What causes ice dams on Yarmouth, ME roofs?

The mechanism

Warm air leaking into the attic heats the underside of the deck. Snow melts, runs down, and hits the overhang, which is over unheated outside air. It refreezes there, builds a ridge of ice, and backs standing water up under the shingles.

Proximity to Casco Bay and the Royal River brings damp air and fog, keeping shingles wet longer. Combined with winter freeze/thaw, that moisture cycle accelerates surface deterioration.

What actually prevents them

• Air-seal the attic floor — bath fans, wiring penetrations, top plates
• Bring insulation up to a consistent depth with no gaps at the eaves
• Keep soffit-to-ridge ventilation clear so the deck stays cold
• Clear gutters in the fall so meltwater has somewhere to go

What ice dams do to shingles

Repeated freeze/thaw at the eave is brutal on aging shingles: it lifts tabs, strips granules and accelerates the drying that makes the shingle brittle in the first place. On Yarmouth roofs we typically see the eave courses aged years ahead of the field of the roof.

Where rejuvenation fits

Rejuvenation doesn't prevent ice dams — attic work does. What it can do is keep the shingles flexible enough to survive the winters while you address the real cause.