Grading Timber
The aim of strength grading timber that is to be used for structural use is to ensure that working stresses allow designers and engineers to use timber in a way that is both safe for use and to the public. It also ensures that money is saved by avoiding over-specification.
There are two methods of grading - visual and machine. There are advantages and disadvantages to both. Visual strength grading uses the grader's experience across a number of different factors, (eg no. of knots, wane and slope of grain, splits and shakes that may have been developed as a result of drying), to predict the load a piece of timber will be able to carry in service and machine grading is more suited to high volumes of timber where the species and cross section are not changed very often. Each time the timber is cut it is regraded.
The strength depends on not only the size but the species of timber - low grade timber from a strong species may be equal in strength to a high grade timber from a weaker species. Strength is, therefore, grouped into classes C14 - C50 for softwood and D30 - D70 for hardwoods. The higher the number the stronger it is.
All timber is then stamped with:
Certification body or logo mark; company reference; species or species group; standard reference ie a BS or AN number (which is a British or european standard); timber condition eg DRY and strength class and if it has been machine graded there will be an M on it.
Timber marked DRY is graded at an average moisture content of 20% and this moisture content must not be exceeded during transportation, storage or installation.
Span tables showing timber sizes and strengths can be obtained from www.trada.co.uk and also by contacting one of our branches, where experts can talk you through your needs.
Using timber for structures is a specialist topic and should you require any further assistance, please do not hesitate to contact any of our branches, where our staff will be happy to help.
