An ice dam forms when snow on the roof melts from heat escaping through the attic, then refreezes at the colder eave, where meltwater has nowhere to go. Stopping it starts in the attic, with insulation, air sealing and ventilation, rather than with anything done to the shingles.
Key takeaways
- An ice dam needs snow on the roof, an upper roof surface above 32 degrees Fahrenheit and a lower eave below it.
- Heat leaking from the house into the attic is what melts the snow from underneath.
- Insulation, air sealing and attic ventilation act on that heat, which is why they come before any work on the shingles.
- Pullman's published design criteria mark ice barrier underlayment as required, which is the sourced basis for treating eave ice as expected on the Palouse.
- Whether Moscow's adopted code requires an ice barrier is a question for the city's Building Safety Division, not a settled answer in this guide.
- Removing ice relieves water that is backing up now but leaves in place the heat that formed the dam.
What has to be true for an ice dam to form?
An ice dam needs snow on the roof, an upper roof surface above 32 degrees Fahrenheit and a lower eave below it. Meltwater that runs down to the cold eave freezes there and backs up behind the dam.
The mechanism is simple to state and easy to misread. Snow lying on a roof insulates the shingles, and heat from the living space warms the roof deck from underneath, so the snow over that warm area melts. The University of Minnesota Extension describes the condition as higher portions of the roof above 32 degrees Fahrenheit while lower surfaces sit below it. Meltwater flows down from the warm area toward the overhang, and the overhang is the coldest part of the roof because little heat from the house reaches it.
Elevation shapes how much snow a roof carries. Moscow's city center sits at about 786 meters, and the towns to the east sit higher than the Snake River valley, so snow there tends to stay longer. Once the eave freezes into a ridge of ice, the water that comes after it has nowhere to drain. It pools behind the dam, works under the shingles and reaches the wall, the ceiling and the insulation. The damage often shows inside first, so a stain on a bedroom ceiling can appear long before anyone sees ice on the roof.
Why is the attic the real source of the problem?
Heat leaking through the ceiling into the attic is what melts the snow from below. Adding insulation, sealing air leaks and improving ventilation reduce that heat, which is why the fix belongs in the attic first.
A cold-climate building science summary of modeling research describes ice dam size as a function of snow load, roof R-value, indoor temperature and the distance from eave to ridge. Snow load is set by the winter and the distance is set by the roof, so the factor a homeowner can reach most directly is the R-value of the attic floor. That research studied the effect of indoor temperature only, so its findings are background rather than a design rule.
Ventilation matters because a cold attic keeps the roof deck cold. Air sealing matters for the same reason. Warm, moist indoor air leaks up through gaps around ceiling penetrations, hatches and wall top plates, and every leak carries heat into the space under the roof. These are attic jobs. They change the temperature of the roof deck directly, and they do it without touching the shingles.
Does replacing the shingles stop an ice dam?
Changing the covering does not address the heat that melts the snow, so it is not a dependable fix on its own. The covering matters for shedding water, but the cause sits underneath it.
A common instinct is to replace the covering when water comes through at the eave. The shingle is the last layer water meets, though, and a new shingle laid over the same warm attic will melt snow in the same pattern. The attic keeps producing the heat that feeds the dam, and the new covering keeps the same cold eave. That is why the fix is designed around heat, air and water on the decking rather than the surface.
Ice barrier underlayment is the part of the roof that resists water when a dam does form. It goes under the covering at the eave. Pullman's published design criteria mark ice barrier underlayment as required, and that is the sourced basis for treating eave ice as an expected condition on the Palouse. Whether the Idaho side requires it is a question for the Moscow Building Safety Division, which can say what the adopted code requires for a given address.
What does a homeowner see before ice forms?
Watch the eaves in the first deep cold after a snowfall and a thaw. Icicles, a band of ice at the gutter line and water stains on ceilings near exterior walls are the usual early signs.
The mechanism needs snow on the roof, a warm upper surface and a cold eave. A snowfall, then a thaw, then a return of cold can produce that sequence, and the first sign is often at the gutter. Look at the eave line after each heavy snow. A ridge of ice where the roof meets the gutter, or icicles that keep growing rather than dripping, shows that meltwater is freezing at the edge rather than draining away.
Inside the house, look at the ceilings and walls along exterior walls, especially in top-floor rooms and close to the eave. A stain, a bubbling paint line or a damp smell near the insulation points to water that has already gotten past the roof. A homeowner can check these signs safely from the ground and from inside the house. Do not climb a snowy or icy roof to chip ice away, because a fall from an eave is the larger risk.
What does an ice dam fix look like on the roof?
A sound fix starts with an inspection of the attic, the eave and the ventilation, and it addresses insulation, air sealing and ventilation before any roof surface work. Removing ice is a temporary measure that does not change the cause.
