6 Ways to Prevent Ice Dams on Roofs

Heat escaping from the building into the attic drives most ice dams, so keeping them off your roof starts at the attic floor and the gaps that let warm air reach the shingles. Sunlight warms a roof enough to matter only in rare cases.

Water that backs up behind the ice works its way through cracks in the roof covering and into attic insulation, ceilings and exterior walls, and wet materials left in place can grow mold and mildew. Interior repairs should wait until the heat loss is fixed and the ceilings and walls have dried, or the damage returns.

What you'll learn in this guide

• The order to tackle attic fixes in, and why insulation comes second

• Which ventilation upgrade can backfire by pulling more warm air into the attic

• Six places in the attic floor where heat leaks are worth hunting first

• What roofers put in a sock to open a drainage channel through a dam, and the de-icer that should never touch your shingles

• How a dam on the roof turns into a slip hazard on the walkway below

• Which ice dam costs an insurance policy may refuse to pay

Ice Dams Start in a Warm Attic

Three conditions have to line up at once, as the University of Minnesota Extension explains. Snow has to be sitting on the roof, the upper roof surface has to average above 32°F, and the lower edge has to stay below 32°F. That last condition needs outdoor temperatures below freezing too.

Once those conditions meet, snow on the warmer upper slope turns to water that runs toward the eave, reaches the colder overhang and freezes there. Each round of melting adds another layer, and the ridge keeps growing until water pools behind it and finds the seams between shingles. In many homes, most of the heat driving that melt rides on air leaking up from the living space into the attic.

Heavy icicles hanging along the gutter line can signal a dam building behind them.

Six Ways to Stop Ice Dams, in the Order We'd Tackle Them

1. Seal the Attic Floor Before Buying Insulation

Fiberglass and cellulose slow heat moving through the ceiling, yet air passes through them and around them with little resistance. Closing the openings in the attic floor with caulk, spray foam and weatherstripping comes first for that reason, ahead of any money spent on more insulation.

Start the search at the spots where builders cut through the top-floor ceiling or left framing open to the attic.

• Recessed light fixtures

• The attic hatch or pull-down stairs, especially if the lid closes without a gasket

• Holes drilled for plumbing vents, wiring and bath fan ducts

• Gaps along the top plates of interior walls

• Around the chimney chase

• Open-topped stairwells, plus the seams where an addition meets the original house

For leaks you can't find with a flashlight, a weatherization contractor can run a blower door test and scan the ceiling with an infrared camera. Furnaces, water heaters and exhaust fans still need enough air to run safely after the ceiling is sealed, so ask the contractor to check combustion appliances as part of the same visit.

2. More Insulation, Once the Leaks Are Closed

With the air leaks closed, added insulation goes to work on conducted heat, the warmth that passes through the ceiling materials themselves. Sloped ceilings over finished rooms need extra attention, like the angled walls in a Cape Cod's upper story. Rafters there leave little depth, so pick material with a high R-value per inch.

At the attic perimeter, install insulation dams so loose fill can't spill into the soffits and choke off ventilation. For new construction, your state's energy code sets the minimum, and meeting or beating it is the target.

 

3. Is Your Attic Ventilation Helping or Hurting?

Air that enters at the soffits and leaves through roof or ridge vents keeps the underside of the roof close to outdoor temperature. Michigan's winter preparedness guidance credits that airflow with holding off repeated melting and refreezing on the roof.

Vents can't make up for a leaky ceiling, and once air sealing and insulation are done well, a modest amount of ventilation covers the rest. Keep the vents open year-round anyway. Attic ventilation also dries out moisture and dumps summer heat, and once heat loss drops, snow that no longer melts can cover roof vents and slow airflow through the attic.

4. Exhaust Fans, Chimneys and Ductwork Above the Ceiling

Bathroom and kitchen exhaust ducts that end a short distance above the roof surface release warm, damp air right where snow is sitting, and in heavy-snow regions they may need extending or rerouting. Chimneys add heat during weeks of steady fireplace or wood stove use.

Leaky or poorly insulated ductwork running through the attic leaks heat all winter long. The knee-wall spaces beside finished upper rooms belong on the inspection list too, since they're part of the attic even when they sit behind a bedroom wall.

