HOMEThe Ice Dam Mistake Almost Everyone Makes7 min read

What an Ice Dam Actually Is

An ice dam is a ridge of ice that builds up along the lower edge of a roof, and it forms in a way most homeowners never expect. When snow sits on a roof, the section above the heated living space warms above 32 degrees Fahrenheit — just enough to melt the snow. That meltwater runs down toward the eaves, which remain cold because no heat rises beneath them. There, the water refreezes and forms a growing rim of ice. As more snow melts and runs down, the rim traps it, creating a pool of standing water on the roof. The ice dam itself is not the real problem. The trapped water behind it is.

Why Your Attic Is the Real Culprit

Most people assume ice dams are a roofing problem. They are actually an attic problem. In a typical home, roughly one-third of all heat loss escapes upward through the ceiling and into the attic space. That warm air heats the wood decking and shingles from below, raising the roof surface temperature above freezing even when outdoor temperatures are well below it. The rest of the roof — particularly the overhanging eaves beyond the attic — stays cold. That temperature difference between the middle of the roof and its edge is precisely what creates the conditions for an ice dam to form. Fix the attic, and the roof surface stays uniformly cold.