An ice dam forms when attic heat melts snow on the upper roof and the water refreezes at the cold eaves. The ridge of ice holds back a pool of meltwater, and that water works under the shingles, through the sheathing and into the attic and the walls below.
Why older roofs here get ice dams
- Heavy snow loads that sit on the roof for weeks
- Thin or uneven attic insulation in older capes and colonials
- Warm air leaking up around chimneys, lights and attic hatches
- Short roof overhangs and gutters that fill with ice



