Ice dams are nasty winter problems for a roof and can cause some serious damage. They form when melting snow on a roof refreezes at the edge of a roof. The snow melts because the underside of the roof in the attic is so much more than 32 degrees Fahrenheit that it warms the outside roof surface to the point that snow melts.

You'll notice ice dams start or get worse after a heavy snow. This is because of the snow's  insulating properties. Since snow is such a good insulator (R-0.5 to R-1 per inch) the outside roof surface is able to warm up easier from the warm attic space, thereby melting the snow faster.

Warm attic spaces occur because of inadequate outside air circulation (ventilation) through the attic (soffit to roof ridge), which is necessary to keep the roof deck cold. This happens when there are not eave vents and baffles installed. Attic warming from poor ventilation is made worse with the introduction of heat from the occupied floor below the attic including sources such as lighting, air leaks, ductwork, etc.

As mentioned earlier, when roof snow is melted by a warm attic space, the water runs between the snow and the warm roof surface. The water then freezes and turns to ice when it gets past the exterior wall and hits a cold unheated roof edge or gutter. The ice dam grows as the snow pack continues to melt, and as water continues to flow down the roof surface. When the water flow hits the ice it creates a larger and larger ice dam.

Your best line of defense in preventing ice dams is to reduce or eliminate any sources of heat in the attic and ventilate the attic space of the roof.

Ice Dam Damage
If the ice dam continues to grow, 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.