How an ice dam forms
University of Minnesota Extension describes it: "Nonuniform roof surface temperatures lead to ice dams. Heat loss from a house, snow cover and outside temperatures interact to form ice dams." Snow over a warm part of the roof melts, the water runs down to the cold eave and freezes, and the ridge of ice grows until water backs up under the shingles.
Where the gutter fits
The gutter is where the ice shows, not why it forms. Clean gutters, guards or new gutters will not stop ice dams; the fix for the dam is in the attic, by keeping the house's heat out of the roof deck. What the gutter itself needs in snowy towns:
- Hangers spaced close enough to carry the weight of ice.
- A sound fascia behind them.
- Clean troughs before the first snow, since leaves hold water that freezes.
Heat cable
Heat cable along the eave and in the gutter keeps a path open for meltwater. It treats the symptom and uses electricity; ask how it is fastened, whether it has its own circuit, and how the gutters were sized to take it.
Is it your town's problem
Each town page gives its normal yearly snowfall and its days below freezing from NOAA normals. In towns with 30 inches of snow or more a year, the page adds this topic.