Ice dams form when warm attic air melts roof snow at the ridge and the meltwater refreezes at the cold eave. The fix is a three-part system: adequate attic insulation (R-49 to R-60 in cold zones) to keep heat out of the attic, balanced ridge-and-soffit ventilation (1:300 NFA ratio) to keep the deck cold, and ice & water shield extending 24 inches past the interior wall line at every eave. This guide explains each layer and links to the matching calculators.
An ice dam is a ridge of ice at the eave that stops meltwater draining. The water backs up behind it, gets under the shingles, and comes through the ceiling. It is caused by heat escaping into the attic, which makes it an insulation and ventilation problem rather than a roofing one.
Ice barrier at the eaves does not prevent ice dams — it prevents the damage when one forms. Code generally requires it from the eave edge to at least 24 inches inside the exterior wall line measured along the slope, and on a low-pitch roof with a deep overhang that means two courses of 36-inch membrane rather than one.
If your roof has had ice dams and the barrier does not extend that far, extending it at the next reroof is the single most valuable thing to specify.
Heat escaping from the house into the attic warms the roof deck, melting snow above it. The meltwater runs to the cold eave, refreezes, and the ice dam pushes water back under the shingles.
Air seal the ceiling plane, insulate to the target R-value for your climate, fit baffles so the soffit intake stays open, and balance intake against exhaust. It is an insulation and ventilation fix, not a roofing one.
No — it prevents the damage when one forms. Code generally requires it from the eave edge to at least 24 inches inside the exterior wall line, measured along the slope.
They melt a channel through the dam and stop the immediate damage, but they run all winter and treat the symptom. A roof rake used from the ground clears the snow that feeds the dam without the running cost.