UK Timber Sizes — CLS and Regularised Reference

CLS (Canadian Lumber Standard) timber has rounded (eased) edges and consistent finished dimensions. It is the standard framing timber used in UK timber frame construction and stud walls. Common CLS sizes include 38 x 63 mm, 38 x 89 mm, and 38 x 140 mm. CLS timber is kiln-dried, planed all round, and ready to use without further processing.

Regularised timber is sawn timber that has been machined on two faces (width faces) to give a consistent depth for use as floor joists, ceiling joists, and rafters. The width faces are smooth but the thickness faces remain rough sawn. For example, a 47 x 200 mm regularised joist has a consistent 200 mm depth but the 47 mm thickness may vary slightly.

Sawn (rough) timber is the nominal size straight from the sawmill. Planed (finished) timber has been machined smooth and is approximately 3-5 mm smaller in each dimension. For example, nominal 50 x 100 mm sawn timber becomes approximately 45 x 95 mm when planed all round (PAR). Always check whether quoted dimensions are nominal (sawn) or finished (planed).

Structural timber in the UK must be strength-graded to BS EN 14081. The most common grades are C16 (general structural use - joists, rafters, studs) and C24 (higher strength - longer spans, heavier loads). C16 has a bending strength of 16 N/mm² and C24 has 24 N/mm². TR26 is used for roof trusses. The grade must be stamped or marked on every piece of structural timber.

Standard regularised joist sizes are 47 x 100, 47 x 125, 47 x 150, 47 x 175, 47 x 200, 47 x 225 mm (and the same range in 75 mm thickness for heavier loads). For floor joists in a typical domestic situation spanning 3-4 metres at 400 mm centres, 47 x 200 mm C16 is commonly used. Span tables in Approved Document A or the TRADA Span Tables booklet give the correct size for any span and loading.