Strength Graded Timber & Span Tables - C16 span tables - guide

Oct 13, 2026

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Strength graded timber products from BSW are used for loadbearing applications such as floor joists and wall studs, requiring strength grading to comply with Building Regulations. Grading is commonly performed to British Standard EN 14081 into strength classes such as C16 and C24, with products marked using a legally required stamp. The document explains the effect of cutting and resawing on grades and provides C16 span tables for domestic ceiling joists and domestic flat roof joists, including permissible clear spans based on loadings, sizes, and spacings, with guidance limits and exclusions.

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Strength Grading and Class

C16 Span Table - Domestic Ceiling Joists

C16 Span Table - Domestic Flat Roof Joists

Strength Graded Timber & Span Tables A step-by-step guide from BSW Timber [www.bsw.co.uk](www.bsw.co.uk) Strength Grading and Class • BSW construction products are frequently used timbers for loadbearing situations such as floor joists and wall studs (sometimes referred to as“carcassing”) . In order to comply with the Building Regulations they must always be strength graded: they are then usually put into a Strength Class. • Strength Grading of BSW construction timbers is often done by machine to British Standard EN 14081 to classes C16 or C24 . There are actually 12 Strength Classes available in Europe, but the UK mainly uses C16 and C24 . • BSW construction timbers are clearly marked with a legally required stamp to help you identify that the products have been graded. The picture below explains the stamp markings. • In your project if strength graded timber is cut to length, there will be no problem-its grade is unaffected. However, if a piece of graded timber is resawn in its cross-section, then its original strength grade is destroyed. Both of the resawn sections must be re-graded before being used for any structural purpose. • The Strength Class of the timber is very important when determining which joist to use for your project. It will help you identify which dimensions of timber to use for a particular span and loading situation. BSW machined kiln dried strength graded construction timbers are available in the following sizes: Nominal Sizes (mm) Finished Sizes (mm) Lengths (mm) | 47 x 75* | 45 x 70 | 2400 | | --- | --- | --- | | 47 x 100 | 45 x 95 | 3000 | | 47 x 125 | 45 x 120 | 3600 | | 47 x 150 | 45 x 145 | 4200 | | 47 x 175 | 45 x 170 | 4800 | | 47 x 200 | 45 x 195 | 5400 | | 47 x 225 | 45 x 220 | | | 75 x 100 | 72 x 95 | | | 75 x 150 | 72 x 145 | | | 75 x 175 | 72 x 170 | | | 75 x 200 | 72 x 195 | | | 75 x 225 | 72 x 220 | *47 x 75 KD Ungraded | C16 Span Table Domestic Ceiling Joists Imposed load not exceeding qk = 0.25 kN/m2 and Qk = 0.9 kN. Gk not more than 0.25 kN/m2 Gk not more than 0.5 kN/m2 | 45 x 120 mm | 2.42 | 2.38 | 2.29 | 2.26 | 2.22 | 2.11 | | --- | --- | --- | --- | --- | --- | --- | | 45 x 145 mm | 3.08 | 3.03 | 2.89 | 2.86 | 2.80 | 2.65 | | 45 x 170 mm | 3.75 | 3.68 | 3.50 | 3.46 | 3.38 | 3.19 | | 45 x 195 mm | 4.42 | 4.33 | 4.11 | 4.06 | 3.97 | 3.73 | | 45 x 220 mm | 5.09 | 4.99 | 4.72 | 4.67 | 4.55 | 4.27 | | C16 | Dead load Gk per square metre excluding weight of joist | | | | | | | --- | --- | --- | --- | --- | --- | --- | | | | | | | | | | | Centre-to-centre spacing of joists | | | | | | | Joist size
(breadth x depth) | 400 mm | 450 mm | 600 mm | 400 mm | 450 mm | 600 mm | | | Maximum clear span in metres | | | | | | | 38 x 89 mm | 1.50 | 1.49 | 1.44 | 1.43 | 1.41 | 1.35 | | 38 x 140 mm | 2.74 | 2.70 | 2.58 | 2.55 | 2.50 | 2.37 | | 38 x 184 mm | 3.86 | 3.78 | 3.59 | 3.55 | 3.47 | 3.27 | | 38 x 235 mm | 5.16 | 5.06 | 4.78 | 4.72 | 4.60 | 4.31 | | 45 x 95 mm | 1.78 | 1.76 | 1.69 | 1.68 | 1.65 | 1.58 | | 72 x 120 mm | 2.95 | 2.90 | 2.77 | 2.75 | 2.69 | 2.55 | | --- | --- | --- | --- | --- | --- | --- | | 72 x 145 mm | 3.72 | 3.65 | 3.48 | 3.44 | 3.37 | 3.18 | | 72 x 170 mm | 4.49 | 4.40 | 4.18 | 4.14 | 4.04 | 3.80 | | 72 x 195 mm | 5.26 | 5.15 | 4.88 | 4.83 | 4.71 | 4.43 | | 72 x 220 mm | 6.02 | 5.90 | 5.58 | 5.51 | 5.38 | 5.05 | | 89 x 184 mm | 5.30 | 5.20 | 4.93 | 4.87 | 4.76 | 4.48 | | --- | --- | --- | --- | --- | --- | --- | | 89 x 235 mm | 6.93 | 6.79 | 6.43 | 6.35 | 6.20 | 5.82 | The above table provides a general summary of common permissible clear spans of simply supported domestic ceiling joists of solid timber for specified loadings, sizes and spacings. The table is solely for guidance on the loadbearing capacity of solid timber members, and cannot be adapted for hardwoods or engineered timber products. In preparing this table, Timber Development UK has assumed the complete design and build will be delivered by c

FAQs

Why must BSW construction timbers be strength graded?

They are used in loadbearing situations and must be strength graded to comply with the Building Regulations, so they are assigned a Strength Class.

What happens to a strength grade if timber is cut to length or resawn?

Cutting to length does not affect the grade, but resawing in the cross-section destroys the original strength grade, requiring both resawn sections to be re-graded.

What information do the C16 span tables provide?

They provide permissible clear spans for simply supported domestic ceiling joists and flat roof joists of solid timber, for specified loadings, sizes, and spacings.

Oct 13, 2026

Upload by jay