Ice Dams sometimes occur on sloping roofs in climates with freezing temperatures. When the temperature in your attic is above freezing, it causes snow on the roof to melt and run down the sloping roof. When the snow melt runs down the roof and hits the colder eaves, it refreezes.
If this cycle repeats over several days, the freezing snowmelt builds up and forms a dam of ice, behind which water ponds. The ponding water can back up under the roof covering and leak into the attic or along exterior walls.
The right weather conditions for ice dams is usually when outside air temperatures are in the low 20's for several days with several inches of snow on the roof.
Research shows keeping the attic temperature below freezing when the outside air temperature is in the low 20's can reduce the occurence of ice dams. Research has show sun exposure in the winter has little effect on attic air temperature. Warm air from living spaces below penetrating into the attic is usually the culprit in the formation of ice dams.
Utilizing a good roof rake and removing the lower 1-2 feet of snow helps in the avoidance of ice dams as well. Once removed, the roof is exposed to the sun and the additional snow on the roof has a chance to melt and fall forward.
