Why do ice dams form on my roof, and what should I do?

An ice dam is a ridge of ice along the edge of the roof. Heat escaping from the house warms the upper roof, snow there melts, and the water runs down to the eaves, which sit over the cold outside air. It refreezes there, and the ridge grows.

The trouble is the water behind it. With nowhere to drain, it backs up under the shingles and leaks into the attic, the walls and the ceilings.

How to tell you have one

• A thick band of ice along the roof edge, often with large icicles hanging from the gutters.
• Water stains or drips on ceilings or walls near outside walls during or after snow.
• Gutters packed with ice, or pulling away from the fascia.

What is safe to do from the ground

• Use a roof rake with a long handle to pull snow off the bottom few feet of the roof, standing on the ground, never on a ladder in snow and ice.
• Keep clear of the area under heavy icicles.
• Inside, catch any drips and move belongings. Take photos for insurance.
• Do not chip at the ice with a hammer or an axe. It damages shingles and gutters.
• Do not climb onto an icy roof.

The real fix

Ice dams are a heat problem more than a roof problem. Air sealing and insulation in the attic keep the roof deck cold, so the snow does not melt from below. A roofer can also check the underlayment along the eaves, which is the layer that stops backed-up water from getting in.

If water is already coming in, call a roofer, and a water damage company if ceilings or walls are wet.