The short version
- Ice dams are caused by heat escaping into the attic
- They force water under shingles
- Heavy snow adds weight
- Do not chip ice off your roof
How ice dams form
Heat from the house warms the roof, snow melts and the water runs down to the cold eave where it refreezes. The ice builds up into a dam, and more water pools behind it and finds its way under the shingles.
The root cause is usually heat loss through the attic.
Damage
Ice dams lift shingles, damage gutters and cause leaks into walls and ceilings. Freeze-thaw cycles crack sealants and loosen flashing.
Look for stains after a thaw.
Snow load
Wet, heavy snow adds weight to a roof. Most roofs handle ordinary snow, but older or damaged roofs can be stressed. Sagging, cracking sounds and sticking doors are warning signs.
Call for help if you notice them.
What to do
Do not chip ice or climb onto a snowy roof. Improving attic insulation and ventilation helps prevent ice dams. After damage, have the roof and gutters inspected.
Sealing air leaks into the attic is a good start.
General information, not engineering, legal, insurance or safety advice. Roofs, policies and local requirements vary, and every situation should be judged on its own facts. If anyone is in danger, call emergency services first.
