What Causes Ice Dams?

Considering all the potential problems that ice dams can cause, it’s surprising home-owners know so little about what causes them.
In fact, it doesn’t take much. Only three conditions must be present:
1. A Heavy Snowfall — enough to to leave several inches of snow on a roof. The more snow, the greater the potential an ice dam may form.
2. Continuously cold temperatures — A heavy snow followed by several days of 40 degree temperatures probably won’t result in an ice dam. The air temperature must remain cold enough for water to freeze. When temperatures fall below 20 degrees, conditions are especially favorable.
3. An under ventilated and poorly insulated attic, factors that create what amounts to a “hot and cold roof.”

When these conditions are in place, here’s what happens:

Heat escapes from the living quarters into the attic. The colder the outside temperature the higher the thermostat is set. The heat builds up at the upper levels of the attic, eventually warming the roof deck. Once the deck is warm, snow on the roof begins to melt. Obviously, if the sun breaks out following a snow storm, melting is accelerated.
Water runs down the roof until it reaches the area over the eaves. Since this area of the roof remains cold, the runoff from the melting snow begins to freeze and the ice dam forms (along with a more easily seen symptom of the problem, icicles hanging from gutters).
As the dam builds, it begins to trap more snow melt, extending the height of the dam. But the real problems begin when water begins to pool, backing up under the shingles. Once that happens, damage can be extensive. Damage to drywall, insulation, ceilings, electric, and paint are not uncommon. Ice Guard roofing membrane used in re roofing can help but not totaly prevent water penetration.

The best solution is prevention. Be sure your attic is heavily insulated and your attic is adequately ventilated, with both intake and exhaust, to help diminish the chances of damage from ice dams. Attic insulation and attic ventilation are your best weapons in the fight against ice dams.