Ice dams: why they form and how to stop them

What an ice dam is

An ice dam is a ridge of ice along the edge of your roof. Warm air from the house heats the upper roof, snow melts, and the water runs down to the cold eaves, where it refreezes. Each thaw adds to the ridge, and water backs up behind it, under the shingles.

Signs you have one

• Thick ice along the gutters or roof edge
• Large icicles, especially with water dripping behind them
• Water stains on ceilings or walls near outside walls
• Wet insulation in the attic

What to do now

• Don't chip the ice. Axes and hammers break shingles and gutters.
• Clear snow from the edge. A roof rake from the ground removes the snow that feeds the dam.
• Watch for leaks inside. Move belongings and catch drips.
• Call a pro for steaming if water is getting in. Low-pressure steam removes ice without damaging the roof.

The long-term fix

Ice dams are a heat problem, so the lasting fix is in the attic and the roof system:

• Air sealing and insulation keep house heat out of the attic.
• Ventilation keeps the roof deck cold, so snow doesn't melt unevenly.
• Ice-and-water shield at the eaves, valleys and around chimneys stops water that does back up from reaching the decking. Many Minnesota cities require an ice barrier on reroofs; Bloomington's handout, for example, requires it to run at least 24 inches inside the exterior wall.

How we help

When we replace a roof, we take the old roof off first, inspect the decking, and install ice-and-water shield and ventilation as part of the job. See roofing, metal roofing (which sheds snow well) or get a free written estimate.