Ice Dams: Causes, Prevention and Snow Load

Ice dams are a common winter roofing problem in cold climates. They form at the eave edge of a roof when the conditions are right, and they can cause serious interior water damage even when the outdoor temperature is well below freezing.

How ice dams form

Heat escaping through the roof warms the roof deck above the insulated living space. Snow on that warm section melts and flows down the slope. When it reaches the cold eave overhang (which hangs over unconditioned space and stays below freezing), it refreezes. Over successive melt-freeze cycles, the ice builds up into a dam. Water behind the dam backs up under shingles and leaks into the attic and ceiling.

The root cause is heat loss, not just snow

Ice dams are primarily an insulation and air-sealing problem, not a roofing problem. Any warm interior air that leaks into the attic through electrical boxes, light fixtures, plumbing penetrations or attic hatches heats the roof deck unevenly and creates melting even when ambient temperatures are very cold.

Prevention: seal and insulate

The most effective prevention is to air-seal all attic bypasses and add enough insulation to keep the entire roof deck cold, so no melting occurs. Most building codes recommend a minimum R-value for attic floors in cold climates; check your local energy code for the requirement in your climate zone. A cold, uniformly cold roof prevents the uneven melting that creates ice.

Ice-and-water barrier at the eave

Most codes require a self-adhering ice-and-water barrier membrane at the eave, typically extending a minimum distance up the roof past the interior wall line. This membrane is a second line of defence if an ice dam does form: it prevents the backed-up water from penetrating the roof deck.

Relationship to snow load

Ice adds significant weight to a roof eave, especially when a large dam builds up over a long winter. A thick ice dam across the full eave can weigh several hundred pounds per linear foot, a concentrated load on the structure directly above the exterior wall. In severe winters, this concentrated load adds to the design snow load and should be considered when assessing overall roof safety.

Short-term remedies

Roof rakes remove snow before it can melt and refreeze. Calcium chloride ice-melt cables along the eave create a drainage channel. Neither is a permanent fix; the permanent fix is insulation and air sealing.