warm roof - deck gap by walls

21 Oct 2014
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West Midlands
United Kingdom
About to start my warm flat roof deck tomorrow - OSB3, VCL, 120mm insulation, OSB3, GRP.

On watching some youtube vids, they suggest a 25mm gap on the deck by walls for expansion, whicjh is covered up by the angled wall trim. But as these are instructional vids, they are just using OSB straight onto mock up joists.

So my question is this. Clearly I'll leave 25mm gap around the edges of the top deck, but what about the bottom deck and insulation? I can't see leaving a gap around the edge of the insulation being a good idea.... bottom deck?
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I don't leave gaps for the insulation and I think half an inch
is plenty for an expansion gap.

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