How to stop ice dams on your roof
Ice dams form when heat escaping from the house warms the roof, melts the snow above it, and the water refreezes at the cold eaves and backs up under the shingles. The lasting fix is to keep the roof deck cold with air sealing, attic insulation and ventilation, not to chip off the ice.
What causes an ice dam?
Three things have to happen together: snow on the roof, a warm patch under the snow and a cold edge at the eaves. Heat leaking from the living space into the attic warms the roof deck in the middle. Snow there melts, runs down to the overhang where the roof is cold, and freezes into a ridge of ice. Meltwater that follows pools behind it, and can be forced up under shingles and into the ceiling and walls. Gutters packed with ice make things worse but are rarely the root cause.
What are the signs of a problem?
Look for thick ice along the eaves, icicles that are large and numerous, bare roof patches while neighbouring roofs stay snow-covered, and damp stains on ceilings near exterior walls. Frost on the underside of the roof boards or on nail tips in the attic is another sign of warm, moist air escaping from the house. Water damage can show up inside weeks later, so inspect after each thaw.
How do you prevent ice dams for good?
Work in this order.
- Seal air leaks. Close gaps around the hatch, wiring, plumbing stacks, fan housings and the tops of walls, as set out in air sealing a house.
- Add insulation. A deep, even layer over the ceiling keeps the deck cold; see attic insulation in Canada, and replacing attic insulation if the old layer is damaged.
- Ventilate the roof. Open soffit vents with baffles feeding an outlet near the ridge move cold air under the deck.
- Vent exhaust outdoors. Bathroom and kitchen fans should not discharge into the attic.
Do the sealing before extra insulation, because insulating over leaks hides them without stopping the moist air.
What can you do about ice right now?
Remove snow from the lower roof with a long-handled roof rake from the ground, taking care near power lines. Do not climb on an icy roof, and do not chip ice with a hammer, axe or heat gun, since that damages shingles and injures people. If water is already coming inside, call a roofer or an ice-removal service that uses low-pressure steam. Heat cables along the eaves can create channels for meltwater and are a stopgap, not a cure. Keep gutters and downspouts clear in autumn, a task on the winter home energy checklist.
What about the roof itself?
When a roof is replaced, ask about a self-adhering waterproof membrane under the shingles at the eaves and in valleys, which gives a second line of defence against backed-up water. Check flashing at chimneys and skylights, which are common leak points. Ice dams are a signal of heat loss, so they also point to higher heating bills, and the order of fixes in the upgrade priorities guide addresses both. A home energy audit with a blower door and thermal camera can find the leaks that feed the problem. Check for programs that support air sealing and insulation through the Government of Canada energy-efficiency pages, your province and your utility.
Quick answers
Are icicles a sign of an ice dam?
A few small ones can be harmless, but long, heavy icicles along an entire eave often show heat loss and melting above. They deserve a closer look.
Will heat cables stop ice dams?
They can keep a channel open and limit damage, but they use electricity and do not fix the heat loss. Sealing, insulation and ventilation are the cure.
Can ice dams damage my house?
Yes. Backed-up water can soak insulation and ceilings, lift shingles and cause mould, so address it early.