Quote:
Originally Posted by songkrai
The best is to use the Grace membrane after stripping all roof materials.
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Doing that isn't necessarily best. It places a vapor barrier above the roof deck, preventing drying in that direction. Drying won't be a problem if the underside of the deck is vented, as with an unconditioned attic space or a cathedral ceiling with insulation up against insulation baffles that provide vent space. If the structure underneath is an insulated, unvented cathedral ceiling, there is no way for any incidental moisture that gets into the roof deck to dry out. However, code does call for a membrane to be applied over the bottom edges, up to a point two feet past where the wall is underneath.
More info in building unvented cathedral ceilings is here:
http://www.greenbuildingadvisor.com/...hedral-ceiling