Timber strength grading and strength classes
The way a tree grows varies with species, but is naturally optimised in response to its environment and growing conditions, so no two trees are identical. Consequently timber, the natural building material obtained from trees, is inherently variable not only between species but also between individual pieces from the same tree. This variability is obvious in the appearance of timber but it affects its stiffness and strength as well.
When using timber to carry structural loads the basic requirement for safety is that the material is more than strong enough for the highest expected load. Strength grading provides a prediction of the strength of individual pieces, so that those pieces that are not strong enough are rejected and the remainder are assigned to an appropriate strength class.
This Wood Information Sheet (WIS) is about strength grading of timbers used for structural purposes. It is an overview of the subject with signposts to more detailed sources which are listed at the end. It does not cover grading for appearance, which is a separate process.
The following key topics are included in this information sheet:
• Types of grading
• Regulations and Standards
• Strength grading and structural design
• Strength grading in practice
• Marking and documentation
• Strength classes
• Moisture content
• Maintaining quality
• Availability of strength-graded material
This Wood Information Sheet was revised in March 2021. Changes include:
• Phrasing adjustments following Brexit
• Updated references to BS EN 14081
• Removal of reference to BS 5268
• Revised information regarding UKCA marking, however please note that this WIS does not address transitional arrangements for CE marking that apply in the UK during 2021, nor the special status of the Northern Ireland market. For details on these subjects please consult TRADA’s Technical Bulletin UKCA marking, available from 1 April 2021 at www.trada.co.uk.
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