What is an ice dam?
An ice dam is a ridge of compacted ice that forms at the eave line of a roof, typically caused by heat loss from the attic warming snow, which then refreezes at the colder eave, creating a barrier that traps meltwater and forces it up under roof shingles.
An ice dam forms at the edge of your roof when a combination of attic heat, outdoor cold, and snow creates a freeze-thaw cycle. Warm air leaking from your attic melts the snow on the upper part of your roof. That meltwater runs down and reaches the unheated eave overhang, where it refreezes into a ridge of ice. As more snow melts and water continues flowing down, the ice dam grows thicker.
The problem emerges when water backs up behind this ice barrier. Unable to drain off the roof edge normally, meltwater gets forced under the shingles and works its way into the roof structure. From there, it can seep into attic insulation, drip down interior walls, stain ceilings, and damage framing. Over time, repeated freeze-thaw cycles can crack shingles and degrade the roof's protective layers.
Ice dams are not simply a winter nuisance. They signal underlying issues, most commonly inadequate attic insulation or air sealing that allows heated indoor air to escape and warm the roof deck unevenly. Attic ventilation problems can also contribute, as can gutter design or roof pitch that traps water. In Ontario's freeze-thaw climate, ice dams are a recurring headache for homeowners and a common reason to call a roof repair contractor. Prevention typically involves tightening attic thermal control rather than trying to manage the ice itself.