Ice Dam

An ice dam is a ridge of ice that builds up along the lower edge of a sloped roof in winter. Water from melting snow higher up runs down to the cold eaves, refreezes, and forms a barrier. Further meltwater pools behind the dam and can work its way up under shingles or tiles and into the building, damaging ceilings, walls and insulation.

How ice dams form

Three conditions must occur together:

1. Outdoor air well below freezing.
2. A layer of snow on the roof, which insulates the roof surface.
3. Heat reaching the underside of the roof deck, usually from the building below.

The upper roof, warmed from beneath, stays above freezing and melts the bottom of the snow. The eaves, which overhang the exterior walls and receive no building heat, stay below freezing. Ice dams are associated with conditions where the roof deck under the snow is above about -1 °C (30 °F) while outdoor air is below about -6 °C (22 °F).

Causes inside the building

Building science research identifies air leakage as the main way heat reaches the roof deck: warm air escaping through ceiling penetrations, leaky ductwork in attics, recessed lights and gaps in the ceiling air barrier. Insufficient insulation also contributes.

Prevention

• Air sealing the ceiling plane and attic penetrations, usually the most effective step.
• Adequate insulation to keep heat in the living space.
• Attic ventilation to keep the roof deck cold, creating a so-called cold roof.
• Roof design: steep, smooth roofs with few valleys shed snow before it can melt and refreeze.
• Eave protection: a self-adhered membrane (often called ice and water barrier) at the eaves is required by US building codes in cold regions. It reduces leaks if a dam forms but does not prevent the dam.
• Metal eave bands provide a slippery surface that may reduce dam formation.

Heat cables and removal

Electric heat cables melt channels through the ice to let water drain, but they use energy, may damage some roofing over time, can be a fire hazard and are disallowed by some insurers. Ice may still form just above the cable. Chipping ice with tools often damages the roof; steam removal by professionals is a common alternative.

Where they occur

Ice dams are common in Canada, the northern United States, Scandinavia, the Alps, northern Japan and other snowy regions, particularly on older homes with poorly sealed attics.