What Causes Ice Dams?

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 run off 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 where it refreezes, forming a ridge of ice or an ice dam. As more snow melts the ice dam grows and eventually the run off water is trapped behind the ice dam. After it builds up, the water works it's way underneath the shingles and begins to rot the roof or even drip right into your house.

Doesn't attic insulation prevent ice dams?

  • Proper attic insulation can help prevent ice dams, 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 of the country), but only above the living area of the home. The areas over the soffits in the overhang should be uninsulated with opening 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. 

Makes sense, so what causes ice dams?

  • The problem is most homes don't have adequate insulation or even if they do, there are still air leaks from inside the home into the attic (plumbing stacks, pot lights, attic access doors). All of these opening allow warmed air to get into the attic, raising the temperature.
  • Once warmer air is in the attic, it raises the temperature of the underside of the roof helping the snow melt. The melted water runs down the warmed roof until it reaches the overhang above the soffits that is the same temperature as the outside air (i.e. colder) 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 backs up under the shingles and into the roof itself. 

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 stable broom to remove snow from the roof. Getting rid of the snow will stop water from running off the roof. Don't go onto the roof to shovel it off, as this is extremely dangerous and can easily damage the 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 from an ice dam. 

Murray Anderson is an experienced freelance writer over 500 articles published on the web as well as in print magazines and newspapers in both the United States and Canada. He writes on a wide range of topics and is a regular contributor to DoItYourself.com. He can be contacted at [email protected].