Ice dams in Ottawa: why they form and how to stop them

Every February, Ottawa homeowners watch ice build along their eaves, see a stain appear on a bedroom ceiling, and call a roofer. The roofer replaces shingles. Next February it happens again. This is because an ice dam is almost never a roofing defect — it is an attic defect that expresses itself on the roof.

The mechanism

Heat leaks from the living space into the attic. The attic warms. Snow on the heated part of the roof melts, even at −15 °C outside. The meltwater runs down the roof surface until it reaches the eave — which overhangs unheated space and is therefore at outdoor temperature — and it refreezes there. That ridge of ice is the dam. Subsequent meltwater pools behind it, and because shingles shed water rather than seal against standing water, it works its way under them and into the building.

So the chain is: air leakage and insulation gaps → warm attic → melt → refreeze at the cold eave → water backing up under shingles. Every effective fix targets the first link.

What the damage looks like

• Ceiling staining, usually along an exterior wall rather than in the middle of a room.
• Peeling paint and damp drywall at the top of exterior walls.
• Rotted soffit, fascia and roof sheathing at the eaves.
• Wet insulation in the attic, which then stops insulating and accelerates the whole cycle.
• Mould growth inside wall and ceiling assemblies — the expensive, invisible outcome.
• Gutters torn loose by ice weight, and icicles heavy enough to be genuinely dangerous below.

An infrared scan finds ice dam damage that a visual inspection misses entirely, because wet insulation and wet framing read cold long after the surface has dried and the stain has been painted over.

The fixes that actually work

1. Air sealing — the one that matters most

Warm air escapes into the attic through every unsealed penetration: pot lights, bath fans, plumbing stacks, the attic hatch, top plates of interior walls, and chimney chases. Sealing these is more important than adding insulation, and it is routinely skipped because insulation is the thing people know to buy.

2. Insulation

Current practice for an Ottawa attic is around R-60. A great many houses here sit at R-20 to R-30, and many that were topped up have gaps at the eaves where wind has scoured the insulation back. Coverage matters as much as depth.

3. Ventilation

A properly vented attic stays close to outdoor temperature, which prevents melt in the first place. That needs balanced intake at the soffits and exhaust at or near the ridge. The most common failure we find is soffit vents blocked by insulation that was blown in over top of them — the vents exist, they are simply buried.

4. Fix the things dumping heat and moisture into the attic

Bath fans and dryer vents that terminate in the attic instead of outdoors. Uninsulated ductwork running through the attic. Recessed lights that are not rated for insulation contact.

5. Ice and water membrane — at the next roof replacement

A self-adhering membrane along the eaves does not prevent ice dams; it prevents them from getting into the house. It is required practice in this climate and it is your insurance policy on the attic work.

Which Ottawa homes are most affected

Houses built before roughly 1990 with original insulation levels. Storey-and-a-half homes, which are common in Westboro, Alta Vista and the older core, and which have notoriously difficult-to-insulate knee wall assemblies. Homes with complex rooflines, valleys and dormers, where snow accumulates and the geometry creates cold spots. And any house where someone has added attic insulation without also air sealing — which increases the temperature difference across the ceiling without stopping the air leakage driving it.

Frequently asked