Ice Dams and Winter Leaks: What to Do When It Happens

An ice dam is a ridge of ice that forms along the edge of a roof and stops melting snow from draining. In Calgary the chinook is the reason, and the underlying cause is usually heat escaping from the house into the attic. If water is coming in, deal with it inside the house, stay off the roof and call someone.

Key points

• A chinook melts the snow, then the overnight freeze builds ice at the cold eave.
• The underlying cause is usually heat escaping into the attic.
• If water is coming in, limit the damage inside, photograph everything and call someone.
• Do not get on the roof, chip at the ice, use rock salt or use a pressure washer.
• Insulation, air sealing and ventilation deal with the cause.

Why chinooks make it worse

Water pools behind an ice dam, sits against the shingles, and works its way underneath. Shingles are designed to shed water running downhill. They are not designed to hold standing water, and once it backs up it gets in. Calgary gets these, and the chinook is the reason.

In a consistently cold winter, snow sits frozen on a roof until spring. Calgary does not work that way. A chinook can push the temperature well above freezing in a few hours, melt the snow on the roof, and then the temperature drops back overnight and freezes all that meltwater in place.

The eave is where it collects, because the eave overhangs open air rather than a heated room, so it stays colder than the rest of the roof. Water runs down the warm part of the roof, reaches the cold edge, and freezes. Do that repeatedly over a season and you build a ridge of ice along the bottom of the roof with a pool of water sitting behind it. That is the whole mechanism. A warm roof and a cold edge.

The underlying cause is usually the attic

Here is the part that surprises people: an ice dam is usually a symptom of heat escaping from the house.

If your attic is warm, it warms the underside of the roof deck, which melts the snow sitting on top of it even when it is below freezing outside. That meltwater runs to the cold overhang and freezes. A properly insulated and ventilated attic stays close to the outdoor temperature, the snow on the roof does not melt from underneath, and you get far fewer dams.

The two things that cause a warm attic are insufficient insulation, which lets heat up into the attic, and insufficient ventilation, which traps the heat that gets there. Intake at the soffit and exhaust near the ridge have to be balanced. Blocked soffit vents are extremely common, often because insulation has been pushed into them at some point. Read more about roof ventilation.

This is why fixing an ice dam by attacking the ice alone rarely works. You remove the ice, the attic is still warm, and it forms again the next chinook.

What to do when water is coming in right now

First, deal with the water inside. Move what you can out of the way, put something down to catch it, and if the ceiling is bulging with trapped water, a small hole at the low point will drain it in a controlled way rather than letting it come down all at once. That feels wrong and it is the right call.

If water is anywhere near light fittings or electrical, kill the power to that circuit.

Photograph everything before you clean up. If this turns into an insurance claim, the photographs taken while it was happening are worth more than anything taken afterwards.

Then call someone. An active leak in winter is not a wait-and-see problem, because the water is already in the insulation and the sheathing, and every freeze cycle makes the path bigger. Call us on 403-616-2605.

See also roof leak in winter: what to do first.

What not to do

Do not get on the roof. A roof with ice on it in winter is genuinely dangerous, and this is the single most common way homeowners get hurt dealing with an ice dam.

Do not chip at the ice with anything hard. Hammers, axes, garden spades and chisels all go through shingles easily, and you will not be able to see what you are damaging under the ice. People turn a leak into a replacement this way every winter.

Do not use rock salt or a de-icer intended for driveways. It corrodes metal flashing and eavestrough, damages shingles, and kills whatever is planted below when it washes off.

Do not use a pressure washer or boiling water. You are adding water to a roof that already cannot drain, in freezing conditions.

If you want to reduce the load yourself, a roof rake used from the ground is the safe option. Pulling the snow off the lower few feet of the roof removes the material the dam is built from. Work from the ground, keep clear of the falling snow, and stay well away from the power line to the house.

Stopping it happening again

Once the immediate problem is dealt with, the fix is upstream of the roof surface.

• Get the attic looked at. Insulation depth and coverage, and whether the soffit vents are actually open. That is the root of most recurring dams
• Seal the air leaks from the house into the attic. Gaps around bathroom fans, light fittings, the attic hatch and plumbing stacks all let warm moist air straight up
• Check where the bathroom and kitchen fans actually vent. If they discharge into the attic rather than outside, you are pumping warm wet air into exactly the wrong place
• Keep the eavestrough and downspouts clear before winter, so the meltwater that does come off has somewhere to go rather than freezing at the edge

And when the roof is next replaced, ice and water shield at the eaves is a membrane under the shingles at the roof edge. It does not prevent ice dams.

What we do

Most repairs take one visit. Qualifying projects carry our 15-year workmanship warranty.

What we cover on leaks and ice damage is on the roof repair page. If the roof turns out to need replacing rather than repairing, the instant estimate will give you a range in about thirty seconds.

If water is coming in now, do not work through the website. Call 403-616-2605.