Timber strength grading and strength classes

TRADA image

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.


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...

23/03/2021 |  Wood Information Sheet  

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...

12/03/2020 |  Wood Information Sheet  

Sole plates in timber frame constructionnavigation-arrow

Sole plates are a vital element in a timber framed building. Their installation has a direct effect on the building's service life, line, level and plumb, and contributes to the speed of construction.

This Wood Information Sheet provides advice on getting the installation of sole plates right first...

25/03/2021 |  Wood Information Sheet