www.rutlandwoodcraft.co.uk/timbers/

Timber is available in a variety of market forms:
Planks
Boards (1.8 m+ in length, approx 200 mm wide)
Strips (1 m+ in length 22x22, 35x35, 47x47 mm PAR (Planed All Round))
Dowel (0.9 - 2.4 m in length, diam. 4,6,9,12,15,18,21,25,28,34,38 mm)
Mouldings (timber preformed into different cross-sectional shapes such as
skirting board).

It is the grain in timber that gives it its strength but is also the cause of its greatest weakness. Wood has greater strength along the grain than across it ~ an important consideration during construction. Wood is also hydroscopic, absorbing and releasing moisture. When it gets damp it swells across the fibres of the grain increasing in size and causing the distortion known as warping. As a natural material it is also susceptible to natural defects such as knots. It is also prone to fungal attacks; dry and wet rot, as well as insect attacks; woodworm, termites etc.

These problems can be overcome, to a certain extent, by manufactured boards.
Clearly natural timber gives rise to several problems: natural defects, inconsistent mechanical properties, limited widths. Timber is, therefore, manufactured into boards of various types for the building industry.

Particle Boards

Chipboard is made by compressing wood chips with resin such as urea formaldehyde. It is regularly veneered to increase its strength and to greatly improve its appearance. This enables it to be used for tables, shelving, work tops etc.

Fibre Boards

Fibreboard is available in a number of different forms:
Hardboard is made by compressing wet fibres at high temperatures. The natural resins in the fibres act as an adhesive. It can be used for facing boards and the underlay of a floor on which vinyl is to be laid.
MDF will be familiar to you all. It is made from the pulp of wood, with resin and wax added, dried under heat and pressure. It is used for modelling and making moulds in the plastics industry because of its smooth finish. It also provides an excellent base for veneers and laminates and is used extensively in the furniture industry.

Laminated Boards

These are made by taking thin sheets or veneers and gluing them together under high pressure and temperature. The most common example is plywood in which each layer, or ply, has its grain at right angles to the next to make the board stronger and more resistant to warp. Plywood is available in a variety of thicknesses with a variety of plys (layers) and can have special properties such as weatherproofing, waterproofing and marine-ply which is suitable for marine use. Skin-ply makes the best use of the strength to thickness ratio of plywood and allows curved surfaces to be formed.
Blockboard is another form of laminated board in which strips of wood, butted together are sandwiched between two veneers.

For Exam questions on this subject, click here

 

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