Ice dams are the winter roof problem in southern Maine. This page explains why they form, what Maine's code puts under the eaves, how to get snow off a roof without getting hurt, how much snow is too much, and what ends ice dams for good.
The National Weather Service puts Portland's normal snowfall, for 1991 to 2020, at 68.7 inches a season, and the city's biggest single storm on record dropped 31.9 inches on February 8 and 9, 2013. The Maine Emergency Management Agency reports 35.5 inches in Gorham from that same storm, the most in the state. Snow that sits on a roof for weeks is where ice dams start.
The building code knows it. In this metro, every town that prints the ice-barrier line of its design table answers Yes, the answer the code reserves for places with a history of ice-dam damage. This page puts the sources together: Efficiency Maine and University of Minnesota Extension on cause and cure, the 2021 code on the eave, and the emergency management agency on snow weight and safe removal.
Three things have to happen at once, University of Minnesota Extension explains: snow on the roof, an upper roof surface above 32°F, and a lower edge below freezing. Snow melts where the roof is warm, the water runs down to the cold overhang, and it freezes there. The ridge of ice grows as long as melt keeps feeding it, and the water that backs up behind it finds the gaps in the shingles.
The warmth comes from the house. Extension calls heat leaking from the house the main cause of uneven roof temperatures, and air leakage, warm air rising through gaps in the ceiling, the biggest single path in many homes. Efficiency Maine tells the same story from the attic up: house air escapes into it, the roof snow above melts, and when enough of that melt freezes again at the cold edge, the dam builds and the pooled water behind it pushes up beneath the shingles and on into ceilings and walls.
Extension's list of things that raise the risk reads like an inventory of a Maine house: recessed lights, skylights, complicated roofs, heating ducts in the attic, chimneys, kneewall spaces, and bath or kitchen exhaust that ends just above the roof.
Table R301.2 in the 2021 code has a line for ice-barrier underlayment, and footnote h tells each town to fill in Yes where ice damming has caused damage locally. Cape Elizabeth, Topsham and Biddeford, the towns here that print the table, all say Yes.
Under Section R905.1.2, the barrier is either a self-adhering polymer-modified bitumen sheet or two layers of underlayment cemented together. It runs from the roof's lowest edges up to a line at least two feet inside the exterior wall. For steep roofs, 8 in 12 and up, the section adds a second minimum of 36 inches measured along the slope.
The barrier limits the damage when water backs up. It does nothing to stop the dam. Roof replacement covers the rest of the layers.
Damp left behind by these leaks can grow mold, Extension cautions, so get wet areas dry right away, and hold off on repairing ceilings and walls until they are dry and the cause is fixed. If water is coming in now, its stopgap is a channel through the dam: on a warmer day, run tap water from a hose to melt a path, working up from the dam's lower edge. The channel stops working within days.
Photograph the dam, the icicles and any inside damage with dates. Whether a policy pays for the water damage is a question for your insurer and your policy; the photos help either way, and they are gone by spring.
The Maine Emergency Management Agency's advice, condensed:
Efficiency Maine's caution applies too: raking helps, but it is hard work and it is easy to miss a storm.
The agency's rules of thumb, credited to the Insurance Institute for Business & Home Safety:
| On the roof | About |
|---|---|
| 10 to 12 inches of fresh snow | 5 pounds per square foot |
| 3 to 5 inches of old, packed snow | 5 pounds per square foot |
| 1 inch of ice | the same as a foot of fresh snow |
| 2 feet of old snow plus 2 feet of new | up to 60 pounds per square foot |
A typical roof, by the same source, takes about 20 pounds per square foot of extra weight, and less if the house or roof is older or has had trouble before.
Design figures are a different measure. The towns here that publish a ground snow load give 50 to 70 pounds per square foot, and Westbrook's applies up to 500 feet of elevation. Since November 2025 the state's code office has had designers look up each address in the ASCE Hazard Tool rather than use one figure for a town. The roof's own design load is worked out from the ground figure, so an engineer, not a table, answers what a particular roof can hold.
Every source here lands in the same place. Efficiency Maine: the best defense is prevention, with insulation and air sealing in the attic or along the underside of the roof keeping the roof temperature even; heat tape works but can be expensive to run, and weatherizing can end ice dams while cutting bills. University of Minnesota Extension: seal the ceiling first, then add insulation; keep some natural ventilation; do not count on powered attic fans.
Air sealing and insulation are their own trades, separate from roofing; Efficiency Maine lists contractors and runs weatherization programs. The roof side, the barrier, flashing and vents, belongs in the roofing scope.
Heat from the house. Air that slips up into the attic warms the roof deck, snow on the high part of the roof melts, and the melt freezes again over the cold overhang. The ice ridge then holds back more water, which can work under the shingles. Efficiency Maine and University of Minnesota Extension both describe it this way.
Not by itself. A new roof brings a new ice barrier at the eaves, which keeps backed-up water out of the house, but the dam still forms if heat reaches the roof. Air sealing and insulation in the attic, with the code's ventilation, are what stop the dam.
From the ground, with care. The Maine Emergency Management Agency says to start at the edge, pull small amounts, leave 2 to 3 inches on the roof, stay clear of power lines, and never use a ladder in snow and ice or heat on roof ice. It also suggests hiring professionals.
By the rules of thumb Maine's emergency management agency repeats, an ordinary roof has room for roughly 20 extra pounds on each square foot. A foot of fluffy new snow comes to about 5 of those pounds, and so does a single inch of ice. Older roofs may hold less.
It can keep a path open for meltwater, and Efficiency Maine says it can prevent ice dams, but it can be expensive to operate and does nothing about the heat loss behind them. Weatherizing the attic does both.
Wide enough to cover the overhang and then two more feet of roof inside the wall line, in any town whose design table answers Yes. Steep roofs, 8 in 12 and up, also need three feet or more of it measured up the slope from the edge.
Ice dams & roof snow across greater Portland and southern Maine, from Casco Bay and the York County coast to the Sebago Lake towns. Each city has its own page with the permit rule and code edition that apply there:
Sources: National Weather Service Gray, climate (Portland normal and record snowfall) · Maine Emergency Management Agency, severe winter storms (snow loads, safe removal, February 2013 blizzard) · Efficiency Maine, Insulation (ice dams) · Efficiency Maine, weatherization tips: attics, roofs, ducts and vents (Nov 10, 2025) · University of Minnesota Extension, Dealing with and preventing ice dams · MUBEC residential code (2021 IRC with Maine amendments), R905.1.2 ice barriers, R908.3 (UpCodes copy) · Town of Topsham, 2021 design criteria with Table R301.2 footnote h · Town of Cape Elizabeth, Chapter 6 construction code (ice shield required) · City of Biddeford, Table R301.2(1) amendment · City of Westbrook, code FAQ (ground snow load to elevation 500) · Maine Office of Community Affairs, Maine Snow Loads guidance (November 2025). Codes, programs and prices change; confirm the current rule with your city before you rely on it.
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