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

18.11.2016
Blog Post
Picture of roofer and roof trusses

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:

A picture of cut roof trusses

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 on earth 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.

View our other "How to get it right" articles 

Comments

(No subject)

Submitted 2 months ago

Standard roof trusses, seen all over the country, are a terrible waste of space. They should be designed as the new standard to allow for making use of the roof void as an extra room at later date.

(No subject)

Submitted 2 months ago

They already are. They are called attic saver roof trusses and ive been fitting them for years. More cost but basically you are forming the attic space as you fix the trusses.

(No subject)

Submitted 2 months ago

TO MANY IDIOTS OUT THERE WHO THINK THEY KNOW BEST.

(No subject)

Submitted 2 months ago

On behalf of a potential purchaser, I inspected a loft space of a house which was for sale. The vendor had cut out the web members where they got in the way of his model railway! Again, the trusses were sitting on the lightweight non-loadbearing partitions which were supported on the first floor structure and the whole structure had racked.
My client walked away from the property so I don't know if and how it was ever remedied.
Do people really think that the builder would have installed timber that wasn't necessary?

(No subject)

Submitted 2 months ago

About 15 years ago I had a similar experience, as a structural engineer I was asked by a building surveyor to visit a property in Basingstoke after they had done a valuation report for a potential buyer. The surveyors report identified that bracing members had been removed from roof trusses, but when I visited it was the internal W chords that had been removed by the owner doing DIY loft conversion, and to his pleasure he mentioned to me that the timbers he had removed he had used to construct worktop & cupboards alongside the eaves within the loft. My immediate reaction was for me to suggest to the owner that we shouldn't be standing on the loft floor!

(No subject)

Submitted 2 months ago

That's why loft conversions should only be done by competent builders, what an idiot.

Webmaster note

Submitted 2 months ago

All comments posted at an earlier date to this one have been transferred from our old website.

Roof trusses

Submitted 1 month ago

Hi,
We are buying a house that has had an extension on the side of the property the existing roof is made of timber so we have considered converting that to rooms. However the extended side is constructed of trusses. My question is can I use this for storage? And if so can I board it out? And how much weight would it take? Can you walk on there?
Sorry if I sound stupid lol but I have only been in old houses where the roofs are timber.

Reply

Submitted 1 month ago

Hello - thanks for your comment.

Truss roofs should not be used for storage unless originally designed to do so. The seller may have the original details or calculations for the roof structure. The trusses will be O.K. to walk on but please do so with care as the timber is of small section and often difficult to access. Light items could be placed but boarding out is not recommended.

Regards,
John, LABC

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