Underlayment for metal roof over unvented attic
We are in Central New York State, remodeling and adding on to a 1977 ranch with 4/12 hipped roof. We will have 5.5″ of closed-cell foam sprayed inside the attic, under the roof deck (need conditioned space for heat pump ceiling cassettes & branch box). We were going to install a metal roof over the existing shingle roof, but have decided to tear-off existing roof because of some sheathing (plywood) rot due to leaks & the poor condition of the shingles. Our contractor wants to install 2 rows of ice & water then a synthetic felt (Raptor). We will be installing Retro-ease in place of wood furring strips to provide an airspace between the metal roof and the roof deck. All my reading indicates that we need an air-permeable roof underlayment because the closed-cell foam will not allow moisture to escape from the plywood. Should we just use felt paper? If so, 15# or 30#? Will the air-space under the metal roof help the plywood dry if water penetrates the roof and underlayment? We are already dealing with snow & rain cycles here… Friday is supposed to be in the mid-50s (it 18 with 3″ of snow on the ground on Wednesday night). We’re going to try to get the roof torn off & the ice & water barrier and underlayment installed on Friday so the ice & water can adhere.
Any thought/comments/advice would be appreciated. I ended up managing this construction project because our selected GC wasn’t available until mid-Sept. Unfortunately, he’s still on the job before ours. We have had one of his carpenters for the last 5 weeks. He and I will be doing the work on Friday…
Replies
If your roof does not receive a lot of direct sunlight and stays somewhat cool standard roofing felt is fine. If exposed to a lot of sun I like to use a breathable, high temp synthetic underlayment. Extreme heat can deteriorate standard felt quickly. I’ve actually seen it melt to the underside of metal roofing before, not good. My region can see extreme sun and heat in summers, so can only speak to my environment. The high temp synthetic has worked well for me here.
Thank you for your response. Half of the roof is sunny, half mostly shady :) I purchased Certainteed Winterguard HT (High Temp). It's what our supplier recommended under metal roofs. If all goes well, we'll have it installed tomorrow. Thanks again!
you need to check your metal roof manufacturer's instructions. Mine, (Union Corrugating) required hi-temp ice/water shield on the whole surface, and over that a synthetic underlayment (which acted as a "slip sheet" so when the metal expands/contracts, that movement is against the synthetic underlayment and not against the ice/water shield). If I didn't do that exactly as spec'd, no warranty. My assembly is a vented shed roof, but it would have been the same had I an unvented assembly.
Thanks for your response. I checked with the metal roof manufacturer (Central States), they did not have a preference. Our local building supply recommended a high temp (Certainteed WinterGuard HT) for under the metal roof, so that's what I purchased. It also can be installed in cooler temps. Our high should be in the mid-50s tomorrow & I have it stored at a neighbor's house (70 degrees). Our house isn't dried in so we don't have heat.
We are installing 2 rows of the ice and water barrier with a synthetic underlayment (Raptor) over the entire roof.
Thanks again for your input.