An ice dam forms when heat from the house melts snow on the roof from below, and the meltwater runs down to the cold eave, where it refreezes into a ridge. The ridge holds back the next melt, which can work under the shingles and into the walls. The fix addresses the heat leaking into the attic as well as the ice at the edge.
Key takeaways
- Heat from the living space that reaches the attic warms the roof boards and melts the snow above them.
- Meltwater runs to the eave, where the overhang is cold, and refreezes into a ridge of ice.
- The ridge holds back the next melt, which can work under the shingles and reach the walls.
- Removing the ice without fixing the attic usually means the dam forms again.
- The attic check looks for frost on the boards, wet insulation and gaps around the hatch and the light fittings.
- Ice should be loosened from the ground with a plastic rake, never chipped with a metal tool from the roof.
How does an ice dam form on a roof?
An ice dam forms in a cycle: snow melts on the warm upper roof, the water runs to the cold eave and refreezes, and each new melt is held behind the growing ridge. The cause is the heat that melts the snow from underneath.
The eave overhangs the walls and is not heated by the house, so it stays colder than the roof above it. Water that reaches the eave in a thaw freezes there when the temperature drops, and the ice grows outward and upward as more water arrives.
The dam is the consequence, not the source. The source is warm air moving from the living space into the attic and warming the underside of the roof boards, which melts the snow above from below.
Why does attic heat matter more than the ice itself?
Attic heat keeps the roof surface warm enough to melt snow even on a cold day. Removing the ice clears the symptom, but the heat keeps feeding the melt that forms the next dam.
Heat reaches the attic through gaps around light fittings, the attic hatch, plumbing stacks and wiring, and through insulation that is thin or displaced. Warm, moist air from a bathroom or a kitchen can add to the load if the fans vent into the attic.
A roof with warm spots on the boards will melt snow in patches, which often shows as bare strips above a frozen eave. That pattern is a sign that the attic needs attention, and it is the clue a technician looks for first.
Look at the attic hatch and the ceiling near it, since a warm, moist draft at the hatch is a common heat path. A simple check is to hold a thin strip of tissue near the hatch edge on a cold day and see whether it moves.
How does the snow itself feed an ice dam?
Snow is the source of the water. A deep, wet snowpack gives the melt more volume to work with, and repeated thaws keep feeding the ridge through the winter.
Wet snow that sits on the roof through a cold spell can melt partly and refreeze, forming a dense layer that holds water against the surface. Each thaw adds to it, and the ridge at the eave can grow over several days.
The Mohawk Valley sees both heavy snow and sharp swings in temperature, and both feed the pattern. A local television news report recorded 18.3 inches of snow in Utica in January 2025, and snow of that depth can feed the eave for days after it falls.
What does an ice dam do to a roof and a house?
A dam holds meltwater at the eave, which can work under the shingles, reach the decking and run down the walls. It can stain ceilings, rot the fascia and the wall sheathing, and in time damage the insulation and the plaster.
Water under the shingles is a common route into the house, because the lowest courses are the least protected. The water reaches the decking, wets the insulation and then drips inside, often at a corner or near an outside wall.
A dam can also pull gutters away from the fascia, since the weight of the ice and the water is carried by the brackets. A bent gutter is a sign that the ice has been heavy enough to damage the roof edge.
Ice at the eave can also shift the gutter brackets over several cycles, so a gutter that once sat level may now slope back toward the wall. Check the gutter line for a change in its slope, since that change holds water at the eave and feeds the next dam.
How can a homeowner tell an ice dam from a simple icicle?
An ice dam is a ridge that builds along the lower edge of the roof and holds water behind it, while an icicle hangs from a single point. The water marks on the wall below the ridge, and any stain inside, show the dam's effect.
Look at the eave from the ground after a thaw and a freeze. A ridge that runs along the full width of the eave, with water marks on the wall beneath, is a dam. A single icicle at a gutter outlet is usually a much smaller matter.
Note the date when the ridge appears, the depth of the ice and any stain inside the house. Those notes help a technician decide how urgent the visit is and where to look in the attic.
What should a homeowner avoid when the ice forms?
Avoid climbing onto an icy roof, avoid metal tools on the ice and avoid melting products or open flames at the eave. Each of these risks injury or damage to the shingles, the gutters or the house.
A plastic or foam rake used from the ground can pull the lowest snow down the slope and reduce the weight at the eave. The rake should not touch the shingles, and the work stops once the pull is no longer safe.
Calcium and other melting products can damage shingles, gutters and the plants below, and they do not address the heat that feeds the dam. A technician can advise on the attic work that reduces the cause.
Keep a record of when the ridge forms, how long it stays and when it melts. That record helps a technician match the dam to the weather and to the heat paths in the attic, and it shows whether the attic work is reducing the problem over time.
What should be checked before the first freeze?
Before the first freeze, check the attic for warm spots and frost, clear the gutters and downspouts, check the insulation depth and seal the gaps around the hatch and light fittings. A roof with these steps done is far less likely to form a dam that feeds a leak.
Clear the gutters and check that each downspout drains well away from the wall. A gutter that holds water at the eave adds to the ridge, and a downspout that freezes at the outlet holds water in the gutter.
Look at the soffit vents and make sure they are not blocked by insulation or paint. Blocked vents keep the attic warm, and a technician can rebalance the intake and the exhaust if the plan calls for it.
A homeowner who has had a dam before should keep the earlier notes with the new ones. A pattern that repeats in the same place, year after year, points to a specific heat path or a specific eave detail, and the technician can then look at that spot first.
When should a homeowner call a technician about an ice dam?
Call when a ridge of ice forms along the eave, when a stain appears inside after a thaw, when the attic shows frost or wet insulation, or when a gutter has pulled away. A technician can check the attic and the eave and plan the fix.
A visit can find the heat loss that feeds the dam and recommend the attic work that reduces it. The findings are written with photographs, and each recommendation is explained with its scope before any work begins.
The homeowner can help by describing the ice, the stains and the weather over the last few weeks. The more precise the description, the better the technician can plan the first visit.
Ask the technician to note the pitch and the shape of the eave on the visit. A low pitch and a long, flat overhang hold water differently from a steep roof with a short eave, and the plan is adjusted to the shape.
Frequently asked questions
Does a Mohawk Valley roof get more ice dams than others?
This page does not claim a local rate, because none is sourced for the valley. What it can say is that heavy snow and sharp freeze and thaw swings feed the mechanism that forms a dam.
Can I stop an ice dam by raking the snow off?
Raking the lowest snow from the ground can reduce the weight at the eave, but it does not stop the heat that melts the snow. The attic has to be addressed for a lasting fix.
Are heat cables a good idea?
Heat cables are used by some homeowners, but they add energy use and can fail or start a fire if installed poorly. A technician can explain the options and whether the attic work would be a better first step.
Why does the leak appear inside the house after the dam forms?
The dam holds water at the eave, which works under the shingles and through the joints. The water reaches the decking, wets the insulation and then drips inside, sometimes days later.
Should I call the insurer if an ice dam has damaged the house?
Whether a claim is covered is decided by the homeowner's policy and the insurer. Keep dated photographs and a written note, and ask the insurer what it needs before any permanent repair.
Does the attic work need a permit?
Whether attic work needs a permit is for the building office that covers the address to confirm. Ask before any structural change, and keep the answer with the written scope.
How long does an ice dam usually last?
It depends on the weather. A dam can clear in a few days of warm weather, or it can hold through a cold spell and rebuild with each thaw. The record of its duration helps a technician plan the attic work.
This guide is general information for Oneida County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.