An ice dam is a thick ridge of ice that forms at the edge of a roof. It prevents melting snow from draining off, forcing the melted water to seep underneath shingles. The water can leak into your home, damaging insulation, walls and ceilings.

Here are some suggestions from This Old House and energystar.gov on how to remove and prevent them.

Why ice dams form

Ice dams form when heat from your attic warms the roof, but not the eaves. The uneven temperatures cause the snow on the warm roof to melt and then freeze on the cold eaves.

What to do if you have an ice dam

Try the following if you have ice dams on your roof:

• Call an expert. A roofing professional can break the dam into small pieces with the least amount of damage to your roof.
• Rake your roof. Remove the snow from your roof using a roof rake or push broom.
• Use calcium chloride to melt the ice. Fill the leg of a pair of pantyhose with ice-melting compound and tie it off. Then place it vertically across the dam, so it slightly hangs over the gutter. This will slowly melt the ice and create a path for water to flow. Don’t use rock salt, which can cause damage.

Put safety first! Don’t get on the roof to clear the dam and don’t stand underneath the dam to chip away at the ice. You can damage your roof or seriously injure yourself.

How to prevent an ice dam

You can clean out your gutters before the first snowfall to reduce the severity of ice dams. But to prevent them, you need to tackle the problem from the inside of your house.

To keep heat from escaping to your roof:  

• Seal air leaks. Cap attic hatches and whole-house fans. Flash around the chimney. Seal ducts.
• Add more insulation in your attic. Keep the heat where it belongs — inside your house. Check with your building department to see how much insulation you need.
• Make sure your attic is well ventilated. Ventilate eaves and ridges. Check to see if insulation is blocking ventilation.

An energy specialist can help you pinpoint the specific source of your problem. You’ll prevent ice dams and make your house more energy efficient in the process.