Retrofitting a Slate Roof
We have a 1900 farmhouse in Zone 5 NW Massachusetts with a basic 13:12 gable roof with two very large dormers, one on each side. Our plan to replace the roof was: remove deteriorating slate, put Grace Ice and Water completely over original board sheathing as air barrier, run 2x4s horizontally on the flat screwed through to rafters with 1.5″ polyiso between them, then a layer of 2.5″ polyiso, a layer of 3″ polyiso, 5/8″ plywood sheathing screwed through polyiso to 2x4s, underlayment, and finally concealed fastener metal roof.
When everything was done except the metal roof installation, we found water leaking inside the attic ceiling and discovered for some inexplicable reason the roofers had built a knee wall on top of the main roof ridge and dormer ridges connecting the original peak to the new sheathing peak, which was filling up with water and then somehow getting through the Grace into the attic. They removed that knee wall and tried buttoning up the main ridge, but another rain found water still coming through, probably where the dormer ridge connects to the main roof.
My general contractor has stopped work with the roofers and is trying to figure out what to do next to guarantee we have both an air tight ceiling in the attic (which is going to be finished) and a water tight roof. One idea is to spray an inch of closed cell foam on the attic ceiling for air barrier (followed by the original plan of fiberglass batts in the rafter bays) and put a layer of Grace over entire outer sheathing. I’m worried about moisture already being trapped between the bottom layer of Grace and the outer sheathing so this could trap it permanently.
Any other ideas besides having them tear everything off and start over?
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