Quick answerIce dams form when heat leaking from the house warms the roof deck, melting snow that refreezes at the cold eaves. The lasting fix is in the attic: seal air leaks, keep soffit vents open with baffles, and insulate to R-50 so the roof stays cold.
How an ice dam forms
- Heat escapes into the attic through air leaks and thin insulation.
- The roof deck warms up and melts the snow sitting on it.
- Meltwater runs down to the eaves, which hang past the warm house and stay below freezing.
- It refreezes into a ridge of ice. More meltwater pools behind it and can back up under the shingles.
Warning signs
- Thick ice along the gutters and eaves
- Large icicles, especially on one side of the house
- Snow melting on the upper roof while the eaves stay frozen
- Water stains on ceilings or walls near exterior walls
- Frost or damp spots on the underside of the roof sheathing
The fix, in the right order
| Step | Why it matters |
|---|---|
| Air seal the attic floor | Stops warm, moist house air from heating the roof deck. This is the biggest single factor. |
| Install baffles at the eaves | Keeps cold outside air flowing from the soffits up under the roof. |
| Insulate to R-50 | Slows the heat that does reach the attic floor. |
| Fix exhaust fans | Bathroom and kitchen fans must vent outdoors, never into the attic. |
What not to do
- Don't chip ice off the roof. It damages shingles.
- Heat cables can help short-term, but they treat the symptom, not the cause.
- If water is already coming inside, call a roofer for the leak first. Then fix the attic so it doesn't happen next winter.
FAQ
Will more insulation alone stop ice dams?
Not always. Air sealing and ventilation matter as much as insulation depth. Adding insulation over air leaks and blocked soffits can still leave a warm roof deck.
Are ice dams common in Toronto?
Yes. Freeze-thaw cycles in GTA winters make ice dams common, especially on older homes with little attic insulation and no baffles.
When is the best time to fix the attic?
Before winter, ideally in fall. Attic work can also be done in winter once the roof isn't actively leaking.