Short answer
Ice dams form when heat escaping into the attic melts roof snow, which refreezes at the cold eaves. Air-sealing the attic, adding insulation and improving ventilation keep the roof deck cold and stop them. Ice-and-water shield limits leaks.
Why they happen
Warm air leaks from the house into the attic through light fixtures, bath fans and attic hatches. That heat melts snow on the upper roof; the water runs down and freezes at the overhang, where the roof is cold.
How to prevent them
The fix is to keep the whole roof cold.
- Seal air leaks from the house into the attic
- Add insulation to recommended levels
- Improve soffit and ridge ventilation
- Install ice-and-water shield along the eaves during reroofing
If you have one now
Rake snow from the lower roof from the ground. Don't chip ice. A roofer can steam channels safely. If water is coming in, see roof leak repair.
Questions
Do heat cables prevent ice dams?
They can keep channels open but don't fix the cause and use electricity. Air-sealing and insulation are the lasting fix.
Are ice dam leaks covered by insurance?
Often the interior water damage is, but the roof repair may not be. Check your policy.
This guide is general information, not professional, legal or insurance advice. Follow your insurer's instructions and local codes, and put safety first.