Ice dams: why they form on Canadian roofs and how to stop them for good

An ice dam is a heat problem, not a roof problem

Nearly every ice dam follows the same script. Snow sits on the roof. Somewhere underneath the upper part of that roof, the attic is warmer than it should be. Heat moves up through the ceiling, warms the underside of the sheathing, and the bottom layer of the snowpack turns to water. The water runs down the slope until it reaches the overhang past the exterior wall. There is no heated space under an overhang, so the eave sits at outdoor temperature, and the water refreezes.

Repeat that for a week of sun and cold nights and the ice builds into a ridge along the eave, sometimes 100 to 200 mm thick. Now meltwater coming down the slope meets a wall of ice and stops moving. It ponds. Shingles shed water that is running downhill; they are not a tank liner, and standing water works its way under the laps and into the deck. From there it travels along a rafter or the top plate of the wall and shows up as a brown ring around a pot light, a damp patch at the top corner of a window, or paint lifting off an interior wall two metres below where it got in.

The ice is the symptom. The cause is warm air getting into a space that is supposed to be as cold as the yard.

How to tell whether you have one

Icicles alone prove little; a few on a sunny March afternoon are ordinary. What matters is a pattern that repeats in deep cold:

• A solid band of ice along the eave or in the gutter while snow further up the slope is still powdery.
• Bare or glazed patches in the snowpack over particular rooms, especially above a bathroom, a stairwell or a recessed light. Even snow cover is the sign of a cold, well-sealed attic.
• Icicles emerging from behind the gutter rather than over its lip, which means water is already getting behind the fascia.
• Water stains on the attic sheathing at the eave end of the rafters.

Compare your roof with the neighbours on a cold morning. Houses on the same street, built the same decade, facing the same way, make a decent control group. If yours is the only one with a bare ridge line, the heat loss is yours.

Where the heat is actually escaping

Homeowners usually assume the insulation is thin. Often it is, but air leakage does more damage than conduction, and it is the part people forget. Warm, humid indoor air rises and pushes through every unsealed gap in the ceiling plane. The usual offenders:

• The attic hatch. Frequently a sheet of plywood resting on trim with no weatherstripping and no insulation on top. A permanently open chimney.
• Plumbing stacks and wiring penetrations. A 90 mm hole cut for a 75 mm pipe leaks for the life of the house.
• Recessed lights. Older non-rated cans vent heat straight into the insulation.
• Bathroom and kitchen fans that terminate in the attic. Common in houses built from the 1960s through the 1990s, and the worst of the lot. A fan running twenty minutes after a shower delivers a large volume of saturated air straight under the roof deck. It must duct to the outside, with the duct insulated so it does not condense inside itself.
• Dropped soffits over kitchen cabinets and the top plates of interior partition walls.

The three fixes, in the order they should happen

1. Air sealing the ceiling plane

Everything else is less effective until this is done. Insulation slows conduction; it does nothing to stop a moving column of air. Blowing another 200 mm of cellulose over an unsealed ceiling buries the leaks without closing them, and the moisture they carry then condenses inside the insulation instead of above it.

2. Insulation, to a consistent depth, all the way to the edge

Two failures are typical. Depth comes first: attics in older houses across Ontario, Quebec and the Prairies were built to a standard that provincial energy requirements have long since moved past. Coverage at the perimeter comes second, where the slope closes down on the top of the wall. That corner is the coldest part of the ceiling and the thinnest part of the insulation, so it is where condensation appears first. Raised-heel trusses solve it in new construction; in an existing attic it means filling that wedge without plugging the air path above it.

3. Ventilation, to keep the deck cold

Once the heat sources are gone, ventilation carries off what remains and keeps the sheathing near outdoor temperature. Air enters low at the soffits and leaves high at the ridge, and both halves have to be open. Adding ridge vents while the soffits are packed with insulation does not increase airflow; it pulls conditioned air up out of the house through whatever gaps are left. Baffles hold a 40 to 50 mm channel open between the insulation and the sheathing.

What does not solve it

Heat cables melt a channel so water has somewhere to go. They treat the consequence, run on electricity all winter and fail quietly when a connection corrodes. A reasonable stopgap on one awkward corner, not an answer.

Roof raking removes the fuel. Pulling the first 1.5 to 2 metres of snow off the eave from the ground genuinely helps in the middle of a bad winter and is the safest thing you can do yourself, but it is maintenance, not a fix.

Chipping ice with an axe damages the roof covering and puts you on a ladder beside falling ice. To stop an active leak before help arrives, lay a nylon stocking filled with calcium chloride across the dam; it melts a drainage channel in a day or two. Avoid rock salt, which is hard on flashing, fasteners and the planting below.

More exhaust vents on their own is the classic mistake. Without matching intake at the soffit, extra openings short-circuit or depressurize the attic. Intake area should at least equal exhaust area.

Where the membrane fits in

Self-adhered eave protection, the rubberized membrane rolled along the bottom of the roof before the underlayment, is required in most of the country and is why many ice dams never reach the ceiling. It is a backstop, not a cure: the ice is still loading the gutters and prying at the drip edge. On a replacement it should extend far enough up the slope to land well inside the inner face of the exterior wall. Confirm the figure your municipality enforces, since it follows the National Building Code as adopted provincially.

Regional differences

Damming is worst where the air hovers near freezing while the sun is still strong: the snow belts east of Lake Huron and Georgian Bay, the Ottawa Valley, southern Quebec and much of Atlantic Canada. On the Prairies winter is colder and more stable, so melting is driven by escaping heat rather than weather, which makes attic sealing the whole story. In coastal British Columbia dams are rare at sea level and routine at elevation in the Interior.

Mid-winter, with water coming in

Catch the water, pull back the snow at the eave where you can reach it safely from the ground, and turn the humidifier down. Then book the attic work for a dry stretch in spring, when sealing, insulation and baffles can be sorted out in one visit. The roof itself may be entirely fine.