image from Wikipedia
Ice dams occur when a snow-covered roof over the attic is warmer than the eave -- the overhang of a roof. If the roof is warm, it causes the snow to melt and run under snow along the roof. When the melted snow hits the cooler eaves, it freezes. As ice builds up on eaves, it eventually traps water behind it. The water backs up under shingles and finds its way through seams in the building paper and roof decking to enter the attic and living area. Wallpaper, plaster, and paint surfaces may be damaged in the process. Fortunately, there are several ways to remedy the problem.
If you have an immediate problem with ice dams, you should remove the snow from the part of the roof directly above the ice dam. This limits the amount of water that can collect behind the dam.
A cooler attic area will help decrease problems with ice dams. Here are two ways to keep attics cool: