Users accustomed to the limited range of steel section sizes will find considerable variation in timber sizes available in the UK. Sawmills producing softwood timber generally do not know the intended end-use for the material at the time it is cut. They therefore produce a range of ‘customary’ sizes. Specifying timber in these sizes is the most economic method, both in terms of material usage and cost.
Ideally, the sizes available would be those listed in the British Standard. Yet when BM TRADA surveyed the supply of structural timber in 2014, it found that the range of timber sizes commonly available from timber merchants did not tie up well with the ‘common’ and ‘complementary’ target sizes suggested in BS EN 1313-1 Round and sawn timber. Permitted deviations and preferred sizes. Softwood sawn timber. Hence the listing of ”customary” sizes in this latest WIS from TRADA.
Moisture content at the time of measurement affects the size of sawn timber, a point to watch when specifying timber. The thickness and width of timber will vary by typically 0.25% for every 1% increase or decrease in moisture content compared to a moisture content of 20%. Length is not significantly affected by changes in moisture content.
It is also important to distinguish between ”sawn” sizes and ”regularised” sizes that are used when softwood is machined to ensure a uniform depth when used as a joist.