Timber I-joists: applications and design

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Timber I-joists are engineered structural components that are widely used in the UK, particularly as floor joists. Its high strength-to-weight ratio and economy in use of materials make them popular when designing for long-spans and multi-storey structures. I-joists shrink or swell less than solid wood when the ambient conditions change.

I-joists are factory produced in the UK and elsewhere, with extensive quality control checking 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) covers the many considerations to be made when working with I-joists, including the potential need for metal strapping or the need for temporary bracing or sheathing during construction due to the tendency for unrestrained joists to twist due to buckling when compressed.



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

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