Quick answer
Ice dams form when heat from the house warms the upper roof above 32°F, melting snow that refreezes at the colder eaves. DOE lists the fix, and University of Minnesota Extension agrees on the first two steps: first seal the air leaks from the house into the attic, then add attic insulation (Minnesota Commerce and NDSU suggest about R-50), and ventilate the roof to keep it cold. For an ice dam that is already there, rake snow off the lower roof from the ground and call a professional if water is coming in.
Why ice dams form
University of Minnesota Extension explains that an ice dam needs snow on the roof while the higher part of the roof surface is above 32°F and the lower part is below it. Water melts on the warm upper roof, runs down, and freezes when it reaches the cold eaves, building a dam. DOE's Building America Solution Center says ice dams can form with as little as 2 inches of snow on the roof.
The heat comes from the house. UMN Extension says that in many homes, warm air leaking into the attic is the main way that heat gets there; heat conducted through thin insulation, chimneys, leaky ducts and exhaust fans that end near the roof add to it.
What the damage looks like
Water backed up behind the dam can leak under the shingles and into the house, damaging walls, ceilings and insulation, and UMN Extension warns the moisture can lead to mold and mildew.
How to prevent ice dams
DOE lists the three best ways, and calls air sealing the most important: fully air seal the ceiling, thoroughly insulate the attic, and ventilate the roof.
- Seal air leaks into the attic. DOE calls this the most important step. The Minnesota Department of Commerce recommends sealing them with caulk or expanding spray foam, and a Michigan township building department's handout says bath, kitchen and dryer fans must never vent into the attic.
- Add insulation. Minnesota Commerce recommends attic insulation of at least R-50 where space allows, and NDSU Extension says attics in northern climates need roughly 15 inches of blown-in insulation, about R-50. Check the minimum for your area with the climate zone lookup and work out how much to buy with the insulation calculator.
- Ventilate the roof. DOE says venting the roof in cold climates keeps it cool and limits the melting that leads to ice dams. UMass Amherst's Building and Construction Technology program calls soffit-to-ridge ventilation the most effective way to cool the roof deck. Size it with the attic ventilation calculator.
Order matters. UMN Extension says that if the air sealing and insulation are done well, only small amounts of roof ventilation are needed, and the Minnesota Department of Commerce says ice dams are not caused by roofing, ventilation or gutter problems but by a house that leaks warm air. A contractor should test the house with a blower door to find the leaks, UMN Extension advises.
If you have an ice dam now
- Remove snow from the lower roof. NDSU Extension suggests a roof rake so you stay off the roof, and warns to watch for overhead power lines. UMN Extension notes a rake can damage roofing, and the Minnesota Department of Commerce suggests hiring a professional if the job needs a ladder or getting on the roof.
- If water is coming in, UMN Extension says a channel cut through the dam, for example with tap water on a warm day, lets the trapped water drain for a few days. This is ladder work on ice, so have a professional do it.
- Hire a professional for anything on the roof. The Minnesota Department of Commerce suggests an ice dam removal service that uses steam if the dam is already causing water damage. UMN Extension warns that anyone on the roof in winter, or working on it from below, risks injury, and advises hiring professionals.
- Wait to repair the inside until ceilings and walls are dry, says UMN Extension.
What not to do
- Don't chip at the ice. The Minnesota Department of Commerce says not to remove ice with chippers, chemicals or heat, and a Michigan township building department's handout warns against chipping away an ice dam.
- Don't rely on salt on the roof. The same handout says not to routinely use salt or calcium chloride to melt roof snow, because they corrode gutters, downspouts and flashing.
- Don't expect heat cables or more vents to fix it. Minnesota Commerce advises against installing heating cables and against adding roof vents, including powered vents, saying they will not eliminate ice dams and often make the problem worse. UMN Extension does not recommend mechanical attic ventilation for ice dams.
- Don't blame the gutters. UMass Amherst's program says gutters do not cause ice dams.
The code requirement for new roofs
Where there is a history of ice forming at the eaves, section R905.1.2 of the 2021 International Residential Code requires an ice barrier under asphalt shingles and several other roof coverings, from the roof edge to at least 24 inches inside the exterior wall line, and at least 36 inches up the roof on slopes of 8 in 12 or steeper. It limits damage from a dam but does not stop one from forming. Your building department knows whether your area requires it.
Frequently asked questions
What causes ice dams?
Heat leaking from the house warms the upper roof above freezing, melting the snow. The water refreezes when it reaches the colder eaves, and the ice traps more water behind it.
What is the best way to prevent ice dams?
Seal air leaks from the house into the attic first, then add insulation (about R-50 in cold climates, according to Minnesota Commerce and NDSU) and ventilate the roof to keep it cold, as DOE recommends, with air sealing the most important step.
Should I put salt on my roof to melt an ice dam?
Government sources advise against it. The Minnesota Department of Commerce says not to remove ice with chemicals, and a Michigan township building department warns that salt and calcium chloride corrode gutters, downspouts and flashing.
Do heat cables prevent ice dams?
The Minnesota Department of Commerce advises against installing them, and the UMass Amherst building program calls heat tape a waste of time and money. Fixing air leaks and insulation addresses the cause.
Will more attic vents stop ice dams?
Not on their own. Ventilation helps keep the roof cold, but UMN Extension and Minnesota Commerce say warm air leaking into the attic is the main cause, and Minnesota Commerce warns that added vents, including powered vents, often make the problem worse.
Calculators for this
Attic Ventilation
How much attic ventilation do you need?
Insulation
Get batt bags for walls, floors and attics, or bags of blown-in fiberglass or cellulose to reach your target R-value, by attic size.
Roof Snow Load
How much does the snow on your roof weigh?
Published 2026-11-05. Sources: University of Minnesota Extension, Dealing with and preventing ice dams; DOE Building America Solution Center, Attic air sealing, insulating and ventilating for ice dam prevention; Minnesota Department of Commerce, ice dam consumer bulletin (Feb 25, 2019); NDSU Extension, Reduce ice dam and icicle damage (Jan 4, 2023); UMass Amherst Building and Construction Technology, Preventing ice dams (archival); Muskegon Township (MI) building department, Ice dams handout; 2021 International Residential Code, section R905.1.2 (Colorado adoption, via UpCodes). See how we calculate.