Ice Dam Forming on Roof Edge — San Francisco, California

An ice dam is a ridge of ice that builds up along the lower edge of a roof, blocking melting snow from draining into the gutters. As heat escaping from the living space warms the roof deck above, snow melts on the upper sections and flows downhill — but when it reaches the unheated eave overhang (which stays at or below freezing), the meltwater refreezes and accumulates into an ever-thickening ice barrier. Once the dam is tall enough, subsequent meltwater pools behind it and backs up under shingles, seeping through the roof deck into the attic, walls, and ceilings below. The telltale sign is icicles hanging from the gutter line combined with water stains appearing on interior ceilings or walls during or just after a cold spell. The root cause is almost always uneven roof temperature: insufficient attic insulation (allowing interior heat to escape), inadequate attic ventilation (trapping warm air at the ridge instead of flushing it out), and air leaks from the living space into the attic through recessed lights, bathroom fans, attic hatches, and top plates of interior walls. A roofer experienced with ice dam prevention can install ice-and-water shield membrane along the eaves (required by code for the first 24 inches in cold climates), add proper ridge-and-soffit ventilation to equalize roof temperature, and coordinate with an insulation contractor to seal attic air leaks and bring insulation to current code depth (R-49 in most cold zones per the IECC).