As the name would suggest, an ice dam is a dam of ice that has formed along the lower edge of the roof line or inside the gutters. This dam prevents the water caused by the melting snow on the roof from traveling to the gutters and downspouts. When the water hits the ice dam it refreezes to form even more ice. These dams can be quite a concern to the home owner because sometimes the water backing up behind the dam can actually leak into the house damaging walls, ceilings, insulation and other areas!
Ice damming generally occurs when there is significant accumulation of snow on the roof. If the attic temp. is above freezing often due to inadequate insulation, venting or heat leakage from the house, it will warm the roof sheathing which in turn melts the snow on the shingles. When the water reaches the roof edge which is not warmed by the attic and is below freezing temperature, it will freeze, which begins the ice dam.
The next time you take a walk through your neighbourhood, take a look at the roofs. The homes with a full roof of snow are probably well insulated and properly vented. If you see a home that has vertical sections of visible roof chances are it is not vented and insulated properly allowing the heat from the house to warm the roof between each of the ceiling rafters which melts the snow creating an ice dam.
You can see the bare roof area in this picture. It seems as though there
is significant heat loss around the chimney area which has melted the snow creating the icicles.
The best way to protect your home from ice dams is to seal all attic air leaks and increase the ceiling/roof insulation to reduce the heat loss. It is recommended that the insulation should have an R-Value of at least 30.
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