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.
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.