Is it ok to not create a structural deck immediately above the ceiling joists when creating a warm deck roof? Instead placing the 100mm rigid board insulation directly onto the ceiling joists and then put the OSB / structural deck on top?
That's how it should be done - ceiling joists, firrings, 18mm OSB, vapour barrier, 120mm celotex, 18mm OSB, roof covering.
Edit - sorry, just reread.... I suppose this is OK providing the insulation is supported enough - I can just see you putting your foot straight through the insulation as you try and lay the top deck as you won't know where your joists are!
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