What causes an ice dam on a roof?

An ice dam on a roof is primarily caused by meltwater that refreezes at the overhang. When snow on a roof melts due to heat from the attic or the sun, the water flows down the slope of the roof. If the roof overhang is colder than the section of the roof where the snow melts, this water can refreeze once it reaches that colder area. This refreezing of meltwater forms a dam of ice, preventing the water behind it from draining off the roof. The trapped water can then seep under shingles, leading to leaks and damage to the roof structure and interior of the building.

While factors like improper drainage systems and faulty insulation installation can contribute to conditions that promote the formation of ice dams, the specific process of refreezing meltwater at the overhang directly results in the creation of the ice dam itself. Excessive roof pitch does not inherently contribute to ice dam formation, as it is more related to how steeply water can flow off a roof rather than how it freezes.