How an Ice Dam Actually Forms
An ice dam is a symptom of a warm attic, not a cold roof. Here's the chain of events during a Western Washington cold snap:
Warm air leaks from the living space into the attic, warming the underside of the roof deck. Snow on the upper, warmer part of the roof melts. That meltwater runs down the slope toward the eaves — which extend past the exterior wall and stay cold because there's no warm attic beneath them. The water refreezes at the eave, building an ice dam that pools water behind it. That pooled water has nowhere to go but up under the shingles, and into your home.
Why Heat Cables Aren't the Answer
Electric heat cables along the eaves are a common band-aid, but they treat the symptom, not the cause. They melt channels through the ice dam, but the dam keeps reforming as long as the attic stays warm. They also add electricity cost, can create fire hazards if installed poorly, and do nothing to stop the water already pooling behind the dam.
The real fix addresses why the attic is warm in the first place.
The Three-Part Permanent Fix
Preventing ice dams means keeping the attic cold so snow melts evenly or not at all, plus sealing the eaves against any water that does back up:
- Balanced attic ventilation — soffit intake and ridge exhaust create continuous airflow that keeps the deck cold and flushes out warm air.
- Proper insulation — enough attic insulation (and air sealing of penetrations) keeps heat from the living space out of the attic in the first place.
- Ice-and-water shield at the eaves — a self-adhering membrane installed at least 2 feet past the exterior wall seals the eave so any backed-up water can't reach the deck.
The Best Time to Address Ice Dams
All three fixes are easiest to install during a roof replacement, when the deck is exposed. Adding ice-and-water shield to an existing roof requires removing the bottom courses of shingles, and upgrading ventilation and insulation can be done anytime but is simplest when the roof is open.
If you're replacing a roof in the Puget Sound, make sure your contractor includes ice-and-water shield at the eaves, valleys, and around penetrations — not just the minimum felt. It's the single most effective defense against ice dam leaks and the wind-driven rain that causes most of our leaks.