5. Rake the Lower Roof After Heavy Snow

Pulling snow off the lower roof takes away what a dam feeds on and lightens the load, which lowers the chance of roof failure from the weight of the snow itself.

1. Stay on the ground with a long-handled roof rake, since a snow-covered roof is no place for a ladder or a person.

2. After each heavy snowfall, clear the first few feet above the eave, where dams start.

3. Use light pulling strokes and keep the rake head from grinding into the shingles, because a rake or push broom can damage roofing when used hard.

4. Watch where the snow lands, and keep it away from doors, walkways and dryer or furnace vents at ground level.

6. Backup Protection for Valleys and Repeat Trouble Spots

During heavy-snow winters, dams can still form after the attic work is done, and they cluster in roof valleys that fill with wind-blown snow and along eaves shaded by overhanging trees. Trimming those branches lets the shaded section warm up with the rest of the roof.

Getting Rid of Ice Dams Without Tearing Up the Shingles

Once a dam has formed and water is backing up, the goal is a drainage channel through the ice, opened without anyone standing on the roof. Chipping is off the table, since axes and shovels gouge shingles and an icy roof offers no safe footing.

Roofers and home-repair guides commonly reach for a calcium chloride sock laid across the dam, letting the brine it releases eat a path through the ice for trapped water to drain. 

1. Fill a nylon stocking or tube sock with calcium chloride ice melt and tie off the end.

2. Using a rake, push it into position so it runs up and down the slope over the ice, leaving a short tail of an inch or two over the edge.

3. Leave it in place until it melts a channel, and add more socks along a longer dam.

When water is already coming indoors, running tap water from a hose on a milder day, working upward from the lower edge of the dam, can open a temporary channel. Expect it to close again within days.

Rock salt has no place in this job, because sodium chloride brine speeds corrosion of nails, flashing and gutters. It can also leave rust stains on shingles and kill the shrubs below the eave. Calcium chloride is the standard fill for the sock, though repeated use is still hard on the same metal parts, so keep it contained and use only what the job needs. For temperature ranges and trade-offs, see our comparison of how calcium, sodium and magnesium chlorides differ.

For large or stubborn dams, professional removal crews commonly use steamers that melt channels through the ice.

 

Drip-Line Ice on Walkways Below the Eaves

Meltwater that gets past a dam, along with steady drip from icicles, lands on the sidewalks and entry steps under the eave and refreezes as the temperature drops. For property managers, that strip of pavement turns into a fall hazard for anyone walking in, and because it forms below the same eaves each winter, it belongs on the crew's regular de-icing route.

Treat the drip line with a walkway-grade snow and ice melt after each thaw-and-refreeze cycle, and use the blue tint in our products to see where coverage is thin. On concrete poured within the past year, spread sand for traction and skip the ice melt, since new concrete isn't yet hard or dry enough to resist freeze-thaw damage.

 

Frequently Asked Questions

Does homeowners insurance cover ice dam damage?

Dwelling coverage may pay for interior water damage when it's sudden and accidental, but policies generally won't pay to remove a dam. Damage traced to poor upkeep is usually excluded, and many policies also exclude pavement, patios and fences damaged by freezing and thawing, so read your exclusions before winter.

Do gutters cause ice dams?

Gutters don't cause them, and taking gutters down won't stop dams from forming at a cold eave. They do take a beating, though. Ice weighs about 57 pounds per cubic foot, and a gutter packed solid can pull loose from the fascia.

Can a metal roof still get ice dams?

Metal sheds snow more readily than asphalt shingles, but any snow left on it still melts over a warm attic and refreezes at the cold edge. Because the cause sits in the attic and has little to do with the roofing material, the same attic work applies to a metal roof.

Will sealing the attic put more snow weight on the roof?

It can, because snow stops melting off from below and sits on the roof longer. Buildings constructed to current code should be designed to carry the snow loads expected where they stand. If you're unsure, your local building inspection office may have the plans on file, and a structural engineer can evaluate the roof.

Why is a newer building suddenly getting ice dams?

Skylights and rooflines broken up by valleys and dormers raise the risk even in recent construction. Snow depth and long cold stretches matter as well, so a building that stayed clear for years can form its first dam in a heavy winter.