Ice dams and gutters: what causes them and what cleaning can and cannot do

Updated November 10, 2026 · KeystoneProGuide editorial team

Do clogged gutters cause ice dams?

No. A University of Massachusetts building science article puts it directly: contrary to popular belief, gutters do not cause ice dams. The Department of Energy's Building America guidance says the same. Ice dams start on the roof, not in the gutter, and cleaning gutters is not the cure for them.

Gutters are still part of the story, because an ice dam forms at the roof's edge, exactly where the gutter hangs. The sections below explain the cause, the damage and what helps.

What causes an ice dam?

Heat escaping from the house. The UMass article says three things are needed: snow on the roof, heat to melt it and cold to refreeze the water. Heat lost from the living space warms the upper part of the roof, the snow there melts, and the water runs down under the snow until it reaches the overhang. The overhang is not warmed by the house, so the water refreezes there and builds a ridge of ice. More meltwater pools behind it.

North Dakota State University Extension adds that ice dams and icicles are most common when snow is on the roof, heat is being lost through it and the outside temperature is just below freezing. It points to the places where heat leaks into the attic: unsealed openings for plumbing, heating and wiring, recessed lights, and attic access doors. The UMass article says ice dams can form with as little as 1 or 2 inches of snow if the roof is poorly insulated and air-sealed and the snow is followed by several days below freezing.

What do ice dams do to gutters and the house?

Water trapped behind the ice can work under the shingles. NDSU Extension says it can leak into the attic or eaves and lead to water-stained ceilings, structural damage and mold growth inside the home.

The UMass article lists the visible damage as water-stained ceilings, dislodged shingles, "sagging ice-filled gutters", peeling paint and damaged plaster. The hidden damage is wet insulation, wet wall cavities, rot and mold, and peeling paint that can appear long after the ice has gone. On gutters in particular, it says that while they do not cause ice dams, they do concentrate ice and water at a vulnerable part of the roof, and as they fill with ice they often bend and rip away from the house, bringing fascia, fasteners and downspouts with them.

The Building America guidance adds a related point for places with heavy snow: gutters can fill with snow, which may freeze and become part of an ice dam that is already forming.

What actually prevents ice dams?

Keeping the whole roof cold. Both extension sources agree on the cure:

• Insulation to slow heat moving from the house into the attic.
• Air sealing of the openings where warm air leaks into the attic.
• Ventilation, ideally from soffit to ridge, so the roof deck stays near outdoor temperature.

NDSU Extension's engineer put it this way: the key is "sufficient attic insulation and enough ventilation to keep the roof near outdoor temperatures", and homeowners too often treat the symptoms with things like heat tape instead.

The building code adds a second line of defense for new roofs. The 2021 International Residential Code, which Pennsylvania adopts, requires an ice barrier under the roof covering in areas with a history of ice forming along the eaves and backing up water, extending from the edge to at least 24 inches inside the wall line. That is a roofing matter, but it is worth asking about if you are replacing a roof on a house that has had ice dams.

What about heat cables?

The UMass author advises against them. He writes that heat tape treats the symptom rather than the cause, often creates an ice dam just above the cable, can make shingles brittle, creates a fire risk and is costly to install and run. NDSU Extension also lists heat tape as treating the symptom.

What can you do safely in winter?

• Rake snow from the lower roof from the ground. NDSU Extension says removing snow from the lower part of the roof reduces ice dams and icicles near the gutter, but that roof snow removal can be dangerous, so use a roof rake rather than getting on the roof, and watch for overhead power lines. Pennsylvania's Public Utility Commission says to stay at least 10 feet from them.
• Do not chip the ice. The UMass article says people attack roof ice with hammers, shovels, picks and worse, and that more damage is done to "life, limb and roof" in the process.
• Clear the gutters before winter. Building America advises making sure gutters are clean as winter approaches in snowy places, so they drain rather than fill with water that freezes.
• Look for leaks. The U.S. Environmental Protection Agency (EPA) says not to ignore wet spots or water stains and to have leaks fixed quickly, or the problem gets worse.

How likely are ice dams in southeastern Pennsylvania?

The sources read do not say. The 2021 energy code places Bucks, Chester, Delaware, Montgomery and Philadelphia counties in climate zone 4A, while counties such as Berks, Lehigh and Northampton are in 5A, but no page read links those zones to ice dam risk. The residential code's own test is history: its ice barrier rule applies where ice has formed along the eaves before. If your house has had heavy icicles and ice at the eaves, treat that as your answer.

For the gutter side of winter, how often to clean covers timing and warning signs covers damage to look for after a freeze. Local businesses are on the gutter cleaning directory page.

Sources

1. Preventing Ice Dams, University of Massachusetts Amherst, Building and Construction Technology
2. Reduce ice dam and icicle damage, North Dakota State University Extension
3. Gutters and Downspouts, U.S. Department of Energy, Building America Solution Center
4. 2021 International Residential Code, Chapter 9: Roof Assemblies, International Code Council
5. 2021 International Energy Conservation Code, Chapter 3: General Requirements, International Code Council
6. Text Version of the Mold House Tour, U.S. Environmental Protection Agency
7. Power Up Your Safety: PUC Essential Electric Tips for Pennsylvania's Warmer Months, Pennsylvania Public Utility Commission
8. A Brief Guide to Mold, Moisture and Your Home, U.S. Environmental Protection Agency