The ProblemThe more snowfall, the more likely the problem. If the climate is very cold, there are less likely to be problems. Hence, ski resorts tend to be perfect areas for ice dams.
Simply put, ice damns are ridges of ice and icicles caused by meltwater from further up the roof re-freezing lower on the roof. The “dam” created by the ridge of ice along the eaves can trap further meltwater and result in significant leakage under and through the roofing, especially shingles and metal roofing. This leakage can cause damage to the sheathing, the roof structure, or the ceiling and walls below. Large icicles along the eaves can become a danger to people below if they fall.
The CausesThe fundamental causes of ice dams are well understood. If part of a roof becomes warm enough to melt snow that is lying on the roof, the snow will melt, and water will run down a sloped roof. If the water subsequently encounters a cold surface, the water will revert to ice. Therefore it can be said that the cause of ice dams is a difference in temperature on the roof surface where the upper part of the roof is warmer than the lower.
Read the full article for further causes of (as well as solutions to) ice dams, at: