What is an ice dam?
Updated August 2026
An ice dam is a ridge of solid ice along the cold edge of a roof that traps the meltwater running down behind it until that water backs up under the shingles and into the house. It builds when the upper roof deck sits above 32°F and melts the underside of the snowpack, while the eave, which overhangs unheated space, stays below freezing. The melt runs down, hits that cold band, and refreezes. Every freeze-thaw cycle adds another layer. Shingles shed running water; they do not seal against a standing pond, so once the trapped water is an inch or two deep it finds the nail holes.
Where it is
Look at the eaves first, especially on the north side where sun never clears the snow, then at valleys, skylight headers, chimney backs, and the roof over an attached garage. A row of thick icicles hanging off the gutter is the outdoor tell. Indoors, the evidence sits at the top of exterior walls: a stain that appears 12 to 18 inches in from the wall, bubbled ceiling paint, damp insulation at the eave line, or dark sheathing near the soffit when you shine a light into the attic. The real source is further inside, at the recessed lights, the bath fan duct dumping into the attic, the open attic hatch, and the wall top plates nobody ever air-sealed.
What it does
An ice dam does two kinds of damage at once. Water pushes back under the shingle course and soaks sheathing, framing, and insulation, which loses much of its R-value the moment it gets wet and starts growing mold within 24 to 72 hours. Meanwhile the ice itself weighs about 57 pounds per cubic foot, enough to tear a gutter off its fascia and split the fascia board doing it. The bills stack up in that order: $500 to $2,500 for ceiling drywall and paint, $1,500 to $4,000 to pull and replace ruined attic insulation, $1,000 to $3,500 for sheathing and shingle repair. A roof that dams every winter eventually needs all three at once. See what a slow roof leak costs when it runs for a couple of seasons.
Maintenance
The durable fix is in the attic, not on the roof. Air-seal every penetration, bring insulation to R-49 or R-60 in cold climates, and keep a clear soffit-to-ridge airflow path with baffles so outside air washes the deck cold. At the next reroof, insist on ice-and-water membrane running at least 24 inches inside the exterior wall line, which most northern codes already require; the reroof interval and expected shingle life tell you how soon that chance arrives. Between now and then, pull the first 3 to 4 feet of snow off the eaves with a $40 to $80 roof rake after any storm over 6 inches, and work the whole cold-weather list from the ground with your roof checklist.
What it costs to replace
A replacement ice dam runs $300–600 for emergency steam removal, or $1,500–4,000 to air-seal and re-insulate the attic so it stops happening.
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Frequently asked questions
Should I chip the ice off my roof?
No. A hammer, hatchet, or ice pick cracks cold asphalt shingles like glass, and you will trade a winter leak for a spring one. Standing on a ladder against an iced gutter is how people end up in the emergency room. Rock salt in a nylon stocking is the other bad idea: it stains shingles, corrodes gutters and flashing, and kills the plantings below. Rake snow from the ground, and hire a steam crew if water is already coming in.
Do heated roof cables prevent ice dams?
They melt a channel through the dam so water can escape, which protects the house without fixing the heat loss that caused it. Treat them as a stopgap on a problem eave, usually $12 to $20 per linear foot installed. Running costs are real: a 100-foot cable draws roughly 500 watts, about $40 to $60 a month if you leave it on all winter. Put it on a thermostat or a snow sensor, and spend the bigger money on attic air sealing instead.
Why does my neighbor get no ice dams and I do?
Their attic is colder than yours. Two identical houses on the same street can differ entirely because one has sealed top plates, sealed can lights, and open soffit vents, while the other leaks warm air into the attic all night. The snow tells on you. If your roof shows bare patches or the snow melts off days before every other roof on the block, that is heat escaping through the ceiling, and it is showing up on your gas bill too.
Does homeowners insurance cover ice dam damage?
Most standard policies cover sudden interior water damage caused by an ice dam, subject to a $1,000 to $2,500 deductible, and many also pay to tear off the ice when removal is needed to stop the loss. What they exclude is the underlying condition: gradual seepage over several winters, rotted sheathing, and the insulation and ventilation upgrade that would prevent a repeat. Photograph the icicles and the stains the day you find them, and file before you start repairs.