Building above rsj and eaves roof line

3 Oct 2017
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United Kingdom
Hi everyone,

I’m close to finishing the roof of my ground floor extension. It’s a mono pitched roof.
Across the back of the extension I’ll be installing long bi fold doors.
Steel is already in which has an 8mm thick x 280mm base plate welded to it which sits the width of the cavity wall above bi folds (100mm inner leaf, 100mm cavity, 100mm outer leaf)
With the roof being low angle (23 deg) the wall plate sits right on top of the steel.
I’m putting 100mm insulation between joists and have notched it to sit over wall plate into the cavity space.
Question is do I need to build up external block work (outer leaf) in between joist ends? They’ll be boxed in eventually by fascia and soffit. If I did I’d only get prob block and a half before I reach to roof underfelt and I’d need to leave at least 50mm for ventilation. Alternate is to just push in some rock wool.
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