Ice dams occur when snow buildup melts and refreezes.  During this melting/freezing cycle, melted water works its way under the shingles, eventually entering the house.  Often the problem occurs  because of a warm attic.  The roof above the attic warms enough to melt the snow, but the roof edge remains below freezing.  When the melted water runs down the roof, it hits the cold edge, creating a rim of ice.  As the rim traps more water behind it, the ice dam grows.  Ice dam results include wrecked shingles, rotted wood, wet insulation and water damage to interior walls and ceilings.

The way to stop ice dams from forming is to keep the entire roof cold.  In most homes this means blocking all air leaks leading to the attic from the living space below, increasing the thickness of insulation on the attic floor, and installing a continuous soffit and ridge vent system. Be sure that the air and insulation barrier you create is continuous.

TIPS:
1.  Close up air leaks caused by gaps in drywall and cracks around light fixtures.  This improves attic insulation by stopping heat from entering the attic through the ceiling.
2.  Unplug/replace blocked soffitt vents.  This improves attic ventilation by allowing any warm air to quickly escape outside.
3.  Clean gutters and downspouts so melted snow can drain easily.
4.  Remove snow before it melts with a roof rake or similar tool. (This will only be effective if you rake all the way to the roof, removing all potential snowmelt)