Meaning that heat loss through air leaks is the big problem. What causes the snow to melt on roofs fast enough to cause ice dams is convective heat loss, not conductive heat loss (remember the three modes of heat transfer from middle school physics?). There's a lot more heat close to the roof.Īnd here's strike three: There aren't that many people in the remodeling industry who understand the difference between insulating and air-sealing. We're trying to maintain the upper floors of our houses in the mid- to high-60s. We expect second-floor bedrooms to be about as warm as the first floor kitchen and living room. Strike two is that we're heating our houses more thoroughly these days. So asphalt shingles-the most prevalent roofing material these days-are one of the least effective barriers against ice dams of any roofing material that's been used in our region. And unlike thatch, they're very thin-any water that gets through the first 1/8' is basically already on the wrong side of the shingle. Unlike cedar, asphalt shingles don't fit any tighter when they get wet they can even shrink a bit when it's cold outside. Asphalt shingles, on the other hand, have a granulated surface which snow clings to pretty tenaciously-not at all like slate. Cedar roofs are pretty robust because the wetter they get, the tighter they fit-it's harder for water to find a way through. With slate, the snow slides off before ice dams even have a chance to form. It leaks a little bit either way, but no more. Thatch, for instance, doesn't leak any more with snow and ice on the roof than it does with rainwater pouring down. Why do we still have problems with ice dams? Wouldn't you think we'd have solved this basic problem by now?įor the first 300 or so of those 400 years, we were using roofing materials that didn't have problems with ice dams. People have been building wood-frame houses with gable roofs in this neck of the woods for almost 400 years now. We'd much rather people call us before they get ice dams, because the best way to deal with them is to take measures to prevent them from occurring in the first place. By this point though damage is already done. Whenever we suffer a severe winter, our phones ring off the hook with calls from people with ice dams who are none too happy about it.

Ice dam roofs