Roof Lantern Structure

26 Dec 2017
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United Kingdom

I am looking to have a flat roof extension.

I wanted to install a roof lantern.

I have been told that I can just double up or triple up the joists around the roof lantern.

I was looking at two potential roof lantern sizes.

Option 1 – 2000mm by 27000mm

Option 2 – 1200mm by 2700mm

The joists would span 4.3 m unsupported. And are made up of

47 x 195mm grade C24 joists at 400 ctrs.

Someone told me that I would need steels for larger roof lanterns. I was not sure if the above would be considered large and therefore require structural steels.

Any views on this. Can I just increase the thickness of the joists if needs be to provide stronger support?

please see attached plans


  • Roof Lanterns (3).jpg
    Roof Lanterns (3).jpg
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I'm no expert but would have thought doubling/tripling up the timbers would be more than adequate. 2x8" are beefy lumps alone so 6x8" would be pretty substantial!
No way.

Joists would be ok but they will deflect over time leading to sagging due to weight, steal beams x 2 needed.
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Can't speak for every BCO but I would accept it, although I'd personally use 3no trimmers on the longer span.
Well I submitted them to BC as part of the Regs plan approval, I didn't even open them mind so could have been anything in there! :)
For roof lanterns that size, you would be better advised having a couple of steel trimmers. As advised above, timber under load is subject to long-term deformation known as 'creep'.
This can sometimes cause ponding on a flat roof, particularly if the fall is not too great to start with.
The steel beams need not be particularly big - probably not as deep as the timber joists.

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