An ice dam is a ridge of ice that forms along the edge of a roof and stops melting snow from draining off. The water pools behind the ice, works its way under the shingles and ends up inside the house. In Chicago it is a January problem with a November fix.
How does an ice dam form?
It takes snow on the roof, heat from the house and a cold roof edge. The order is always the same.
- Warm air from the house leaks into the attic and heats the upper part of the roof from below.
- Snow on that part of the roof melts and runs down toward the edge.
- The water reaches the eave, which hangs out past the wall and has no heat under it, and freezes again.
- The ice builds into a ridge. More meltwater arrives, pools behind it and backs up under the shingles.
How do I know I have one?
- A row of icicles along the roof edge. They look good on a Christmas card and bad on a gutter.
- Gutters packed solid with ice.
- A stain at the top of an outside wall, or on the ceiling beside it.
- Water or ice coming out from behind the gutter or through the boards under the eave.
What should I do right now?
- Pull the snow off the lowest three or four feet of the roof with a roof rake, standing on the ground.
- Put a bucket under any drip and move things out of the way.
- Leave the ice alone. Cold shingles crack, and a hammer or hatchet will do more damage than the dam.
- Skip the rock salt. It is hard on shingles, gutters and whatever is planted underneath.
- If water is coming into the house, call a roofer. The number here is (773) 676-3830.
What fixes it for good?
The cure is to keep the roof cold, so snow stays snow until it melts off evenly. Three of the four fixes happen in the attic.
| Fix | What it does |
|---|---|
| Seal air leaks into the attic | Stops warm house air reaching the underside of the roof. |
| Add attic insulation | Keeps heat in the rooms and out of the attic. |
| Keep the attic vents clear | Lets cold outside air carry off the heat that does get up there. |
| Ice barrier under the shingles | A self-sealing membrane along the roof edge. It does not stop ice forming, but it stops backed-up water getting through. |
Building codes in cold climates call for that membrane to run from the roof edge to at least two feet inside the line of the outside wall. It can only be fitted with the shingles off, which makes a roof replacement the moment to get it right.
Why are so many Chicago houses prone to it?
Many Chicago bungalows and frame houses have rooms built into the attic, with short walls close to the eaves. That puts heated space right under the roof, with little room for insulation in between. The snow above melts from below, and the cold eave does the rest.