Timber strength grading and strength classes

TRADA image

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

Suggested Reading

Principles of green oak constructionnavigation-arrow

There is growing interest in green oak framing for a variety of reasons, from satisfaction in hand craftsmanship to the ecological advantages of using a renewable material.

This Wood Information Sheet (WIS) is an overview of the design and construction principles that specifiers should consider in order to ensure...

13/07/2015 |  Wood Information Sheet  

Timber frame: Fact sheet no. 14navigation-arrow

This fact sheet, published by wood for good gives readers an overview of building with timber frame.

The fact sheet contains a number of basic points regarding timber frame including the benefits which make it popular, some standard studding and joist sizes and a number of statistics in the...

01/01/2004 |  Info from other organisation  

Introduction to timber frame constructionnavigation-arrow

Timber frame construction of some description is a method which has been around for millennia; however over time it has become a tremendously refined practice to the point that - as buildings of up to six storeys become widely used in England and Wales - timber frame constructions of today...

04/04/2016 |  Wood Information Sheet