Ice Dams and Roof Ventilation

Here is what is happening up there.

Heat is getting into your attic. It warms the underside of the deck, the snow sitting on that part of the roof melts, and the water runs down to the overhang — which hangs past the heated part of the house and is therefore cold. It refreezes. A ridge of ice builds up at the eave, and the next lot of meltwater pools behind it until it works its way backwards under a shingle lap.

Shingles shed water running downhill. They do nothing whatsoever against water sitting still and being pushed uphill. So it comes in, runs along a rafter, and shows up as a brown ring on a bedroom ceiling several metres from where it started.

The roof did not fail. The attic did.

Four places the heat is getting in

• Recessed light fixtures. Older ones are essentially holes in your ceiling with a warm appliance sitting in them.
• A bathroom fan ducted into the attic instead of outside. Warm, wet air, delivered straight onto the cold underside of the deck.
• The attic hatch — very often a sheet of plywood with no weatherstrip, in the ceiling of a heated hallway.
• Insulation stuffed into the soffits. Usually done by somebody topping up the attic, and it blocks the air intake at exactly the point the ventilation path needs to start.

None of those is on the roof. That is rather the point.

Why your neighbour does not get them

Two identical houses on the same street, one damming every February and one never. It is almost always air sealing rather than luck — different owners, different renovations, different decades of somebody putting pot lights in a ceiling.

It also depends heavily on where you are. Stratford averages about 218 cm of snow a year and Listowel about 192, so cover sits on the roof for weeks. Chatham gets 79 and Leamington 86, and rarely holds it long enough for a dam to form at all.

Steam, and nothing else

If somebody turns up at your house in February with a hammer, a chisel or a pressure washer, send them away.

Chipping ice takes granules, shingles and quite often the eavestrough with it, and the damage does not appear until April — by which point nobody connects the leak to the person who was paid to stop one. Steam is slower and it costs more on the day and it is the only method that removes ice without removing your roof along with it.

Heat cable and salt pucks

Both manage a symptom. Heat cable earns its place on a geometry that genuinely cannot be ventilated — a valley over a cathedral ceiling, a dormer cheek with no attic behind it — and it costs you electricity every winter it runs. Neither one finds the leak that is causing the problem.