Grace Ice & Water Shield on entire low-slope roof
Hi. I’m Lizzie.
My husband and I are gut renovating a house in New England. Most recently we learned we need to completely replace our roof, including decking, headers, joists due to years of water damage & rot.
We have a low-slope, 2/12 roof. We previously used closed cell spray foam to insulate the underside of the decking. We plan to use asphalt shingles (GAF; their site indicates they can be used on a 2/12…). Our roofing contractor has suggested we use Grace Ice & Water shield on the entirety of our roof under the shingles. A structural engineering/architect friend told us that this is a bad idea since our closed cell spray foam acts as an air and vapor barrier. He said that if the ice & water shield has any leaks (and that eventually it will), moisture that enters won’t have a way out and we risk rotting decking again.
I’m inclined to agree with our friend and I think this would be a no brainer if we had a steeper pitched roof. But I do worry about wind/rain getting under the shingles with such a low pitch. Any guidance and insight is much appreciated! Thank you.
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> He said that if the ice & water shield has any leaks (and that eventually it will), moisture that enters won’t have a way out and we risk rotting decking again.
While their heart is in the right place, the risk mitigated by your roofer's suggestion outweighs (in my opinion) the incremental risk your friend is highlighting.
Two things:
(1) Many synthetic roofing underlayments used today are also impermeable (meaning they would inhibit drying, similar to Ice & Water shield.
(2) Asphalt shingles are also impermeable.
This article will probably help: https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers
If it were my 2/12 roof, I would be putting Ice and Water on the whole thing.
Thank you for your response and the link!
I agree with Patrick. I put a full layer of I&W under all shingle roofs, regardless of the slope.
Keep in mind that the shingle installation requirements will be different for low slope application than for a steeper slope.
Thank you!
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If you want to increase the assembly's resilience, and make your engineer happy, you could install a vapor-permeable WRB over the existing sheathing, install 2x sleepers and a new layer of sheathing to create a vent space, then install full I+WS and shingles. But it's probably overkill.
Be aware that installation instructions for shingles on a 2:12 slope are usually different than standard installation instructions, and your roofer may or may not follow those instructions.
In any case, full I+WS is a good idea for a situation like yours. Even better would be a different type of roofing, such as EPDM, made for low-slope roofs.
Appreciate your insight. I'll chat with the roofer re: shingle installation. I downloaded the installation guide from the manufacturer.
LizzieR,
Be aware that although you can use shingles at a 2/12 pitch, they are at the very edge of their viability - meaning that they will not have as long a useful life, and are much more prone to leaking than they would otherwise be. That's probably why you are having to do so much work on the roof now. I&WS is the least protection you should add. My own preference would be to use something other than asphalt shingles.
Thank you
Lizzie, I downloaded their installation guide as well, as I was curious what they specifically recommend for a 2:12 pitch. It simply referred to the instructions on the inside of the shingle packaging. Not very helpful when you're in the design stage. Maybe they have other information that I didn't find.