Ice Damming in Ontario: How to Prevent It Before Winter (2026 Guide)

Ice damming is a ridge of ice that builds along the cold edge of a roof when snow higher up melts and the meltwater refreezes at the overhang. The dam then holds standing water on the roof, and shingles are built to shed water running down, not water sitting still or backing up under the courses.

Canada Mortgage and Housing Corporation sets out the mechanism plainly in Attic Venting, Attic Moisture and Ice Dams: if the attic is above freezing it warms the roof sheathing, which melts the snow lying on the shingles. That water runs down the slope until it reaches the overhang, which the attic does not heat and which therefore sits at the temperature of the outside air. If that air is below zero, the water turns to ice, and every melt after it runs into the back of what is now a dam.

The dam grows uphill. Water pools behind it, creeps sideways under the shingle courses, soaks the sheathing, and eventually finds the ceiling. That is why the damage bill is rarely the roof alone. It is drywall, insulation, and paint, and the same job left to run through a winter routinely lands between $1,200 and $4,000 once water has reached the deck.

The clearest proof that this is a heat problem rather than a roof problem sits at the two extremes. CMHC points out that an attic with no insulation at all generally does not get ice dams, because the heat pouring through melts the snow off before it can pile up. A well sealed and well insulated attic does not get them either, because the roof stays cold. The houses that suffer are the ones in the middle, which is most of the housing stock in Ontario.

More often than most people expect. On Environment and Climate Change Canada's 1991 to 2020 climate normals for Toronto Buttonville Airport in Markham, 55 of the 121 days between December 1 and March 31 rise above freezing and drop back below it. Each of those days is one more chance for meltwater to run down and set.

Here is the part that surprises people. January is not the dangerous month. A normal January at that station has 18 days that never climb above zero at all, so the snow on the roof simply sits there, and only 10 days actually cycle. March cycles 18 times. March is also when we take the most calls about ceiling stains, and the two facts are the same fact.

Deep cold does not build an ice dam. A crossing does. This is why a mild, sloppy Ontario winter can be harder on a roof than a brutally cold one, and why the roofs that get through February untouched are so often the ones that fail in the first week of March.

The cheapest ice dam diagnosis in Ontario is free. After the first light snow or the first heavy frost, go outside and look at your roof before the sun gets to it. Wherever the snow melts off first is where heat is leaving your house, and that is where the dam will form in January.

CMHC recommends exactly this, and it works because a thin, even layer of snow is a better thermal camera than most thermal cameras. A roof that melts evenly is fine. A roof with bald patches is telling you what is underneath them.

That last row catches people out. Natural Resources Canada notes in Keeping the Heat In that dark siding on a southerly wall, heated by low winter sun, can push warm convection currents up into the eave and cause ice damming above it. If one end of your house dams every year and the other never does, walk around and check which way that wall faces, and what it is clad in, before you blame the roof. Our guide to siding in Ontario covers what sits behind the cladding, which is where the air actually moves.

Because ventilation clears heat that has already escaped instead of keeping it in the house. CMHC is blunt about this: reaching for more attic ventilation first is the wrong approach, and extra vents can pull more warm, moist house air up through the ceiling and make the attic wetter than it was.

Ventilation still matters, and the Ontario Building Code still requires it. Roof spaces need unobstructed vent area of at least 1/300 of the insulated ceiling area, with a quarter of the openings high and a quarter low, distributed evenly. What the code does not claim is that vents cure ice damming. They keep the deck dry and the shingles cool, which is why we check them on every roof, but they are the last line of the fix and not the first.

CMHC goes further than most contractors are comfortable with and calls attic ventilation overrated, citing research in which identical attics, one unvented and one vented to code, held much the same temperature and humidity. Take that as a caution about sequence rather than a reason to block your soffits.

The February sales pitch is a row of new roof vents. It is quick, it is visible from the street, and on a house pushing warm air through twenty holes in the ceiling it will not move the needle. Get the order right and the venting you already have is often enough.

Four steps, and the order is not negotiable. Seal the air leaks out of the ceiling, insulate the whole attic floor to depth, fix the thin spot where that floor meets the outside wall, then confirm the intake and exhaust venting is clear. The first two do most of the work.

1. Air seal the ceiling below the attic. Natural Resources Canada puts it as directly as a government document ever does: the importance of air sealing cannot be overstated, and you should not rely on batt, blown-in or poured insulation to reduce the need for it. Target the attic hatch, plumbing stacks, chimney chases, wiring holes, bathroom fan ducts, and the joint where interior walls meet the ceiling.
2. Insulate the whole floor to depth. New builds in Ontario are built to about R-60 in the attic. Older houses are often at half that, and settled loose fill is thinner than it looks.
3. Fix the squeeze at the eave. This is the step everyone skips.
4. Then check the venting. Clear soffit intake, a working exhaust path, nothing stuffed with insulation.

Step three deserves its own paragraph, because it is the one that decides whether the ice forms right where it does the most harm. CMHC notes that once a baffle is holding a ventilation channel open at the eave, there is often only 100 mm of depth left for insulation at that point, and that thin band melts snow just above the overhang. NRCan asks for 38 to 50 mm of clear air between the top of the insulation and the underside of the sheathing, with a baffle in every rafter space, so the answer is not to squash the insulation or to skip the baffle. It is to use the best insulation you can get into that gap and to keep the channel open.

