Timber I-joists: applications and design

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Timber I-joists are engineered structural components for use in walls, roofs and floors. They are widely used in the UK, particularly as floor joists. Their high strength-to-weight ratio, low weight, ease of installation and economy in use of materials make them popular, especially for long spans and multi-storey structures.

I-joists have better dimensional stability than solid wood so they shrink or swell less when ambient conditions change. I-joists are the main structural component in the off-site manufacture of some panellised systems.

I-joists are factory produced in the UK and elsewhere, with extensive quality control to ensure constancy of performance. The route to market is via third-party certification that establishes safe design values for use with design codes.

This Wood Information Sheet (WIS) is an overview of the subject with signposts to more detailed sources that are listed at the end.


  • Product certification
  • Structural principles
  • Application of I-joists
  • Details for design and construction
  • Engineering design


This Wood Information Sheet was revised in July 2021 and minor amendments were made to reflect post-Brexit changes to CE marking and the updated European Assessment Document. Sections amended include: structural principles, stability during construction, strutting, control of deflection, stability in service.


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