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One dormer flat roof and one bay felt roof. I'd like to upgrade the insulation on both and have GRP fitted instead of felt. All I read about is warm roof being best, but adding 100mm+ Celotex to the bay will look strange I feel. Not so concerned about the dormer roof.
On the bay, can I have 50mm Celotex above the joists and extra 50mm in-between the joists? Can I do the same to the dormer window or best to do it proper.
On the dormer roof it would mean adjusting the tiles at the rear edge either way.
I know Part L demands 100mm+ above the joists, but I also read there can be exceptions.