Short Answer
An ice dam forms when snow melts on a warmer portion of the roof and refreezes near colder roof edges.
Detailed Explanation
An ice dam is a ridge of ice that forms at the edge of a roof, preventing melting snow from draining off the roof. The process: heat from the attic warms the roof deck, melting snow on the upper portion of the roof. The melted water runs down the roof toward the eave, which is colder (because it extends beyond the exterior wall and isn't warmed by the attic). At the cold eave, the water refreezes, forming a dam of ice. As more snow melts and runs down, it pools behind the ice dam and can back up under the shingles, entering the roof deck and the home. Ice dams are primarily caused by a warm roof deck — which is caused by inadequate insulation, air leakage from the living space, or inadequate ventilation.
Why It Matters
Ice dams are a common cause of winter roof leaks in cold climates. Understanding how they form helps homeowners prevent them through insulation, air sealing, and ventilation.