What causes ice dams on Bloomington, MN roofs?

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Ice dams form when heat escaping into the attic melts snow on the upper roof, the water runs down to the cold eaves and refreezes, building a ridge of ice that traps meltwater behind it. That trapped water backs up under shingles and leaks into walls and ceilings. The root causes are insufficient attic insulation, air leaks from the living space, and poor attic ventilation, all common in older Bloomington homes during our long, cold winters. The real culprit is a warm attic. When indoor heat leaks past gaps around light fixtures, ducts, and the attic hatch, it warms the roof deck unevenly. Snow melts high and refreezes low at the cold overhang, creating the dam. An inspection identifies these air-sealing and insulation gaps. Ventilation matters too. A properly vented attic stays close to outdoor temperature, keeping the roof deck cold and uniform so snow does not melt prematurely. Blocked soffit vents or missing baffles defeat this and contribute to dam formation. Ice dams can cause hidden damage, including soaked insulation, stained ceilings, rotted roof sheathing, and mold. A thermal imaging scan during inspection can reveal heat loss patterns and past moisture intrusion that signal a home prone to ice dams.

What causes ice dams on Bloomington, MN roofs?

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