A technician's first look is usually in the attic. The check covers insulation depth, gaps around ceiling penetrations, the vent path from the soffit to the ridge, and any staining on the sheathing. The roof surface is checked next, with attention to the eave, the valleys and the flashing where water is most likely to back up. What is found in those places sets the scope of the work and the basis of any quote.
Removing ice from the eave can relieve water that is backing up now, but it leaves the heat that formed the dam in place. Prevention means air sealing the ceiling plane, adding insulation to the attic floor and keeping soffit and ridge ventilation clear so the roof deck stays cold. The service this points to is ice dam prevention and removal, and the prevention half is the part that changes what happens next winter.
How do the Pullman and Moscow rules differ on ice barriers?
Pullman's published criteria require ice barrier underlayment. On the Idaho side, the Moscow Building Safety Division confirms what the adopted code requires for a given address, and that answer should be in writing.
Pullman and Moscow are two cities in two states with two sets of criteria. Pullman publishes its design criteria with ice barrier underlayment marked as required, under the 2021 International Residential Code with Washington amendments. Moscow's adopted requirements are not stated in the materials this guide uses, so the guide does not say whether the Idaho side requires an ice barrier, and it does not say that it does not.
Ask the Moscow Building Safety Division which requirement applies to your address, and whether the local design criteria table designates the eave for ice. Write down the answer and keep it with the permit. The same question belongs in Pullman for work in Washington, and in any town outside a city limit, where the county building official confirms the rule. Do not assume that a Pullman answer carries over to the Idaho side of the line.
Does ventilation alone solve an ice dam?
Ventilation helps keep the roof deck cold, but it works best alongside insulation and air sealing. Ventilation on its own cannot stop heat leaking from the living space into the attic.
Soffit intake and ridge exhaust move air across the underside of the roof deck. When that air stays cool, the snow above the deck melts less from below. A vent cannot exhaust heat that is leaking out through a ceiling hatch or a recessed light, though, so ventilation has to be paired with air sealing at those openings. The two jobs work on the same heat from different directions.
Blocked vents undo the work. An easy mistake in an attic is to let insulation fill the space at the soffit, which stops air from entering the vent path. A technician should check that the baffles are in place and that the path from the eave to the ridge is clear. That check takes little time and matters more than the vent count.
What can a homeowner do before the first deep freeze?
Clear the gutters so meltwater can leave the eave, look for obvious attic gaps and book an inspection before the snow arrives. These steps are a sensible start, and they do not replace a proper attic check.
Clearing the gutters before the first snow lets meltwater leave the eave instead of freezing there, which is the first step in the local seasonal calendar. Work from the ground or from a safe platform rather than a ladder on a slope. Keep the downspouts clear too, because water that cannot exit at the bottom backs up into the gutter and freezes at the edge.
In the attic, look for insulation that is thin or missing, bathroom and kitchen fans that vent into the attic instead of outside, and ceiling openings such as hatches and recessed lights. Those are the usual air leaks. A pre-winter look with Moscow Roofing is a reasonable way to get the attic checked before the cold sets in, and a visit the same day is possible only when a technician is free. No arrival time can be promised.
Frequently asked questions
Can I rake the snow off the roof to prevent an ice dam?
Raking the lower slope can reduce the weight on the eave, but it does not change the heat that melts snow from underneath. A dam can return after the next storm for that reason. Raking also puts a person at the roof edge with a tool and often on ice. Use a roof rake from the ground only where the eave can be reached safely, and call a technician when the eave has already iced over.
Does an ice dam mean my roof is failing?
Not by itself. An ice dam shows that heat is escaping from the house and that the eave is cold enough to freeze meltwater. Staining inside the house, or ice that has backed water under the shingles, is the sign to look at more closely. A technician can tell the difference between a dam that needs attic work and a roof surface that needs repair.
Why does ice form at the eave and not at the ridge?
The eave overhangs the wall, and little heat from the house reaches it, so it stays below freezing while the upper roof above the warm attic stays above it. The University of Minnesota Extension describes that temperature split as the condition for an ice dam. Water running down from the warm zone freezes as it reaches the cold edge, and then the dam grows.
Is ice barrier underlayment the same thing as a fix?
No. Underlayment is a layer that keeps water out if a dam does form, so it works as a backstop. It does not stop the heat that melts the snow. Pullman's published criteria require ice barrier underlayment. On the Idaho side, ask the Moscow Building Safety Division what the adopted code requires for your address before deciding anything.
Will a metal roof stop ice dams?
A metal surface does not cause or cure an ice dam, because the dam is driven by attic heat and a cold eave. Snow that melts from below will still refreeze at the edge unless the attic is addressed. Whether metal suits a Palouse winter is covered in its own guide, which looks at snow shedding and snow retention.
When should I call a technician?
Call when water has reached a ceiling, when ice has backed up under the shingles or when the eave is too icy to walk safely. Phones are answered around the clock, and a visit the same day is possible only when a technician is free. Do not wait for the ice to melt on its own if water is already inside the house.
Sources
This guide is general information for Latah County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.