An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation.
Here's a simplifed explanation of how they form:
1. Warm air rises in the attic.
2. Outside, it's below 32 degrees and snow and ice accumulate on the roof.
3. Throughout the winter, as snow and ice refreeze, water can get trapped under the roof shingles (see image).
4. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish.
To help prevent ice dams:
1. Proper insulation reduces the warm air in the attic.
2. Roofing underlayments designed to shield ice and water can create a watertight barrier between roof and shingles.
3. Proper roof ventilation moves warm air out of attic and keeps it cold.