As the temperature dips below freezing, we are given a yearly reminder of the damage caused by ice dams. Left unchecked, ice cams can damage shingles and gutters, and lead to leaks causing damage to interior walls and ceilings, mold, and other serious problems.
Ice dams are caused by heat loss through your ceiling into your attic. When an attic air is warmer than the freezing outside air, heat is transferred through your roof deck and shingles, melting the snow on the roof. As the water from the melted snow flows down the slope of the roof, it eventually reaches the colder eaves (as they are not receiving the heat from the warmer attic air), and the water re-freezes. The ice continues to build up, damming the water on the roof, causing leaks.
Fortunately, ice dams are relatively easy to prevent by improving on one, or all, of three areas.
Ventilation: proper ventilation will help exhaust any warm air trapped in your attic, pulling cool air from soffit vents. Proper ventilation also vents unwanted moisture year-round, preventing any mold build-up.
Attic air leaks: gaps between the interior and the attic space near fixtures, chimneys, fans, attic hatches, wiring holes, etc. All of these should be sealed, keeping warm air in the interior.
Insulation: the ceiling is the most important area to insulate, as 45% of heat loss on an un-insulated house is through the ceiling. A level of at least R-38 is recommended, and easy to achieve over a lower level of existing batt or blown-in insulation. Additionally, the energy savings will be immediately felt, both in comfort and savings on energy bills.
Obviously, the best solution is to properly ventilate, seal, and insulate an attic so that the air in the attic matches the temperature of the air outside. If you find yourself in a bind, salt applied directly to the ice dam will cause it to melt, allowing the water to flow to the gutters. Read the following link to a blogger in Minnesota battling ice dams on his home: