Ice dams are large masses of ice that collect typically on the lower edge of a roof and in the gutters. As melting snow (or rain) runs down the roof, it meets this ice mass and backs up, sometimes under the shingles and then leaking into the attic.

Ice damming usually occurs when there is a significant amount of snow on the roof. The snow itself, especially when deep, can act as an insulator. If the attic temperature is above freezing, it warms the roof sheathing, which melts the layer of snow that is in direct contact with the roof. Melt water runs down the roof until it meets the overhang.

If the air and the overhang are below freezing, the water freezes on the roof and starts the ice dam. In addition, southerly exposed dark wall siding heated by the sun can cause warm convection currents to enter the eave and roof area above it, causing ice damming.