How to Prevent Ice Dams on Your Roof

An ice dam is a ridge of ice that builds up along the lower edge of a roof. Melting snow higher up the roof runs down, hits the dam, and backs up under the shingles, where it can leak into the attic, walls and ceilings. Ice dams are common across Ontario because winters bring repeated freeze-thaw cycles. Large icicles along the eaves are often the first warning sign.

The root cause is heat escaping from the house into the attic. That heat warms the roof deck, so snow on the upper roof melts even when it is below freezing outside. The meltwater runs down to the eaves, which stick out past the heated walls and stay cold, and it refreezes there. Common heat leaks are gaps around light fixtures, plumbing stacks, wiring holes and chimneys, an uninsulated attic hatch, too little attic insulation, and bathroom fans that vent into the attic instead of outdoors. Blocked gutters can make a dam worse, but they are rarely the cause.

Use a roof rake with an extension handle (CAD $40–$90) from the ground to pull snow off the lowest 1 to 1.5 m of the roof after each heavy snowfall. Do not climb onto an icy roof, and do not chip at the ice with a hammer, axe or shovel, which damages shingles. To open a drainage channel through an existing dam, fill a nylon stocking with calcium chloride ice melt and lay it across the dam so it runs from the roof down past the edge. Do not use rock salt (sodium chloride), which damages shingles, metal flashing and plants. Indoors, put buckets under any drips and move valuables away from affected walls.

1. On a cold day, check the attic with a flashlight. Frost or water stains on the underside of the roof sheathing, damp insulation, or warm air around fixtures all point to heat leaks.
2. From outside, look at the roof after a light snowfall. Patches that melt quickly or unevenly show where heat is escaping.
3. Seal air leaks from the living space into the attic: gaps around plumbing stacks, wiring holes and top plates. Use airtight, insulation-contact (IC) rated fixtures for pot lights, and fire-rated sealant around chimneys and flues.
4. Weatherstrip the attic hatch and fix a block of rigid insulation to its top.
5. Add insulation where it is thin. Many older Ontario homes have far less attic insulation than new homes are built with today.
6. Make sure bathroom and kitchen fans vent outdoors through the roof or wall, not into the attic.
7. Keep soffit vents clear by installing baffles at the eaves, so cold outside air can flow along the underside of the roof.
8. Clean the gutters in late fall so meltwater can drain.

Electric roof and gutter de-icing cables can keep a channel open along the eaves, but they treat the symptom, not the cause, and they add to your electricity bill. Install them exactly as the instructions say and plug them into a GFCI-protected outdoor outlet. Air sealing and attic insulation fix the cause, and Ontario homeowners can often get rebates for that work; check the current programs from Enbridge Gas and Save on Energy before you start.

Call a roofer if water is leaking inside, if the ice dam is on a second-storey or steep roof, or if gutters are pulling away from the house. If ice dams come back every winter, book an energy audit: an energy advisor uses a blower door test to find where heat is leaking into the attic, and an insulation contractor can seal and insulate it properly. Never go up on the roof in winter to deal with ice yourself.