Insurance & Storm Damage · FAQ

What actually causes an ice dam to form?

Two ingredients: snow on the roof and heat escaping into the attic. The attic heat melts snow on the upper portion of the roof; meltwater runs down to the eave (which sits over unheated soffit space, not the attic); the meltwater refreezes at the colder eave; and ice builds up over multiple melt cycles, eventually forming a dam that traps water behind it. The trapped water finds its way under the shingles and into the ceiling.

Notice what’s not on this list: ice or snow themselves. A roof with no attic heat loss can hold three feet of snow indefinitely without forming a dam. Fixing the heat loss (ventilation and insulation) is the actual prevention; everything else (heat cables, melt-tape, ice and water shield) is mitigation that helps when the prevention isn’t perfect.