Ice dam
A ridge of ice at a roof eave, formed when heat escaping into the attic melts snow that then refreezes over the colder overhang.
What is Ice dam?
Ice dams are an insulation and air-sealing problem that presents as a roofing problem. Warm air leaking into the attic — through recessed lights, bath fans, the attic hatch, top plates — melts the underside of the snowpack. The meltwater runs down to the unheated eave, refreezes, and builds a dam that backs water up under the shingles. Treating it by adding heat cable or replacing the roof does not address the cause. The durable fixes are air sealing the ceiling plane, adding insulation, and ensuring the attic is ventilated so it stays close to outdoor temperature. Ice-and-water shield at the eaves limits the damage but does not prevent the dam.