Cause · heat in the wrong place

Ice dams: why they form, how to remove them safely, and why the fix is in the attic

An ice dam is a Canadian roof problem with a Canadian cause. The house leaks heat into the attic, the heat melts the snow on the roof, the melt runs down to the cold overhang and freezes there. The ridge of ice grows, and the next melt has nowhere to go but back up under the shingles and into the ceiling.

This page sets out what Natural Resources Canada says about how ice dams form, where to look for them, what membranes do and do not do, the quick fixes and why NRCan says they fall short, how to deal with ice in the eavestrough, and what insurance does with water an ice dam lets in. The roof leak itself is on the roof leak page; this one is about the dam.

What the roofer will call it

How an ice dam forms

NRCan’s explanation turns the usual assumption around. An ice dam is not caused by too much snow; it is caused by heat in the wrong place. Uninsulated attics, NRCan says, usually do not get ice dams, because the snow melts as it lands. A well-sealed and insulated attic gives a cool roof and generally does not either. The dam forms in between: enough heat reaching the roof deck to melt the snow, and a cold overhang to refreeze it.

The roof’s shape matters too. NRCan says ice dams are more frequent where valleys and dormers complicate the roof or there is a large overhang, because complicated roofs hold more snow and have more places for heat to leak out.

Where to look for them

NRCan’s method costs nothing. After the first heavy frost in the fall, or the first light snow, go outside and see where the roof clears first. That spot is where heat is reaching it. On a 1½ storey house it is often a straight horizontal line where the short knee wall meets the ceiling. Elsewhere it is above a roof-ducted exhaust fan, a plumbing vent, a skylight or an attic hatch that leaks air. Each of those is a place to seal from inside the attic before winter — and the same air leaks are the ones that cause attic condensation and mould.

Removing one, and what not to do

NRCan lists the quick fixes and the catch with each. Chopping at a dam with an axe or an ice pick damages the shingles you are trying to protect. Electric heating cables melt channels for the water to escape through, sometimes enough to help, at a significant electricity cost. Taking the eavestroughs down stops them filling with ice but sends the roof’s water back against the foundation. And raking snow off the roof helps but is not always easy or safe. Contractors offer steaming, which melts the ice without the blade; this site found no published Canadian price to quote for it.

For ice in the troughs themselves, the federal flood guidance gives a method that keeps you off the roof:

Work with a partner, pour hot water over ice in the eavestrough or break it gently with a roof rake. The eavestrough page covers the troughs and downspouts the rest of the year.

Prevention: the attic, not the roof

The membrane is worth understanding precisely because it is the thing most people think solves the problem. It is a waterproof layer under the shingles at the eaves, and building codes require it on the lower roof of new houses. It keeps the water that backs up behind a dam out of the house. It does nothing to stop the dam forming, and NRCan notes the dam can still damage shingles and gutters.

NRCan’s preferred solution for most houses is the one that removes the cause: keep house heat out of the attic by air sealing and insulating. Where the attic is too tight at the eaves for full insulation, it suggests the strongest insulation possible at that edge, with a gap kept for ventilation against the sheathing. Ventilation itself helps keep the roof cold — NRCan describes a colder, well-vented attic as less prone to ice dams — but it is the second line of defence, not the first.

What insurance does with ice dam water

IBC draws the line at repetition, not at ice dams. The first time melt water backs up under the shingles and comes through a ceiling, standard policies commonly cover the sudden damage. Ongoing or repeated ice dam damage is treated as a long-term maintenance issue. That makes the first claim the moment to fix the cause in the attic, because the second one may be argued as maintenance. IBC also says basic policies might not cover ice dam damage at all, so read your wording.

Questions people actually ask

Where to read next

• Roof leaks
• Attic mould and condensation
• Eavestroughs and downspouts
• Frozen pipes
• Twelve water situations compared