A buildup of ice at the eaves of a sloped roof, caused by heat escaping from the building interior and melting snow on the upper roof. As the meltwater runs down to the unheated overhang, it refreezes. The resulting dam traps subsequent meltwater, which can then back up under the shingles and leak into the building envelope.
Heard on the job
We need to install more edge venting and insulation, or they'll get an ice dam every winter.
Insider tip
The presence of an ice dam almost always indicates inadequate attic insulation or poor roof ventilation rather than a failure of the roofing material itself.