Ice dams are the accumulation of ice at the eaves and valleys of roofs, and in some winters create major problems. Insufficient attic insulation and/or attic ventilation can cause roof snow to melt and slide down to the eaves, where the roof surface is cooler and the melting snow freezes to the roof. Subsequent melting causes water to pool behind this dam and sometimes the water seeps between the shingle layers, wetting the roof deck and possibly the walls and ceilings below.
What are some possible solutions? See Solutions used for Ice Damming.