Ice dams develop from melting snow on snow-covered roofs. Heat loss from thermal bridging at poorly insulated wall top plates can melt the snow during mild, near freezing weather. The water from the snow moves down the roof to the colder area of the roof above the soffits. As it runs over this colder roof area, it can freeze. The newly frozen water can act as a dam to the snow which continues to melt at the exterior wall and ceiling intersection.
Large ice dams can back water up under the shingles beyond the eave protection normally placed below the shingles. Staining of interior finishes, deterioration of wood roof sheathing and ice accumulation in attics can result from this problem.
Better insulation and proper ventilation of both the soffits and upper portions of the attic can improve this condition but in some older homes it is not possible to totally eliminate it due to the design of the roof structure and the absence of air vapour barrier or total seal of the same where in place.
Icedams can also be a safety hazard as can be seen on picture 2.
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