Timber

There are many types of timber, but not all are suitable for construction. Smaller projects may only need a few sawn timbers cut to size. Larger projects such as a loft conversion would normally need site drawings with an associated materials list. Both of these projects would generate a simple list of timber specifications, timber dimensions, and quantities required. These lists of timber should include any structural requirements (C24, C16 or ungraded) and details of any necessary timber processing.

The most common type of timber for a structural building would be pressure treated and graded at C24. These structures are invariably made watertight using cladding, ply sheets and building felt. While the number of timber extensions using C24 is considerable there are numerous fences, decks and small buildings constructed using sawn timber. These timber may be structurally ungraded but should be pressure treated and machined ready for use.

Timbers are machined in different ways to try and slow down rot and insect decay. The choices of timber preservation is partially affected by price, availability and performance – a proposed build needs to be cost effective. C24 is the most commonly used graded timber. Weakness in timber usually occurs where moisture and oxygen are allowed to mix freely. Providing the timber is treated to UC4 (Use Class 4) before installation should improve performance considerably.

Timber planks and posts rot quickly when used without the counteracting effects of preventative chemicals. In the UK timber growing regions experience high moisture and relative warmth, the same growing conditions favoured by fungi and insects. Posts rot very quickly if they haven’t been treated in pressurised tanks of timber preservative. Timber’s resistance to rotting is also dependent on the timber being kept away from moisture. If stored for some time then it should be a dry, well-ventilated building – condensation can be as destructive as a roof leak.