Question 1

What is CLS timber?

Accepted Answer

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.

Question 2

What is regularised timber?

Accepted Answer

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.

Question 3

What is the difference between sawn and planed timber sizes?

Accepted Answer

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).

Question 4

What timber grade do I need for structural work?

Accepted Answer

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.

Question 5

What are standard joist sizes in the UK?

Accepted Answer

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.

Finished Size (mm)	Imperial Equivalent	Common Use
38 x 63	1.5" x 2.5"	Battens, noggins, light framing
38 x 89	1.5" x 3.5"	Stud walls (non-load-bearing), partitions
38 x 140	1.5" x 5.5"	Load-bearing stud walls, sole plates
38 x 184	1.5" x 7.25"	Heavy-duty stud walls, headers
38 x 235	1.5" x 9.25"	Deep stud walls, platform frame floors
38 x 286	1.5" x 11.25"	Deep floor joists, headers

Nominal Size (mm)	Finished Size (mm)	Common Use
47 x 100	45 x 95	Ceiling joists, light rafters
47 x 125	45 x 120	Ceiling joists, flat roof joists
47 x 150	45 x 145	Rafters, floor joists (short spans)
47 x 175	45 x 170	Floor joists, rafters
47 x 200	45 x 195	Floor joists (standard domestic spans)
47 x 225	45 x 220	Floor joists (longer spans), trimming joists
75 x 150	73 x 145	Purlins, heavy floor joists
75 x 200	73 x 195	Purlins, ridge boards, trimmer joists
75 x 225	73 x 220	Large purlins, ridge boards

Sawn Size (mm)	Common Use
25 x 38	Tile battens (BS 5534)
25 x 50	Counter battens, plasterboard battens
38 x 50	Roofing battens (heavy tiles), grounds
47 x 50	Wall plate, noggings
50 x 50	Fence post rails, studwork

UK Timber Sizes — CLS and Regularised Reference

CLS Timber Sizes

Regularised Structural Timber (Joists, Rafters, Studs)

Battens and Carcassing

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