Ice Dams: Why They Form and How to Prevent Them

An ice dam forms when heat from the house warms the roof deck under the snow, the meltwater runs down to the cold eave, and it refreezes into a ridge that backs up more water. Preventing it means stopping that heat: seal the air leaks into the attic first, then add insulation, then confirm the ventilation.

The roof is where the problem shows, but the cause sits in the attic floor below it. Minnesota’s Department of Commerce puts it in one sentence: “The real problem is a home that is leaking warm air.”

How an Ice Dam Forms: Warm Deck, Cold Edge

DOE’s Building America guide to constructing roofs and attics for ice dam prevention gives the condition: “Ice dams occur if snow is present on a roof and the roof deck under that snow reaches above freezing temperatures.” University of Minnesota Extension’s guide to dealing with and preventing ice dams names the cause behind that: “Nonuniform roof surface temperatures lead to ice dams.”

Here is the sequence. Warm air from the house leaks into the attic and heats the underside of the deck. Snow touching the warm deck melts from the bottom, and the water runs downhill under the snow blanket. When it reaches the eave overhang, which sits outside the heated walls and stays as cold as the outdoor air, it freezes. Each day adds a layer, and the ridge of ice grows.

The damage comes from what happens behind that ridge. Shingles are built to shed water running downhill, not to hold back standing water. Meltwater that pools behind the dam can work its way up under the shingle laps and into the deck, the wall and the ceiling below. If a winter storm has already left stains or other damage, what to check first after a storm sets the order.

Where the Heat Comes From

The Building America guide calls air sealing “the most important step,” which tells you where the heat travels: through gaps between the living space and the attic, carried by warm air rising. Insulation slows heat moving through the ceiling, but air leaking through a gap carries heat straight past it.

If the insulation is loose, pebble-like vermiculite, EPA’s page on vermiculite attic insulation advises treating it as possibly containing asbestos and leaving it undisturbed, so look for heat-loss signs from the hatch and leave any air sealing over that insulation to a professional. Otherwise, from the attic hatch, with a flashlight, standing only on framing, look for these heat paths:

• Gaps around recessed light fixtures and ceiling electrical boxes
• The attic hatch or pull-down stairs, if they have no seal or insulation
• Openings around plumbing vent pipes and the bathroom and kitchen fan housings
• The gap around a chimney or flue chase
• The tops of interior walls, where the wall cavity opens into the attic
• Heating or cooling ducts that run through the attic, with loose joints or thin wrap
• Insulation that is thin, matted or pushed back near the eaves, where the roof comes closest to the ceiling

Dark, dirty insulation is a useful clue. Air leaking up through insulation leaves dust behind, the way a furnace filter darkens. Frost on the sheathing in winter points to the same leaks.

The outside of the house tells the same story. During the next snow, look at the roof from the ground and photograph where the snow melts first. Bare patches above a bathroom, along a line of recessed lights or around the chimney mark the spots where heat is escaping.

Prevention in Order: Air Seal, Insulate, Then Ventilate

The guidance puts air sealing first. University of Minnesota Extension says: “First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space.” DOE’s measure guideline on attic air sealing adds that “air sealing is completed prior to insulating,” and sealing gaps is far harder once new insulation covers them. Ventilation comes after both, to keep the deck cold by moving outdoor air under it. How air sealing is done step by step, and the insulation levels recommended for your climate zone, are covered in the guide to insulation levels by climate zone and how to seal first.

Ventilation is the third step for a reason. A vented attic helps the deck stay close to outdoor temperature, but venting alone may not keep up with a steady stream of warm air leaking through the ceiling. A powered fan does not change that: University of Minnesota Extension says “mechanical attic ventilation is NOT recommended,” and a fan that pulls air out of the attic can draw more warm air up through those same leaks, as the page on whether attic fans help or hurt explains. How intake at the soffits and exhaust near the ridge work together is covered in how attic ventilation keeps the roof deck cold.

Rooms with sloped ceilings are the hard case. Where the roof deck sits directly over the living space with no attic between, there is no hatch to work from, and sealing and insulating have to be done from inside the room or from above at the next re-roof. Dams that form only above those rooms point to that section.

Ask for an air-sealing estimate that names each penetration to be sealed and the materials, and photograph the attic before any insulation is added so the sealing work can be checked.

What an Ice Barrier Does and Does Not Do

An ice barrier is a self-adhering membrane installed under the shingles at the eaves. It does not prevent ice dams. It limits what the water behind a dam can do, by sealing the deck and the nail holes where meltwater would otherwise get through.

Where codes require it, placement is defined relative to the wall. Lincoln, Nebraska, whose re-roof rules are built on the 2018 International Residential Code (IRC), spells out the depth in its re-roof policy: the barrier must extend “not less than 24” inside the exterior wall line of the building.” DOE’s guide to roof eaves sealed with self-adhering membrane in cold climates describes installing it “from the eaves to at least two feet in from the interior plane of the wall,” with its lower edge covering the top edge of the metal drip edge; the same page also shows drip edge installed over the membrane, so the product instructions decide the order.

Both rules aim the membrane at the zone where dams form: the cold overhang and the first stretch of roof over the heated house. Confirm the requirement with your local building department; code triggers, valleys and other placements are covered in where ice and water shield is required and where it helps.

Fixes That Only Treat the Symptom

Some fixes deal with the ice, not the heat, and the dams come back:

• Heat cables. Minnesota’s Department of Commerce, in its ice dam guidance, says: “Do not install heating cables.” A cable may keep a channel open over a short stretch of eave while the attic keeps melting snow above it.
• Raking snow off the lower roof. Removing snow from the ground takes away the supply of meltwater and is a reasonable winter tactic, as long as the rake stays clear of overhead power lines and you stand clear of the snow as it comes down. University of Minnesota Extension cautions that a roof rake and push broom “may damage the roofing materials,” so it treats the symptom at some cost.
• A new roof alone. A new covering with an ice barrier limits leaks from the next dam, but if the attic still leaks heat, the dams return on the new roof too.

Which Fix to Start With After a Winter of Ice Dams

Start with air sealing if the snow melted first in patches above lights, bathrooms or the chimney, or if the attic shows dark insulation or frost. Add insulation next, then confirm that the soffit intakes are open and the exhaust vents work.

If the attic has already been sealed and insulated and dams still form in the same valley, below the same dormer, on a low section under a taller roof or behind the chimney, the cause may be the roof’s shape rather than heat loss: those spots hold deeper snow and more shade. Ask a roofer to look at how snow collects at that spot and whether the detail can be changed at the next re-roof. If you are re-roofing anyway, specify an ice barrier at the eaves to the depth your local code requires, and treat it as protection against leaks, not as the cure.

If an ice dam is sitting on the roof right now, deal with it safely first; how to remove an ice dam safely covers the methods that do not damage shingles. Then, during the next snow, photograph where snow melts first from the ground, check the hatch for gaps around ceiling penetrations, and get an air-sealing estimate.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

Sources

• Construct Roofs and Attics for Ice Dam Prevention, U.S. Department of Energy Building America Solution Center
• Dealing with and preventing ice dams, University of Minnesota Extension
• Measure Guideline: Guide to Attic Air Sealing, U.S. Department of Energy Building America
• Re-Roof Policy, City of Lincoln, Nebraska Building and Safety
• Roof Eaves Sealed with Self-Adhering Membrane in Cold Climates, U.S. Department of Energy Building America Solution Center
• Got ice dams? Learn what to do and how to prevent them to save energy, Minnesota Department of Commerce
• My attic has vermiculite insulation in it. Am I at risk? Should I take it out?, U.S. Environmental Protection Agency