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Q: We just love our house, which we designed and built in 1993, but we have a problem with ice dams. We've checked attic insulation levels, installed continuous ridge and soffit vents, and even paid an expert $500 to find heat leaks using infrared thermography. We plugged every leak he found, and still last winter we had more ice dams. My poor husband is tearing out what little is left of his hair, so I hope you can help!

— Liz, Sauk Rapids, MN

A: Tom Silva replies: You're on the right track. Ice dams are caused by air from the house warming the underside of a snow-covered roof in freezing weather. As the snow above the warmed area melts, water trickles down to the eaves, which are typically colder than the rest of the roof. If the water turns to ice on the eaves and builds up enough to form an ice dam, the meltwater coming down the roof pools up behind it and gets under the shingles and into your house.

The best way to stop ice damming is to keep the entire roof as cold as the eaves. Ventilating the underside of the roof, insulating the attic floor, and plugging air leaks are all worthwhile steps to take.

But completely sealing off the attic from heat leaks isn't that easy to do, as you discovered. The only fail-safe, works-every-time solution I've found to problems like yours is to force the cold attic air to circulate using a fan or two. Whenever I aim a fan at the area nearest the ice dam, leaking stops within an hour because heat is no longer warming the underside of the roof. For a temporary fix, all you need is a standard fan. But the best long-term solution calls for a thermostatically controlled fan that kicks on whenever the temperature at the eaves falls below freezing.

This trick works only in attics with some way of letting cold air in, at the gable, the eaves, or the roof ridge. Otherwise, the fan ends up pushing warm air around.
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