Ice dams form when melting snow on a roof runs off and refreezes at the edge of a roof. This condition occurs when the snow is melted by a warm roof, creating water running between the snow and the warm roof surface, then freezing and turning to ice when it gets past the exterior wall and hits a cold unheated roof edge or gutter. As the bottom of the snow pack continues to melt, water continues to flow down the roof surface until it hits the ice, thereby creating a larger and larger ice dam.
If this situation continues, the ice can work its way back up the roof edge, get under shingles, melt and leak into the exterior wall, home or attic. Damage from ice dams may not be readily apparent. As the ice melts and possibly drips into the wall or attic, insulation can be become wet and lose its ability to perform. In some cases if the right temperature and humidity exist, mold may begin to grow in the attic. Often paint will peel or blister weeks or months after the ice dam has melted as moisture from the leak in the wall or ceiling cavities tries to leave and pushes outward.
SOLUTION: The most common long term solution is to reduce or eliminate any sources of heat in the attic and ventilate the attic space of the roof. The underside of the roof deck must be close to the temperature of the exterior side of the roof. Ideally ventilation should be installed using a continuous soffit-and-ridge vent system with baffles at the lower side of the roof. Provide at least a 2-inch space between insulation and sheathing. By providing adequate ventilation the temperature of the attic will be lowered thereby lowering the underside roof deck temperature.