Ice Dams: Why Winter Roof Leaks Happen, and How to Stop Them
If your ceiling starts dripping near an outside wall in the middle of winter, when it hasn’t rained in weeks, you’re probably looking at an ice dam. They’re one of the main reasons Ontario searches for roof leak repair peak in December and February.
How an ice dam forms
Natural Resources Canada describes it in three steps:
- Heat from the house warms the attic. If the attic is above freezing, it warms the roof deck and melts the layer of snow touching the roof.
- Meltwater runs down the roof until it reaches the overhang at the eaves, which sits outside the heated house and stays cold.
- It refreezes at the eaves. “If the air and the overhang are below freezing, the water freezes on the roof and starts the ice dam.”
Each melt adds to the ridge of ice. Eventually water pools behind it, and because shingles shed water downhill but aren’t watertight against standing water, it backs up under the shingles and into the house.
Why Ontario is prone to them
Ice dams need snow on the roof plus repeated melting and freezing. Ontario has plenty of both, though the amount varies a lot by place (Environment Canada climate normals):
| Area | Snow / yr | Days crossing 0°C / yr |
|---|---|---|
| Kitchener-Waterloo | 160 cm | 94 |
| Barrie | 223 cm | 91 |
| London | 194 cm | 86 |
| Hamilton | 157 cm | 82 |
| Ottawa | 223 cm | 77 |
| Windsor | 129 cm | 72 |
| Downtown Toronto | 122 cm | 55 |
Signs you have one
- A thick ridge of ice along the eaves, often with large icicles.
- Water stains or drips on ceilings or walls near exterior walls, often on the top floor.
- Damp or frosty insulation in the attic above the eaves.
If an ice dam is leaking now
- Stay off the roof. Icy roofs and ladders are dangerous.
- Contain the water inside and take photos. The Insurance Bureau of Canada recommends documenting damage and acting to limit further damage.
- Don’t chip at the ice with tools. It’s easy to damage shingles and eavestroughs. A roofer can safely open a channel so the water drains.
- Call for help: roof leak repair or, if water is pouring in, emergency roof repair.
How to stop them coming back
The real fix is usually in the attic, not on the roof. NRCan puts it plainly: “A well-sealed and insulated attic results in a cool roof and generally will also not have ice dams.”
- Air-seal the attic floor. Seal gaps around light fixtures, plumbing stacks, the attic hatch and top plates so warm air can’t leak up.
- Insulate to current levels.
- Ventilate so the attic stays close to outdoor temperature.
- Protect the eaves. When the roof is next replaced, make sure ice-and-water protection is installed along the eaves.
Try this: after the first light snow or heavy frost of the season, look at your roof. NRCan suggests noting where the snow melts first, because whatever is under that spot is losing heat. Common culprits are attic hatches, exhaust fans vented into the attic, plumbing vents, skylights, and the knee walls of 1½-storey homes.
Want someone to take a look?
A roofer can check the eaves, flashing and attic during a free roof inspection. Late fall, before the snow, is the ideal time.
Call 1 (833) 435-4261 Book a free roof inspection
Source: Natural Resources Canada, Keeping the Heat In: Section 5, Roofs and attics.