What Causes Ice Dams — and Does Insulation Fix Them?
Ice dams form when heat escaping into the attic melts snow on the upper roof, the water runs down and refreezes at the cold eaves. The cause is heat loss and air leakage, not the gutters. Air sealing, insulation and ventilation address the cause; heated cables only manage the symptom.
Why the roof melts unevenly
A properly performing attic stays close to outdoor temperature, so snow sits on the roof and melts from the top down when the sun hits it. When warm air leaks into the attic, the roof deck warms from below and melts snow even in freezing weather.
That meltwater runs to the eaves, which are still at outdoor temperature because there is no heated space beneath them. It refreezes there, builds a ridge, and the next round of meltwater backs up behind it — which is how water gets under shingles and into ceilings.
What actually stops them
The order matters: seal the air leaks, then bring insulation to level, then confirm the soffit-to-ridge ventilation path is open. Baffles are what keep that path clear after new insulation is blown in.
Roof cables and mechanical removal have their place on a roof that cannot be fixed any other way, but they are a yearly cost that does nothing for your heating bill. Fixing the attic reduces both problems at once.
Follow-up questions
I have ice dams but no leak. Can I wait?
Repeated freeze cycles work water into the assembly before you see a stain. Ontario winters cross the freezing point often enough that damage tends to appear suddenly.
Could it be ventilation alone?
Sometimes. Blocked soffits with adequate insulation can produce dams on their own, which is why we check the whole path rather than only measuring depth.
General guidance for homes in the Greater Toronto Area. Every building is different — treat this as background, not as a specification for your project.
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