There have been many calls this winter about ice damming so I wanted to take a minute to explain the process. For ice dams to form an accumulation of at least a few inches of snow must be on your roof. The heat from your home rises up through the attic and melts the bottom layer of snow on your roof. As the melted snow (water) travels down the surface of the shingles and reaches the overhangs and gutters, the water is no longer warmed by the air in the attic, so the water begins to re-freeze. When we have a large drop it temperature, like a couple weeks ago (1 degree), the water refreezes almost immediately in the gutter. As the ice builds higher and higher above your gutter line, it creates a dam. The water can no longer go over the gutter, so it starts backing up under your shingles. When water gets under your shingles, it can easily get through your attic and cause water stains and drywall damage to your ceilings.
Two important things to think about in preventing ice damming are insulation and ventilation.
A properly insulated attic lets very little heat into your attic space. If the attic is lacking in insulation, the heat rises up through the ceiling and quickly melts the bottom layer of snow. The melting snow is the first step in forming ice dams, so adding insulation to your attic will slow the process.
If you do not have enough ventilation in your attic, heat will be captured in your attic space. If the heat cannot quickly escape the space, it starts melting the bottom layer of snow which is the first step in ice dams forming.
You can reduce your chances of ice damming by lessening the heat that gets into your attic and quickly venting the heat out of your attic.
Lastly, when it is time to replace your roof, have the contractor install an Ice and Water Barrier along the gutter lines and in any potential ice damming area. It is a material that is rolled out along the gutters and then shingled over. It forms a leak barrier so if there is water pushing under your shingles, it cannot make it into your attic space.