Timber strength and spans

Date Published

5 September 2022

Document Type




Timber used for structural applications must be graded and clearly marked to show it complies with the correct standards and strength requirements laid down by building codes and regulations.

The load a member can carry depends on several factors, including its span, thickness, width and species. Usually the deeper/wider the section, the longer the span.

Different species have different strength properties – a factor that needs to be considered when choosing your timber.

Key Information

Strength grading is needed to ensure timber is strong enough for a particular job, e.g. a floor joist or a roof truss. It also saves money by helping to avoid over-specification. Grading can be carried out visually or by machine: Visual strength grading uses the grader’s experience and knowledge to assess each piece of timber.

Machine strength grading is best suited to high volumes of timber where the species and cross section are not changed very often. When a piece of timber is machined into a small cross section, it may need to be re-graded. Cross cutting is permitted.

The rules governing strength grading and structural uses of timber are laid down in British and European standards.

Case studies

Arcadia Nursery was created to provide early-years education for children of university staff, students and the general public and caters for up to 113 children ranging in age from six weeks to five years.

Timber is not a material commonly used in largescale office refurbishments in central London, but the Republic Masterplan at East India Dock demonstrates how it can successfully transform
a couple of tired 1990s office blocks into high quality sustainable and affordable workspaces and help to create a green, relaxing and biodiverse landscape around them.