7m span for flat roof joists?

[Tom Marshall]

Hello,

In the process of extending our bungalow, and have been advised by our builder that you can use joists that will span 7m to suitably hold a flat roof with two lanterns in. I am aware that they will be large, potentially 9x2"

Has anybody seen a span of this size without an rsj supporting it?

Thanks.

 

[tony1851]

You won't span 7m with ordinary joists - steel would be required, particularly for trimming around the lightwell.
On the other hand, you can consider glulam beams or posi-joists or similar, but they will be £££££££££££s.

 

[John D v2.0]

Span tables only allow 6.58m Max for the lightest load on a 3 by 9 joist with the closed sourcing, so as above you'll either need steel or sky hooks.
You would be able to use those trussed joists with the gaps in the middle but your roof would get very thick especially as a warm roof.
How long is the other direction and could you half the span with a cross beam? For our loft, the SE specified huge timbers bolted together in pairs to avoid having to install steel. Apparently it's a lot safer to lug wood than steel up to any height.