What causes ice dams on my roof?

Ice dams form when heat from your home melts snow on the upper roof, which then refreezes at the colder roof edges and gutters, creating a barrier that traps water behind it. This trapped water can then back up under your shingles and leak into your home, causing significant damage to ceilings, walls, and insulation.

The primary culprit is inadequate attic insulation and ventilation. When warm air from your heated living spaces escapes into the attic, it warms the roof deck from below. This causes snow on the upper portions of your roof to melt, even when outdoor temperatures are below freezing. The melted water flows down toward the gutters and roof edges, which remain cold because they extend beyond the heated portion of your home. When this water hits these cold areas, it freezes solid, gradually building up into a dam of ice.

Poor attic ventilation compounds the problem by preventing cold outdoor air from circulating through the attic space to keep the roof deck at a consistent temperature. Without proper intake vents at the soffits and exhaust vents at the ridge, warm air gets trapped in the attic, creating those problematic temperature differences across your roof surface.

In Toronto's climate, ice dams are particularly common during our freeze-thaw cycles in late winter and early spring. Century homes in neighborhoods like The Annex and Cabbagetown are especially susceptible due to their older insulation standards and retrofit challenges. Even newer homes in Mississauga, Vaughan, and Markham can experience ice dams if the builder skimped on insulation or ventilation during construction.

Heat loss from recessed lights, bathroom fans, and gaps around chimneys also contributes to uneven roof temperatures. Many Toronto homes have these penetrations in the ceiling that weren't properly sealed during construction, allowing warm air to escape directly into the attic space.

Prevention requires addressing the root cause - heat loss from your living space into the attic. This means adding insulation to achieve R-50 or higher (as recommended by the Ontario Building Code), sealing air leaks between your home and attic, and ensuring proper ventilation with both intake and exhaust vents. While you might be tempted to remove snow from your roof or install heat cables, these are temporary band-aids that don't solve the underlying problem.

Professional assessment is crucial because ice dam prevention often involves working with insulation, electrical (for proper sealing around fixtures), and roofing components. A qualified contractor can perform a thermal imaging assessment to identify exactly where heat is escaping and develop a comprehensive solution that addresses your specific home's issues.

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