What Are Ice Dams? Causes and Warning Signs
Quick answer
An ice dam is a ridge of ice that forms at the edge of a roof and stops melting snow from draining off. It happens when heat escaping into the attic melts snow higher up the roof, and that water refreezes at the cold eaves. The trapped water then backs up under the roofing and can leak into the home.
What are ice dams on a roof?
An ice dam is a wall of ice that builds up along the eaves — the lower edge — of a roof in cold, snowy weather. Snow on the roof melts, the water runs down toward the eave, and there it refreezes into a growing ridge of ice. Because the ice blocks the roof's edge, the next round of meltwater has nowhere to go and pools behind the dam. It is a common winter problem on homes in snow country, and while the ice itself looks harmless, the trapped water behind it is what causes the trouble.
What causes ice dams?
A temperature difference across the roof. Warm air escaping from the living space into the attic heats the underside of the roof deck, melting the snow sitting on the upper roof. That meltwater trickles down to the eaves and gutters, which overhang the exterior wall and stay at outside temperature — well below freezing — so the water refreezes there. Repeat over days of snow and cold, and the ice ridge grows. The true root cause is heat loss into the attic from poor insulation, air leaks, and inadequate ventilation, which is why ice dams are ultimately an attic problem, not just a weather one.
What do ice dams look like?
The classic signs are a thick band of ice built up along the roof's edge and large icicles hanging from the eaves and gutters — those dramatic icicles are often the first visible clue. You may also see ice filling the gutters, and, inside the home, water stains or drips on ceilings and walls near exterior walls, or peeling paint, which mean water has already backed up under the roofing. Big icicles plus interior water stains after a snowy cold spell are the warning combination to act on.
Why are ice dams a problem?
Because water always finds a way in. The meltwater trapped behind an ice dam sits on the roof and works its way up under shingles or into seams, then leaks into the roof deck, attic, insulation, ceilings, and walls — causing stains, rot, ruined insulation, and mold. The weight of heavy ice can also tear off gutters, and falling icicles are a genuine safety hazard to people below. Left unaddressed, a single bad winter of ice dams can cause thousands of dollars of interior and structural damage. Our guides to preventing and removing ice dams explain what to do.
Frequently Asked Questions
How do ice dams form on a roof?
Heat escaping into the attic warms the upper roof and melts the snow on it; the meltwater runs down to the cold eaves and refreezes into a ridge of ice. As the cycle repeats, the ice dam grows and traps more water behind it. It takes snow on the roof, a warm roof surface above, and freezing eaves below — all three together.
Are ice dams dangerous?
They can be, in two ways. The trapped water can leak into your home and cause hidden rot and mold, and the heavy ice and large icicles they create can fall and injure people or damage gutters below. The ice on the roof is a symptom; the water damage inside and the falling ice are the real dangers.
When do ice dams form?
They form during cold, snowy weather — typically when there is snow on the roof and outdoor temperatures are below freezing while the attic and upper roof are warmer. They are most common after a significant snowfall followed by a stretch of sub-freezing days, and in climates with repeated freeze-thaw cycles.
Sources
- US Department of Energy — Attic insulation, air sealing and ice dams
- University of Minnesota Extension — Ice dams: causes and prevention
Explore Interlock Metal Roofing
Last reviewed 2026-07-12 · Reviewed by Scott Plumptree, Director of Marketing