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Hi Rafael
A lot of builders that do flat roofs use polyisocyanurate, but that is subject to a reduced performance in cold weather, so some builders have begun including a final layer of rockwool on top of the polyiso to keep it warmer so it performs as best as possible. As for how much to install, we have come to realize that for the most part, regional building codes are about half what we ourselves would do. So our recommendation would be to find out what is specified in the regional Building Code (which is a bare minimum) and probably double it.