What causes ice dams?

Ice dams form when convective heat is present in an attic and melts snow on the roof. This melted snow (water) runs down the roof, under the blanket of snow, to the edge of the roof where colder conditions cause it to freeze, forming ice dams. If ice dams are not removed, water may be caught behind them, forced under shingles or spill over to form icicles. This can result in costly damage to your home: wet (ineffective) insulation; stained or cracked plaster or sheet rock, damp, rotting walls, and stained, blistered or peeling paint.