Ice dams form when snow continually melts at the roof edge. When snow accumulates on a roof, the heat in the attic will cause it to thaw and the resulting water will run down to the eaves where there is no heat and it will refreeze. This can occur on a daily basis until large icicle form at the eaves. If no protection was installed when the roof was put on, the ice can eventually back up under the shingles and cause leaks. It can also get bad enough to rip the gutters right off of the building. Ice dams can allow moisture to damage attic insulation which reduces the R-value of the insulation and raises the energy bills, they allow water to penetrate wall cavities which can cause paint and plaster to peel and also rust nails, electrical boxes, or any other non-rustproof metal building material located in walls.
There are three good ways to help prevent ice dams or the damage caused by ice dams. Proper ventilation will help maintain the ambient air temperature at the roof level thereby not allowing the snow located on the roof above the living areas of the home to thaw. Heavy attic insulation will help insure that very little heat gets into the attic. The installation of an ice and water protection membrane to the eaves and valleys of the roof which will help prevent damage, but doesn’t treat the root of the problem, which is heat loss. Heat tapes are often used as a solution but rarely prove effective.