How Do Ice Dams Form :Ice dams form when a roof’s surface is unevenly heated. When conditions are right to create an ice dam, the upper part of the roof is warmed from the attic below, which causes snow to melt. This water then runs down to the roof edge—where the surface is cooler—and refreezes.
This creates the block of ice that prevents melting snow from draining to the gutter system. As runoff from melting snow continues to run down the roof, it can back up along the roof edge, causing further problems.