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Ice dams: why the roof leaks when the snow melts

An ice dam is not really a roofing failure. It is heat escaping into the attic, melting snow in the middle of the roof and refreezing it at the cold edge, until the backed-up water finds a way in.

A heat problem that looks like a roof problem

Ice dams have a reputation as a mysterious winter hazard, and they are actually one of the most mechanically understandable problems on this site. Snow sits on the roof. Heat escaping from the house warms the roof deck in the middle of the slope, above the heated space. Snow there melts. The meltwater runs down toward the eave, which overhangs unheated outside air and is therefore several degrees colder, and it refreezes. Do that for a few days and you build a ridge of solid ice along the eave.

What makes it a leak is what happens next. Water pooling behind that ridge is standing water on a roof designed to shed running water. Shingles are lapped to handle water moving downhill; they are not a waterproof membrane, and standing water works its way back up under the laps by capillary action and gets onto the deck. That is why ice-dam leaks so often appear at the top of an exterior wall or in the corner of a room, and why they can appear in a roof that is in perfectly good condition.

The giveaway is timing. Ice-dam water comes in during a thaw, or on a sunny afternoon after a cold night, and stops when it refreezes. If a ceiling gets wet in January in an otherwise sound roof, this is the first thing to consider.

Because it is a heat problem, the durable answers are heat answers: air sealing the ceiling plane so warm air stops leaking upward, insulation that is actually continuous, and ventilation that keeps the deck cold. Heated cables and the rest of the winter hardware manage the symptom; they do not stop the melting.

And a caution that belongs at the top of the page rather than the bottom. Ice-dam season produces an enormous amount of advice about getting onto a snow-covered roof with a hammer, an axe or a shovel. A roof covered in snow and ice is one of the least safe places you can stand, chipping at ice reliably damages the shingles underneath it, and a roof rake used from ground level does most of the useful work with none of the risk. We do not publish instructions for going up there. We do explain what a thaw is telling you, what makes next winter better, and where the line is.

Ice dams guides

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.