Roof Lantern Structure

[Judith Birkett]

Hello

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

 

[domdee]

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!

 

[freddiemercurystwin]

No problem.

 

[Londoner2]

No way.

Joists would be ok but they will deflect over time leading to sagging due to weight, steal beams x 2 needed.

 

[frutbunn]

Can't speak for every BCO but I would accept it, although I'd personally use 3no trimmers on the longer span.

 

[freddiemercurystwin]

How could you refuse an SE's spec and calc's assuming the calc's were done properly?

 

[frutbunn]

How could you refuse an SE's spec and calc's assuming the calc's were done properly?

That's assuming it has some.

 

[freddiemercurystwin]

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!

 

[tony1851]

That's exactly how I work........

 

[tony1851]

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.