Ask the Carpenter: Why metal roofs are not a cure-all for ice dams
Ask the Carpenter’s Rob Robillard weighs in on the havoc that is winter. Get more home improvement advice at realestate.boston.com.
Q. We recently built a home in Michigan. This is our first winter in the home, and we noticed within the first few days of a snowstorm that our metal roof developed what looked like ice dams — icicles and then a buildup of snow over the gutters. We are hoping you can give us some insight. We want to be sure we understand what’s going on in case the builder tries to tell us everything is fine. We had ice damming at our old home on a four-season porch addition that was not properly insulated or ventilated. It had a shingled roof. This is our first time having a metal roof, but it is only on the front porch.
FRAN
A. Metal roofing can help, but it is not an ice dam panacea. A metal roof will generally shed snow better, but snowmelt running down it will still freeze on the unheated overhangs, creating an ice dam. Metal roofs are just as susceptible to this as any other roof.
Ice dams are not a roofing material problem; they are a heat loss and insulation issue. Proper ventilation will help mitigate the heat loss but will not cure what is the substantive source of the problem. Air-sealing and insulation combined with the proper attic ventilation will.
Q. In the winter, the outside flapper to our bathroom exhaust freezes shut after someone takes a shower. How do we keep if from doing this? I believe this has contributed to a condensation problem in our attic.
Bill H., Bridgewater
A. To prevent the flapper from freezing, install a high-powered fan (160 cubic feet per minute) and a smooth, metal, 4-inch-in-diameter duct with the shortest run possible. Wrap any duct in unconditioned spaces with pipe wrap insulation. Install a metal termination wall cap instead of a plastic one. Metal is highly conductive and warms up faster when the fan is running.
Additionally, install a fan timer switch to run your fan longer after showering. I set mine for 30 minutes. Also check the damper at the fan box to ensure it is working. If it stays open, moist air from the bathroom can leak into the duct, where it will condense and freeze when the fan isn’t running.
Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to [email protected] or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.
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