What Causes Ice Dams?

large icicles hanging from the side of a roof

It's hard to believe a warm sunny day in the winter can actually be causing major damage to your home, but it's true. That's because the runoff water from the melting snow (both from the heat of the sun and heat inside your attic), will run down to the edge of the roof. If it gets stuck there and refreezes, it can form a ridge of ice, aka an ice dam.

As more snow melts, the dam grows, trapping still more water in a vicious, chilly cycle. As it builds up, the water can work it's way underneath the shingles and begin to rot your roof, or even drip right into your home.

Does Attic Insulation Help?

Yes, proper attic insulation can help reduce the likelihood of ice dams forming, but insulation alone can't do it. Preventing ice dams also requires proper attic ventilation that allows airflow and movement through the attic underneath the roof.

A properly insulated house should have at least R-38 insulation (or even higher in northern areas), but only above the living area of the home. The areas over the soffits in the overhang should be uninsulated, with openings to allow for air movement

Outside air comes in through the soffits at the bottom of the roof, works its way up the underside of the roof and out the top through roof or ridge vents.

This cold outside air moving through the attic helps keep the temperature of the underside of the roof similar to that of the outside air.

icicles on a roof edge

What Causes Ice Dams?

The problem is that most homes don't have adequate insulation. But even if they do, there are usually still air leaks from inside the home into the attic (from plumbing stacks, pot lights, and attic access doors, etc.). All of these opening allow warmed air to get into the attic, raising the temperature.

As attic air warms, it raises the temperature of the underside of the roof, helping the snow melt. The melted water trickles down the warmed roof until it reaches the overhang above the soffits. Here, the temperature starts to match the outside air (i.e. colder than the air in the attic) and it freezes. The tiny ridge of ice grows and an ice dam begins to form, blocking the water from running off the roof. Eventually the runoff water can back up under the shingles and into the roof itself.

icicles on the side of a house

What Can I Do if I Get an Ice Dam?

In the short term there are a few things you can do.

Use a roof rake or push broom to remove snow from the roof. Getting rid of the snow will reduce the amount of water running off the roof. Unless you don't have a choice, it's usually not a great idea to go onto the roof for this activity. In very cold weather this can be dangerous, and your stomping around can easily damage frozen shingles.

Using a hose or warm water to cut some 'channels' through the ice dam will provide openings for the melted snow to run off the roof. Obviously this is only a temporary fix, but it will help minimize the potential damage.