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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This situation sounds too unique to provide general advice. Can you share photos or drawings that describe your situation clearly? If photos, some should be from far enough away to provide context.
here are some pics
Here's a diagram of the plan. It doesn't show the ridge, but the intention was for the insulation to wrap over the ridge in continuous layer for thermal break.
The peel and stick layer over the original roofing needs to be only air tight, not liquid tight. It doesn't take a lot to also make it liquid tight, so it leaking on a relatively simple roof sounds problematic. I've had roofs with nothing but peel and stick make it through major storms without a drop getting in.
It looks like the valley Grace section was not installed correctly, it should have gone under the rest of the peel and stick not over it. This creates a reverse lap and water will easily find a way in. Hard to see details on the other picture.
Since the Grace is not the main underlayment under the metal roof, it doesn't need to be liquid tight, just air tight, so as long as the final underlayment is installed properly you should not have issues.
Can the Grace be air tight without being liquid tight? I assumed if water was getting through the Grace then it's not air tight.
Not quite. A 3/8" hole in roof will leak a LOT of water but the amount of air leaking through the same hole will not be measurable with a blower door on the house.
Similar idea with a slate/tile roof. If over skip sheathing, in most attics you can clearly see daylight between them, so they definitely leak a lot of air but will not leak any water.
If in doubt about the air sealing, get a blower door test done and see if your roof is leaking. It is probably cold enough in the mornings nowadays to clearly see major air leaks with an FLIR camera.
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The part of your description that confused me at first and that I still don't understand is your description of kneewalls. Can you show on a photo or drawing where these kneewalls are?
I agree with Akos that a roof can be watertight (I might say water-resistant) without being airtight, but in this case I think it's the opposite--the Grace in your valleys looks like it was installed over the field Grace, and has wrinkles that will direct water into the house. Perhaps there is a second layer of Grace installed in the valley before the field, but if that's the case, it's not visible.
You can see in this picture at the ridge they built a structure like a short wall between the original ridge and the new ridge instead of lapping the rigid foam panels over the ridge to thermally break as planned. They said they did this so they could anchor their ropes through the ridge to the attic floor. I could see daylight through the ridge from inside the attic. I was told this was removed, sealed and filled with insulation, but when it started leaking discovered the cavity was still there, filled with water.
I see. That is an unfortunate situation. I definitely would not spray foam the interior until you are sure the leaking has been addressed. Can you install a full membrane (Grace or otherwise) over the entire roof, creating a new air control layer? If not, I'd suggest removing the new assembly near the ridge and redoing it the way you had planned.