Ice Dams
An ice dam is not a roofing fault. It is a heat-loss fault with a roofing symptom, and that is why clearing it every winter never ends.
Short answer
Heat escaping into the attic melts snow on the upper roof. The meltwater runs down to the cold eave, refreezes, and builds a dam. Water then backs up behind it and gets under the shingles. The cure is air sealing the ceiling, insulating properly and keeping the soffit-to-ridge airflow clear — not chipping ice off the eave in February.
Why they form here
Toronto gets a long stretch of below-freezing weather with repeated snowfall, which is exactly the condition that builds a dam over weeks.
Warm air leaks into the attic through pot lights, top plates, the attic hatch, plumbing stacks and bathroom fan ducts.
That warms the roof deck above the heated space, melting the snow lying on it.
The eave overhangs unheated space, so it stays at outdoor temperature and the meltwater refreezes there.
Each cycle adds to the dam, and the water behind it finds the shingle laps.
And a blocked soffit intake makes it worse, because the attic cannot flush out the heat it is receiving.
Icicles are the tell
A roof with a row of icicles along the eave is losing heat into the attic. A properly sealed, insulated and vented roof in Toronto keeps its snow and does not grow icicles. They are not a decoration; they are a diagnosis.
What the water does
Backs up behind the dam and gets under the shingle courses, which are designed to shed running water rather than to hold standing water.
Soaks the sheathing and the eave framing, which is where the rot starts.
Runs into the soffit and down inside the wall, which shows up as staining well away from the roof.
Wets the insulation at the eave, which then stops insulating and makes the next winter worse.
Shows up on the ceiling as a stain, usually at the exterior wall line, in February rather than during the storm.
And over several winters produces mould in the eave and the attic, which is a bigger job than the roof repair.
The fix, in order
Air seal the ceiling plane: pot lights, top plates, the attic hatch, plumbing and wiring penetrations, the chase above a dropped ceiling.
Duct every bathroom and kitchen fan to the outside, not into the attic.
Clear the soffit intake and fit baffles so insulation cannot slump back into it.
Insulate to current depth, evenly, with the eave area done properly rather than skipped.
Confirm there is exhaust at the top — ridge or roof vents — balanced against the intake.
And only then consider heat cable at the eave, which manages a symptom and is not a solution.
Common questions
Why do I get ice dams every winter?
Heat is escaping into the attic. Until that is sealed, the dam comes back.
Are icicles a problem?
They are a symptom of heat loss into the attic. A well-sealed roof does not grow them.
Does heat cable fix it?
It manages the symptom. The air sealing and the venting are the actual fix.