Ice dams in Montana homes: causes and prevention

What is an ice dam, exactly?

The Insulation Institute describes it simply: ice dams form when the temperature in the attic is above freezing and there is snow on the roof. Snow on the warm part of the roof melts, the water runs down toward the edge, and at the colder eave it refreezes and builds a ridge. Water trapped behind that ridge can seep into the ceiling or walls.

The Minnesota Department of Commerce, which runs that state's energy information programs, says ice dams are not caused by roofing, ventilation or gutter problems; the real problem, in its words, is a home that is leaking warm air. Heat escaping into the attic warms the underside of the roof deck and melts the snow from below.

Why do ice dams form on some houses and not others?

Because the amount of heat reaching the roof differs. Where heated rooms are not well sealed from the attic, warm air leaks upward, which is the leakage the Minnesota agency blames. Where insulation is thin, heat conducts through it as well. Both warm the roof deck from below, and both are invisible from the street, which is why an assessment is the first step.

Western Montana weather sets up the rest. Valley winters bring long cold spells with snow on roofs, followed by sunny days and nights that drop back below freezing. Each cycle melts a little and refreezes a little at the eave. Two neighboring houses under the same snow can behave completely differently because one keeps its heat inside and the other does not.

What actually prevents ice dams?

Keeping heat out of the attic. The Minnesota Department of Commerce recommends sealing air leaks with caulking or expanding spray foam and insulating the attic to a minimum of R-50 as space allows, and says an advanced home energy assessment, or audit, is the first step; auditors may use an infrared camera to find where warm air is leaking. It recommends getting bids from at least two licensed contractors.

The Insulation Institute agrees that air sealing and insulating the attic is the best way to prevent ice dams, and adds that attic ventilation gives remaining heat somewhere to go and moderates attic temperature. The Minnesota agency is more skeptical of ventilation as a fix on its own, warning that adding roof vents, including powered vents, will not eliminate ice dams and often makes the problem worse. The common ground is clear: seal and insulate first, and treat ventilation as part of a design a qualified contractor sets, not a quick add-on.

Do heat cables stop ice dams?

The Minnesota Department of Commerce advises against relying on them, saying they will shorten the life of the roof and add to energy costs. They treat the symptom at the edge rather than the heat loss that causes it. If you are considering them anyway, ask a roofing contractor and an electrician how they would be installed on your roof and what they would and would not fix.

What should you do if an ice dam has already formed?

Get the snow off safely, and get help if water is coming in. The Minnesota agency suggests removing snow from the roof with a snow rake but recommends hiring a professional, especially if the job requires going on the roof or using a ladder, and says to put safety first. If an ice dam is already causing water damage inside, it recommends a removal service that uses steamers.

FEMA's Snow Load Safety Guide adds safety points for anyone using a roof rake from the ground: use a non-metallic rake, stay outside the range where roof snow can slide, and watch for dislodged icicles, which can cause injury even when they fall from a short height. OSHA warns that snow-covered roofs hide hazards and recommends methods that keep people off roofs. Our guide on when roof snow needs removing explains when the snow itself becomes a structural concern.

What should you never do to an ice dam?

Do not chip at it. The Minnesota Department of Commerce warns that removing ice with chippers, chemicals or heat can damage shingles, gutters and other building components. Hatchets, picks, and climbing onto an icy roof all add personal risk to the property damage.

Also avoid making the ground below worse. Ice weighs a little less than 5 pounds per square foot per inch, according to FEMA, and slabs and icicles that come off an eave land on whatever is underneath: steps, walks, a deck, a parked car. Keep people and vehicles out of that zone while ice is present, and see our guide to icicles and roof snow slides near entrances.

What signs suggest you have an ice dam problem?

A thick ridge of ice along the eaves with large icicles hanging from it is the obvious one, especially if snow higher up the roof has melted while snow at the edge has not. Inside, look for water stains or dampness on ceilings near exterior walls, peeling paint, or wet insulation in the attic. Uneven melt patterns on a snow-covered roof, with bare stripes or patches over certain rooms, show where heat is escaping.

Those signs are worth noting even if no water has come in yet, because they tell an energy auditor and a contractor where to look.

Are gutters to blame for ice dams?

Not according to the Minnesota Department of Commerce, which lists gutter problems among the things that do not cause ice dams. Gutters full of ice are usually a symptom: they are simply the coldest, last place the meltwater reaches before it refreezes. Clearing ice out of a gutter does nothing about the heat loss that keeps sending water down to it, and hacking at frozen gutters is one of the ways they get torn off.

That does not make gutters irrelevant. Where downspouts discharge matters a great deal at ground level, because meltwater from a roof with ice dams keeps flowing on sunny days and refreezes wherever it lands. A downspout that empties across a walk or the top of a driveway creates a sheet of ice every evening. Redirecting that discharge to a spot that drains away from paths is often the simplest fix on the ground.

What should you ask an energy auditor or contractor?

Ask where the warm air is escaping and how they found it, since the Minnesota agency notes that auditors may use an infrared camera to locate attic leaks. Ask what the attic's current insulation level is and what they recommend, how they will seal the gaps before adding insulation, and how their plan handles ventilation. Ask whether any part of the roof, such as a cathedral ceiling, a low-slope section or an addition, needs a different approach.

Get the answers in writing and compare at least two bids from licensed contractors, as the Minnesota Department of Commerce recommends. A good contractor will explain what the work should change on your roof next winter, which gives you a way to judge whether it worked.

When is the best time to fix the cause?

Before the snow. Attic work is easier and safer when the roof is dry and the attic is not frozen, and the effect can be checked against the first snow cover of the season. If you saw ice dams last winter, late summer and fall are the time to book an assessment, not the week the icicles reappear.

Who should you call about ice dams?

For the cause, an energy auditor and an insulation or air-sealing contractor. For damage to the roof and for removal, a roofing contractor. For electrical heat cable questions, a licensed electrician. Your utility may offer or recommend home energy assessments, so it is worth asking. If water has damaged ceilings or walls, talk to your insurer about what your policy covers.

We do not do roofing, insulation or attic work, and we do not remove ice from roofs. What we can do is described below.

What can HJ do around the house?

Ice dams create problems on the ground: meltwater dripping onto walks and steps and refreezing, ice falling from eaves, and downspouts that freeze and dump water across a drive. Our residential snow removal arrangements include walks, steps, landings and decks where access requires it, with ice management on the surfaces where refreeze is a real risk. Our guide to where to pile snow explains why piles should stay away from downspouts and foundations.

Trees matter too. Branches overhanging a roof drop snow and debris onto it and shade sections that then hold snow longer; our tree services team can prune them back. Our full snow and 24-hour storm services cover Missoula and the Bitterroot Valley. This guide is part of our Montana Winter Property Guide. Call (406) 215-7319 or contact us to talk through what you need on the ground.

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