Heat loss melts snow mid-slope and the cold eave refreezes it. Why that makes a sound roof leak, and what actually changes it.
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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 We do not publish instructions for going up there.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
Why chipping, salt and climbing make it worse, what is safe from the lawn, and why removal is a patch.
A roof rake from the ground does one job well. Most roofs need nothing more, and some signs mean call an engineer.
Why heavy icicles mean heat is leaking into the attic, and why knocking them down is the wrong reflex.
Shingles, gutters, wet insulation and ceiling stains: what each sign looks like and how to check from the ground.
Air seal, insulate and ventilate so the deck stays cold, and why cables and salt only treat symptoms.
A sticky backup layer under the shingles at the eaves, and why it does not stop ice dams from forming.
How to confirm it from inside, limit the damage safely, and fix the heat loss behind it.
Find the heat leak, seal, insulate, ventilate, and skip the fixes that only treat the ice.
What buys you a few days, what stops the dam for good, and which one you need right now.
What a gutter full of ice tells you, the damage to watch for, and why chipping it out is the wrong move.
Patchy melt, nighttime melt and bare stripes: how to tell harmless sun from heat loss, from the ground.
Why a downspout freezes, what the overflow tells you and why boiling water is the wrong move.
Why flat roofs hold snow longer, the signs of overload from inside, and why you stay off the roof.
What a heat cable melts, where it fails and why the attic comes first.
The route ice dam water takes through insulation, ceilings and walls, and what to do first.
How to tell a harmless glaze from an ice dam by looking from the ground.
How a steam crew works, what to ask before booking and what you can do from the ground.
Check for leaks first, then decide whether to wait or call a roofer.
From watching and raking to steam crews, and the three methods to skip.
Four usual causes, and what the location of the drip tells you.
Why the ice collects at the gutter and what its pattern tells you.
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.
- NOAA / National Weather Service. Winter — National Weather Service safety hub
- NOAA / National Weather Service. What To Do If You're Caught in a Winter Storm