Timber strength grading and strength classes

The growth pattern of any tree is optimised according to its species in response to its environment and growing conditions, so no two trees are identical. Consequently, timber, the natural material obtained from trees, is inherently variable not only between species but even between individual pieces of timber from the same tree. This variability is obvious in the appearance of timber but also affects its stiffness and strength.

When using timber to carry structural loads it is important to ensure that the material is at least strong enough for the highest expected load. Strength grading provides a basic prediction of the strength of individual pieces, so that the weakest pieces are rejected and the remainder are assigned to an appropriate strength grade and/or 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:

• Strength and appearance
• Regulations and standards
• Strength grading and structural design
• Strength grading in practice
• Marking of strength-graded timber
• Strength classes
• Moisture content
• Maintaining quality
• Availability of strength-graded material