Timber strength and spans

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Timber used for structural applications must be graded and clearly marked to show it complies with the correct standards and strength requirements laid down by building codes and regulations.

The load a member can carry depends on a number of factors, and different species have different strength properties which will be a major factor when choosing which timber to specify.

This Choose and Use sheet offers an overview of the strength grading process, including the two methods of grading (visual and machine strength grading).

This sheet also contains a table which details the grading rules and strength classes of some of the more common softwood species used in UK construction. It also contains an explanation of how span tables can be used in relation to strength classes. NEW in this version: common sizes and spans for domestic floor joists and flat roof joists.


  • How timber is graded
  • Visual strength grading
  • Machine strength grading
  • What do the strength stamps and grades tell me?
  • How do I use span tables in relation to strength classes?
  • Spans of some common joists
  • Spans for engineered wood products and I-joists

Suggested Reading

Engineered wood: Fact sheet no. 11navigation-arrow

This wood for good fact sheet is an introductory guide to engineered wood.

This sheet briefly explains engineered wood and then discusses the creation process and a number of the benefits of the different types of engineered woods available, included in which are: engineered timber beams (I-beams), heat treated...

01/01/2004 |  Info from other organisation  

Engineered wood productsnavigation-arrow

With engineered timber becoming an increasingly prevalent component in construction, it is important for members of the construction industry to understand not only the benefits and properties of Engineered wood products, but also the different types.

With modified wood products being improved upon on a seemingly daily basis, this...

01/01/2012 |  Choose and Use  

Plywood: Fact sheet no. 13navigation-arrow

This fact sheet published by wood for good is an overview of plywood as a construction material.

The sheet details the main two types of plywood and offers details on the thickness and grading types of different plywoods.


  • Main types of plywood
  • Thickness
  • Grades
  • Plywood for...

01/01/2004 |  Info from other organisation