Ice Dams on Toronto Roofs: Causes, Damage and Prevention

Long rows of icicles may look nice, but on a Toronto roof they are often a warning sign. Thick ridges of ice along the eaves, called ice dams, trap melting snow on the roof. That water has nowhere to go except under the shingles and into your home. Here is why ice dams form on so many homes in Toronto and the GTA, and what actually prevents them.

How an ice dam forms

1. Warm air from the house leaks into the attic through gaps around light fixtures, pipes, chimneys and the attic hatch.
2. The warm attic heats the roof deck and melts the snow on the upper part of the roof.
3. Meltwater runs down to the eaves, which hang past the warm walls and stay cold.
4. The water refreezes into a ridge of ice that grows with every melt.
5. Water pools behind the ice, backs up under the shingles and leaks into the ceiling and walls.

Why ice dams are common in Toronto

Our winters bounce between freezing and thawing, sometimes several times a week. Each thaw adds more water and each freeze adds more ice. Older homes in Toronto, North York and East York often have less attic insulation and limited ventilation, which makes the problem worse. Clogged eavestroughs also hold ice at the edge of the roof.

Signs you have an ice dam problem

• Thick ice along the eaves and in the eavestroughs
• Large icicles, especially in the same spots every winter
• Water stains on ceilings or upper walls near outside walls
• Wet or frosty insulation and roof boards in the attic
• Peeling paint or water marks on the soffit and fascia

How to prevent ice dams

Seal air leaks into the attic

This is the most important step. Sealing gaps around pot lights, plumbing stacks, bathroom fans, chimneys and the attic hatch keeps warm air out of the attic.

Add attic insulation

Good insulation keeps heat in the living space and the roof deck cold.

Improve roof ventilation

Air should enter through clear soffit vents and leave through roof or ridge vents. Baffles keep insulation from blocking the soffits. Damaged or painted-over soffits can be replaced as part of our soffit, fascia and eavestrough services.

Keep eavestroughs clear

Clean gutters in late fall so meltwater can drain. Our guide to eavestroughs, soffit and fascia explains why the roof edge matters so much.

Use ice and water protection when you re-roof

When a roof is replaced, a self-adhering waterproof membrane should be installed along the eaves and in valleys. It won’t stop ice from forming, but it gives a strong second line of defence if water backs up. Make sure it is included in any roof replacement quote.

What to do if you already have an ice dam

• Don’t climb on the roof and don’t chip at the ice with a hammer, axe or shovel. You can easily damage the shingles and hurt yourself.
• Don’t use rock salt on the roof. It can damage roofing materials, metal and plants below.
• Keep people away from the area under large icicles and ice sheets.
• If you can do it safely from the ground, use a roof rake to remove snow from the lowest few feet of the roof.
• If water is coming inside, call a professional. Our roof leak repair team offers 24/7 emergency service.

Frequently asked questions

Are heating cables a good solution?

Heat cables can keep channels open for water to drain, but they treat the symptom, not the cause. Air sealing, insulation and ventilation are the long-term fix.

Can ice dams damage a new roof?

Yes. A new roof with ice and water protection is much better protected, but if heat loss continues, ice will still form. That is why we look at ventilation during every roof inspection.

Should I remove all the snow from my roof?

Usually not. Clearing the lower edge from the ground is enough for most homes. Leave heavier snow removal to professionals.

Stop ice dams before they start

Roofing Pros helps homeowners in Toronto, North York, Scarborough and Vaughan fix the causes of ice dams and the leaks they leave behind. Call 905-252-PROS (7767) or book an inspection. You can also read our fall roof checklist to prepare before the snow.