Bungalow Roofing in Portage Park, Chicago

The finished attic puts warm air against the roof deck, so snow melts near the ridge. The meltwater runs down to the cold eave overhang, refreezes, and builds a dam. Water then backs up behind it, under the shingles and into the house. Stopping the heat reaching the deck stops the cycle.

The wide overhanging eaves stick out past the heated house, so they sit at outdoor temperature with no warmth beneath them. Meltwater from the warmer roof above reaches that cold edge and freezes. That temperature difference is exactly why bungalows with finished attics get ice dams.

They share a type, so the problems repeat: ice dams, finished-attic ventilation, dormer flashing. But each house has been altered differently over a century, so the specifics vary. An inspection still matters, even on a street of near-identical homes. A roofer who knows the bungalow belt will know where to look first.

Not on their own. Ice dams here are caused by heat reaching the roof deck, and new shingles do nothing about that. What helps is attic ventilation and insulation, plus ice and water shield at the eaves on the new roof, so water that does back up cannot reach the deck.