Answers · Is something wrong?
What causes ice dams, and how do I stop them?
The short answer
Heat escaping into the attic melts snow on the upper roof, the water runs down to the cold eave and refreezes, and the resulting ridge of ice backs water up under the shingles. The cause is air leakage and insulation, not the roof, and raking snow or chipping ice treats the symptom.
The three-part fix, in order
- Air sealing. Stop warm air getting into the attic: the attic hatch, recessed lights, plumbing and wiring penetrations, and ducts. This does more than anything else and it is usually the cheapest.
- Insulation. Bring the attic floor up to a consistent depth, with no gaps at the eaves where it is usually thin.
- Ventilation. Soffit intake and ridge exhaust working together keep the deck cold and even. Ridge vent without soffit intake does very little.
What the roof can contribute
A self-adhering ice and water membrane at the eaves is the defence against the water that gets behind the dam, and it is required by code in many cold-climate jurisdictions. It does not prevent dams; it stops the ones that form from getting into the house. Specify it at reroof time and ask how far up the slope it runs.
Also asked
- Do heated cables fix ice dams?
- They manage them rather than fix them: a channel melted through the ice lets water off the roof, which is useful on a building where the underlying heat loss is hard to correct. They run on electricity every winter and they do not address the cause, so they are a mitigation rather than a repair.
General information about roofs and about hiring, not advice about your building. Roofing contractor licensing, permits, insurance practice and deductible rules are set state by state and province by province across the United States and Canada, so your own authority is the current answer.