Ice dams in southeast Wisconsin: what causes them and how to stop them

An ice dam is a ridge of ice along the lower edge of a roof that holds back water behind it. It needs three things at once: snow on the roof, an upper roof surface warmer than 32°F, and a roof edge colder than 32°F. Snow over the warm part of the roof melts, the water runs down under the snow blanket, and it freezes again when it reaches the cold overhang at the eave.

The heat almost always comes from the house. Warm air leaks into the attic through gaps around ceiling light fixtures, plumbing and wiring holes, chimney chases and the attic hatch, and heat conducts through thin or unevenly spread insulation. Ductwork running through the attic adds more. The overhang sits outside the heated space and stays at outdoor temperature, so the roof ends up warm in the middle and frozen at the edge.

The damage happens behind the dam. Shingles are built to shed water running downhill, not to hold back standing water. Water pooled behind the ice backs up under the shingle laps, finds nail holes and seams, and runs into the wall or ceiling below. The USDA Forest Products Laboratory also lists ice dams as a cause of peeling paint and staining on siding under the eaves.

The raw ingredients are plentiful here. Milwaukee averages 48.7 in. of snow a year (1991–2020 normals), most of it from December through February, and Milwaukee County averages 18 nights a year at or below 0°F. Snow that sits on a roof for weeks gives a leaky attic plenty of time to build a dam.

Totals also vary sharply over short distances. In the lake-effect storm of February 15–16, 2021, lakeshore parts of Milwaukee, Racine and Kenosha counties got 10–16 in. while areas inland got 1–4 in. Two houses a few miles apart can have very different winters on the roof.

The housing matters too. Much of the area's older stock, including Milwaukee's bungalows built mostly between about 1900 and 1930, went up long before anyone air-sealed an attic, and insulation in newer homes settles, gets disturbed or was never brought to current levels. The result shows on the street: ice ridges on one house, clean eaves next door.

Gutters get blamed because that is where the ice is easiest to see. But the dam forms on the roof edge above the gutter, because the overhang is cold. A house with no gutters at all will still form dams if the attic is warm. Taking gutters down doesn't remove the cause, and a gutter packed solid with ice adds weight at the eave and can pull away from the fascia.

Clean, properly pitched gutters still matter. A gutter clogged with leaves holds water that freezes early in the season and gives a dam a head start. Keep them clear in the fall, but don't expect new gutters or guards to solve an attic problem.

Heat cable zig-zagged along the eave can keep a drainage path open at a known trouble spot. It is a way to manage the symptom: it uses electricity every time it runs, it does nothing about the heat escaping into the attic, and ice can still build above the cable. If a cable is the only thing between you and a leak every winter, the attic needs attention.

ENERGY STAR's sealing and insulating guide lists a history of ice dams as an indication of serious air leaks.

The building-science sources agree on the sequence. University of Minnesota Extension says to make the ceiling airtight first and then consider adding insulation. The Department of Energy's Building America program calls air sealing the most important step. Ventilation comes last, as a supplement: Building Science Corporation notes that trying to ventilate away large air leaks is very difficult and bad practice.

Air sealing and insulation are usually done by insulation or weatherization contractors, often after an energy audit with a blower-door test. The roof's job is different: to keep water out when a dam forms anyway. That is what the eave ice barrier does. It is a self-adhering membrane labeled to ASTM D1970 (often called ice-and-water membrane), or sheet metal, installed under the shingles at the eaves. If water backs up behind the ice, the membrane seals around the nails and keeps it out of the deck and the wall below.

Wisconsin's Uniform Dwelling Code requires it in one specific case: shingle or shake roofs with a slope of 4:12 or less that extend over a heated area of the home or an attached garage. There it must run at least 30 in. up the roof from the edge and at least 12 in. up the slope past the inner face of the exterior wall (SPS 321.28(4)). The '24 in. inside the wall' figure that circulates online is International Residential Code wording, not Wisconsin's.

Steeper roofs are not covered by that clause. On them, an eave membrane is a matter of good practice and manufacturer instructions: CertainTeed, for example, strongly recommends an ASTM D1970 underlayment on all slopes wherever ice build-up is possible. Building Science Corporation suggests running the membrane high enough to hold back 6–8 in. of water above the line of the wall insulation, which on a deep overhang can take more than one course.

The UDC applies to homes started on or after June 1, 1980, and to alterations of those homes. For older homes, your municipality's requirements govern; check with the building department before a re-roof, and see our guide to permits in southeast Wisconsin. A complete roof replacement estimate should name the ice-barrier coverage, the underlayment, how flashing is handled and the ventilation plan — the items that drive the cost of a new roof as much as the shingle does.

Taking snow off the lower part of the roof removes one of the three ingredients. University of Minnesota Extension lists removing snow as a first step, and it also warns that anyone working on a roof in winter, or on the roof from below, risks injury and can damage the roof and house.

Water coming through a ceiling or down the inside of a wall in January means the dam is already holding water. The goals are to limit the damage inside, give the trapped water somewhere to go, and deal with the cause when the weather allows.

After a bad winter, have the parts of the roof that ice attacks checked: eave membrane, drip edge, flashing at walls and chimneys, and the bottom courses of shingles and decking. A useful roof repair estimate names what was damaged, how eave protection and flashing will be handled and what ventilation changes are planned — and says plainly when the real fix is air sealing and insulation.

C&L Property Improvements provides free estimates, looks at the house in person and puts the scope in writing. If you are weighing a repair against a full replacement, the roof replacement cost guide explains what moves the price.