How to get it right: Roof truss "alterations" (or why not to cut your roof trusses!)

^{Article updated on 31 January 2024}

Roof trusses offer a quick and efficient alternative to a conventional timber roof but they're quite delicate when not in their fully braced vertical position.

They work through a combination of tension and compression to ensure that the timber chords and web sizes can be kept to a minimum and keep costs down.

The central "W" shape is how most of the loads are distributed and this is where longitudinal and diagonal bracing is needed to reinforce the roof diaphragm.

So imagine the shock one of our building control surveyors had when he encountered this roof recently:

The owner had decided to carry out their own loft conversion for a bit more space and just removed the central web chords that were "getting in the way".

Once the web chords had been cut, instead of transferring all roof and wind loads to the wallplate they were taken down the internal walls. The owners called a builder in when they noticed cracking in the bedroom walls within days of the "alteration".

Trusses are always built with an upward camber so that once loaded they deflect about 6mm over time to become straight. 

Without the internal walls, the calculated deflection would have been as much as 170mm but only whilst the gang nail truss connectors stayed in place.

So how was it fixed?

The roof tiles had to be taken off completely to allow the remedial work and the ceilings were propped down to the ground floor so that they could be retained in place while new trusses were inserted alongside, and bolted to the trimmed roof members.  

The buildings insurance providers refused the home owner's claim leaving them with a very expensive repair bill and nothing to show for it...

If in doubt always contact your local building control team

Other building blunders we've come across...

• Basement work that avoids house collapse
• Removing a chimney the right way (with video showing the wrong way...)
• How not to carry out localised underpinning

View further 'How to get it right' articles for building tips and guidance

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.