One honest note on scope. We do not install insulation, so we have nothing to sell you on steps one and two, and we will tell you plainly when that is the work your house needs. What we do handle is everything at the roof edge: soffit and fascia that actually breathes, eave protection membrane when the roof comes off, eavestrough that is not being torn off its fascia every March, and the roof repair when water has already got in.

Air sealing plus a blown-in top-up to R-60 runs roughly $1,500 to $3,500 on a typical Ontario attic, and the provincial Home Renovation Savings Program pays $1,000 to $1,250 back on attic insulation with no energy assessment required. That makes the permanent fix cheaper than two seasons of emergency ice removal.

The figures below are Ontario market ranges for 2026, not a Sherak quote, since most of this work belongs to an insulation contractor rather than a roofer.

Two of those lines are worth comparing directly. Two winters of steam removal costs about the same as air sealing and insulating the attic once, except that one of them is still working in 2040. If your roof is also near the end of its life, price the two jobs in the same month and read our roof replacement cost guide before you sign anything, because there is no roofing rebate in Ontario and the insulation money is the only rebate in this story.

Pull the snow off the lower roof with a roof rake from the ground, and if the dam is already thick, book steam removal. Do not climb up, do not chip at it, and do not throw salt on your shingles. The dam is the symptom, so deal with the cause once the thaw arrives.

A roof rake with an extension pole clears the bottom metre or so of the slope from the driveway, and no snow at the eave means no ice at the eave. Rake after each significant snowfall, keep both feet on the ground, and watch for the overhead wires. This is the one winter job on the list that a homeowner can safely do alone.

Steam is the only removal method that does not cost you shingles. It melts the ice out without heat high enough to damage the roof, and it runs $400 to $1,200 a visit through the GTA depending on how much ice there is and how bad the access is.

What not to do, and CMHC is unusually direct on all four:

• An axe, a hammer, or an ice pick. A good way to shorten the life of your shingles, and a good opportunity to fall off a ladder.
• Salt or ice-melt pucks in the trough. Corrosion of the eavestrough and the flashing, plus runoff into whatever is planted below.
• Heating cables. They melt a channel through the ice rather than removing it, draw real power all winter, and look like what they are.
• Taking the eavestroughs off. They are not the cause, and they are what keeps roof water away from your basement.

If water is already coming through a ceiling, that is a different phone call and a faster one. Our roof leak guide covers what to do in the first hour, and our crews run 24/7 emergency roof repair across York Region and the GTA through the winter.

Eave protection membrane, a metal roof, and a sound eavestrough all reduce what an ice dam costs you. None of them prevents one. CMHC is explicit that a self-sealing membrane under the shingles does not stop ice dams, it only stops the water backing up behind one from getting through the sheathing.

That distinction is worth holding onto, because eave protection is sold as an ice dam solution constantly. The Ontario Building Code requires it on shingle roofs from the roof edge up the slope to a line at least 300 mm inside the inner face of the exterior wall, and we run it at the eaves and the valleys as standard. It is the difference between an ugly winter and a ruined ceiling, which is a real difference, but the dam still forms.

Metal helps more, and still does not cure it. A smooth standing seam plane sheds snow before the melt-freeze cycle can get going, which is why metal roofs see fewer ice dams than shingle roofs on the same street. We have still pulled ice off metal roofs sitting over badly sealed attics, and the standing seam cost guide makes the same point: the material reduces the symptom, the attic cures the disease.

Your shingle warranty has an opinion too. Water entry caused by ice damming is a written exclusion on the products we install, alongside damage from inadequate attic ventilation, which is covered in our guide to the best asphalt shingles for Ontario. Nobody is coming to pay for this on a warranty claim.

The eavestrough takes the physical damage. On Lake Drive in Jackson's Point in March 2025 we replaced a system that an ice dam had pulled clean off the house, which turned into new eavestrough, soffit, fascia, siding, and leaf guard, because once the trough goes it takes the fascia with it. Our seamless eavestrough comparison explains why the sectional runs give up first. Going into winter with the trough still loaded makes all of this worse, because leaves left in November freeze into a solid plug along the eave, which is why our guide to eavestrough cleaning puts the last clear of the year in late November rather than October.

Ice damming is a September problem that people treat as a February emergency. In February the attic is full of snow-damp insulation, every insulation crew in the GTA is booked, and the only thing anyone can sell you is a truck with a steamer on it. In September the same work is calm, cheap, and permanent.

Our summer and fall maintenance checklist has the two attic checks worth doing before the snow arrives, and if you want a second opinion on what your roof is telling you, we will come and look. Homeowners in Newmarket, Aurora, and Richmond Hill call us every spring about the same stains, and almost all of it was preventable in the fall.

At Sherak we are based in Gormley and work across York Region, Simcoe, Kawartha, and the GTA. Every quote is free and itemised, we carry $5 million liability insurance, our workmanship is backed by a 10 year warranty, and you pay the last 10 percent only once you are satisfied. Get a free quote and we will tell you honestly whether the answer is a roofer or an insulator.

Sherak Ltd, Head Office

12275 Woodbine Avenue, Gormley, ON L0H 1G0

Phone: +1 905 490 8800

Email: info@sherak.ca