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.

Contents:

  • 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

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