Timber I-joists: applications and design

Date Published

13 August 2022

Document Type




Timber I-joists are Design > Engineer;ed 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.

Key Information

As there is no harmonised standard for l-joists, the current route to CE marketing for the European single market is by complying with EAD 130367-00-0304.

l-joists are readily available in longer lengths than solid timber and with better strength and stiffness-to-weight ratios and deeper depths.

Decked floors do not usually need strutting, but its use may improve vibrational performance. If strutting is required use full-depth l-joist sections or proprietary metal cross-bracing.

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Case studies

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