More Q&A Spotlight
In most places in North America, spring has arrived, and for a while, winter is a thing of the past. It’s a great time to think about the improvements that can be made before next winter shows up. To that end, “montlamothe” posted a question about the design of his new home in southern Washington state. The plans are for a one-story house with a low-sloped roof supported by exposed glulam beams, with 3-ft. to -4-ft.-deep overhangs. The unvented roof will be insulated with 9 in. of polyiso rigid foam (R-60) and covered with a standing-seam metal roof. Because the location has an annual snow load that averages 134 psf (about 107-in. deep), montlamothe is worried about ice dams, and wonders if he should follow Dr. Joe Lstiburek’s advice to vent roofs in high-snow areas.
Low slope, big snow, needs venting
Ice dams are a potential source for roof leaks during the winter. Heat radiating up from the interior can melt snow on the roof. The snow melt drains down towards the eaves, where it can freeze, forming a dam and blocking further drainage. Any additional snow melt will get backed up, often leading to leaks in the roof.
Reader responses are unanimous in their support for a vented roof. Akos, writes, “Snow is an excellent insulator. With that much snow sitting on your low-slope roof, no matter the R-value, an unvented roof will have ice dam issues.” He questions the need for R-60 insulation, and cautions that a tongue-and-groove ceiling will need to be air-sealed beyond the plane of the walls to prevent ice dams.
Akos, DennisWood, and benwolk discuss the details of layering the roof deck to be airtight and insulated from below; it will also provide ventilation from eave to ridge. Akos mentions a product…
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3 Comments
I am thinking of using basically this exact assembly on a roof in Colorado Climate Zone 7. I'm curious how you would run electrical for lighting? Thanks!
One would want to run it in conduit on the underside of the T&G. I have seen folks route it through the beams, but this can be time consuming and one would need to check with one's structural engineer if going this route.
If you are in CZ 7 in Colorado, I would certainly suggest designing the building with a higher roof pitch. This will mitigate a number of issues the reader is seeing with a low slope roof in snow country.
Dear Chuck: First, thanks for this excellent article which covered key details. I’m sure this will be helpful for many GBA readers.
One caveat: Sunlight can lead to melting snow on the roof surface, whether it is metal or shingles, and particularly if the surface is darker in color. Ironically, this can lead to ice dams further down on a highly insulated roof, with or without venting. In some conditions venting will increase the opportunity for melt water to freeze - forming an ice dam. And the ice dam (and water leaks) can form at a variety of points under the snow - because the entire roof should be cold. Not just along the lower edge, which is typical with poorly insulated roofs.
I have noticed this omission in a series of articles about insulate roofs and ice dams, so you are in good company,.
The ONLY solution is a fully sealed surface under your roofing. We ran into this problem with our roof in Massachusetts after it was fully insulated. So I installed Grace Ice & Water Shield on the entire roof surface, before new shingles were installed. Products like this are not expensive, so it is cheap insurance against costly water leaks at any point on the roof. With metal roofs, this also prevents water leaks from poorly sealed seams, etc. - at any point on the roof.
As a result of the success with our previous home in Massachusetts, I had the same sealant installed on our new beach house in Massachusetts. Plus this creates a very effective air seal for the entire roof surface.
I hope this is helpful,
Mark
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