Ice Dams in Ontario: Why They Form, Why the Roofer Cannot Fix Them, and the Three Things That Do

Roofing

Ice Dams: A Roof Problem That Is Not a Roofing Problem

Every February someone in Ontario pays a roofer $800 to steam an ice dam off the eave, watches it come back in ten days, and then pays for a new roof that gets an ice dam the following winter. The roof was never the problem. The dam is made by heat leaking out of the house into the attic, and the fix is below the shingles, where roofers do not work. Here is how a dam forms, what it is doing to the house, and the three things that stop it, in order.

Building science as Natural Resources Canada and every Ontario building inspector describe it; the costs are October 2026.

How a dam forms

Snow sits on the roof. Heat from the house gets into the attic (warm air through gaps around light fixtures, the attic hatch, plumbing stacks and top plates; and conducted heat through thin insulation) and warms the underside of the roof deck above the heated part of the house. The snow on that part melts from underneath and runs down the roof under the snow. At the eave, which hangs past the heated walls and is as cold as the air, the water refreezes. That ridge of ice grows with every melt-freeze cycle until it is a dam, and the water behind it has nowhere to go but up, under the shingles, through the nail holes, into the deck, the soffit, the wall and the ceiling. The big icicles at the eave are the overflow. A house with no heat loss into the attic has a roof as cold as the air, snow that does not melt from below, and no dam, which is why barns do not get them.

What it does to the house

  • Water through the deck at the eave: stains on the ceiling near exterior walls and above windows, usually first noticed in a thaw.
  • Water into the wall cavity: wet insulation, mould behind drywall, rot at the top plate and the window heads, found years later when a window is replaced.
  • Rotten fascia and soffit, and the eavestrough torn off by the weight of ice.
  • Shingles lifted and seal strips broken at the eave, so the roof leaks there in rain the following summer too.
  • In the worst cases, $20,000 to $40,000 in interior and wall repair from one bad winter. It is the damage claim insurers in Ontario see most in March.

What does not fix it

A new roof. New shingles over the same warm attic make the same dam. What a new roof does do is put ice and water shield at the eaves, which keeps the backed-up water out of the house while the dam is there; that is damage control, and the underlayment page explains it. More roof vents alone. Ventilation moves attic air out, but if warm house air is pouring in through a dozen gaps, the attic stays warm and the vents are just moving your heating bill outside. Natural Resources Canada says this plainly: ventilation does not correct the problems caused by significant air leakage. Steaming. Removes this dam. The next one starts the same afternoon. Heat cables. Melt channels through the dam so water can escape; useful on a problem spot you cannot fix (a north valley over a cathedral ceiling), useless as a plan, and $200 a winter in electricity.

The three things that do, in order

  1. Air sealing. Find every path warm air takes into the attic and close it: foam around plumbing stacks and electrical penetrations through the top plates, a gasket and insulation on the attic hatch, airtight boxes or caulked trim around pot lights (or replace them with sealed IC-rated ones), the top of interior walls where the drywall stops, the chimney chase, the bathroom fan housing. Done by an insulation contractor or an energy auditor’s crew with a blower door to find the leaks: $800 to $2,500 for a typical house, and it is the single most effective thing on this page. The bathroom fan dumping into the attic is its own page: fan venting.
  2. Insulation. Ontario attics built before 1990 often have R-20 or less. Current code is R-60 for a new house. Blown cellulose or fibreglass over the existing to R-50 or R-60, with baffles at the eaves so the soffit vents stay open: $1,800 to $4,500 for a typical house, and your site already has the attic insulation cost page and the R-value guide for the details. Insulation slows the conducted heat; it does not stop the leaked air, which is why it is second.
  3. Ventilation. With the air leaks closed and the insulation deep, the attic is now cold, and ventilation keeps it that way and dries it out: soffit intake along both eaves, ridge vent or roof vents at the top, 1 sq ft of vent per 300 sq ft of attic floor split between high and low, nothing blocked by insulation. $600 to $2,000 if done with a roof; more as a standalone job. The ventilation page.
Do them in that order. Ventilation without air sealing pulls more warm air out of the house into the attic (the attic becomes a chimney) and can make the dam worse. Insulation without air sealing gets wet from the air leaking through it and loses half its value. Seal, then insulate, then vent. A home energy auditor with a blower door and a thermal camera will show you exactly where the heat is going for $300 to $600, and most of the Ontario rebate programs require the audit anyway.

Roofs that get dams no matter what

Cathedral ceilings with no attic (the insulation is between the rafters and the deck is warm by design); roofs with a valley that dumps onto a shaded eave; flat-to-sloped transitions; a second-floor wall that sits on the first-floor roof. These need the full ice and water treatment (membrane well up the slope, sometimes the whole plane), a heat cable in the worst spot, and a roofer who has seen it before. On a cathedral ceiling, the real fix is a vented over-roof or spray foam between the rafters, which is a project, not a repair.

This winter, while you plan

Rake the bottom 3 to 4 ft of snow off the eaves after each big fall with a roof rake from the ground ($50 to $80; never from a ladder). No snow at the eave, no dam. If a dam has formed and water is coming in, a roofer with a steamer ($400 to $1,200) removes it without damaging the shingles; a chisel, a hammer or salt will damage them. Then book the air sealing for spring. The eavestroughs, by the way, do not cause ice dams and do not prevent them; they just hold the ice where you can see it.

Questions people ask

What causes ice dams on a roof?

Heat leaking from the house into the attic melts the snow from underneath; the water refreezes at the cold eave and builds a dam that backs water up under the shingles. The cause is heat loss, not the roof.

How do I stop ice dams?

Seal the air leaks from the house into the attic, bring the insulation up to R-50 or R-60 with baffles at the eaves, and ventilate the attic with soffit intake and ridge or roof exhaust. In that order.

Will a new roof stop ice dams?

No. It will put ice and water shield at the eaves, which keeps the water out of the house while the dam is there. The dam still forms until the attic is fixed.

How much does it cost to remove an ice dam?

$400 to $1,200 for a roofer to steam it off. It will return until the heat loss is fixed. Raking the eaves after each snowfall prevents most dams for the cost of a roof rake.

Are heat cables worth it for ice dams?

On one problem spot that cannot be fixed from inside (a shaded valley over a cathedral ceiling), yes. As the plan for a whole roof, no: they cost $200 a winter to run and treat the symptom.

Fix the attic

Seal, insulate, vent. The roof will stop making ice the first winter after, and the heating bill drops while you wait for it.

Attic ventilation, done rightAttic insulation cost

Sources and further reading