What is an ice dam?
An ice dam is a ridge of ice that forms at the edge of a roof when heat escapes from the attic and melts snow, which then refreezes as it runs toward the cold eaves.
In New England winters, ice dams form when snow on a roof melts due to heat loss through the attic space, then refreezes into a solid ridge along the eaves where the roof overhangs. The process begins when the upper portion of the roof warms enough to melt snow, but the overhang remains cold. Meltwater flows down and backs up behind this frozen barrier, unable to drain off the roof edge as it should.
The trapped water then seeps under the roof shingles, sheathing, and flashing, making its way into the attic and interior walls. Homeowners often discover ice dams when water stains appear on ceilings or walls during a thaw, or when icicles hang heavily from gutters and eaves. Ice dams are particularly common in older homes with poor attic ventilation or inadequate insulation that allows warm air to escape.
Preventing ice dams starts with reducing heat loss through proper attic ventilation, adequate insulation, and air sealing to keep the roof surface cold and even. Once an ice dam is present, water damage may already be occurring inside the home. To assess whether your roof is at risk or if damage has already happened, consult a qualified roof inspector who can evaluate your attic conditions and recommend corrective measures.