How do you prevent ice dams on a roof?
By Best Roofing Answers · Published May 2026 · Updated July 2026
Ice dams are prevented by (1) sealing every attic-to-house air leak, (2) upgrading attic insulation to R-49 or higher per IECC climate zones 5–7, (3) balancing soffit-to-ridge ventilation so the roof deck stays cold, and (4) installing an ice-and-water shield underlayment at eaves per IRC R905.1.2 — heated cables are a last resort, not a fix.
The physics
Per DOE and IECC guidance, ice dams form when the attic side of the roof deck is warm enough to melt snow while the eave is cold — usually because heat is escaping the living space into the attic. Insulation slows the heat loss; ventilation removes what escapes; ice-and-water shield contains any water that does back up.
IRC R905.1.2 requires an ice-and-water shield from the eave to a point at least 24" inside the exterior wall line in cold-climate zones — a code minimum that solves the leak even when the dam still forms.
Sources
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