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
Availability of strength-graded material
Timber can be graded either visually or by machine. There is normally a supplementary visual assessment for defects the machine cannot detect, which will vary depending on the machine technology.
Once the strength grade is known, together with the species and the source, the piece can be assigned to a strength class, which leads to the characteristic values needed for structural design.
Structural timber falls within the framework created by the UK Construction Products Regulations (UK CPR), and there is a legal obligation on manufacturers to UKCA mark the product for it to be placed on the market.