How a dam forms
Heat escaping upward from living space warms the roof deck above it. Snow sitting on that warmer section melts and runs down toward the eave, which sits over an unheated overhang and stays below freezing even while the section above it does not. The meltwater refreezes there into a ridge of ice, and the water backing up behind it eventually finds its way past the ice barrier, or past nothing at all where there is none, and into the wall.
The South Coast gets a pattern of freeze and thaw rather than one long deep cold snap, which is close to ideal for dam formation. Older Fall River triple-deckers with finished top-floor ceilings pressed close to the roof, and capes with knee-wall attics and thin insulation, are the two worst cases seen locally.
Reading the damage
Inside, the tell is a stain or peeling paint near the top of an exterior wall, a damp spot above a window, or a soffit dripping in February on a day nothing is falling from the sky. On the roof itself, the first two or three courses at the eave curl, granules wash out of them, and the deck underneath starts to go soft. Gutters pull loose under the added weight of ice sitting in them.
Three fixes, in order of value
Air sealing comes first and does the most for the least cost. Recessed lights, the attic hatch, plumbing chases and top wall plates leak warm air upward far faster than insulation alone can offset, and sealing them is inexpensive relative to the difference it makes.
Insulation comes second. Current Massachusetts code targets roughly R-49 in an attic, and a lot of older South Coast homes sit closer to R-11 or R-19. Adding blown cellulose or fiberglass over what is already there, with baffles installed at the eaves to keep soffit vents clear, brings the deck temperature down closer to outdoor conditions.
Ventilation comes third. A continuous ridge vent paired with open, unobstructed soffit intake moves outside air across the underside of the deck so it tracks closer to the outdoor temperature rather than sitting warm. Gable vents alone do not accomplish this. The target is roughly one square foot of net free vent area per three hundred square feet of attic floor, split evenly between intake and exhaust.
What the roof itself still needs
Ice and water barrier at every eave, running at least twenty four inches past the interior warm wall line, is the code minimum and the last line of defense if the attic work above it is not perfect. Valleys and any low-pitch section get full coverage as standard practice on a re-roof. On an existing roof with visible dam damage, the bottom courses are stripped and barrier retrofitted as part of the repair rather than skipped to save a day.
Heat cables and other partial fixes
Self-regulating heat cables along the eave can keep a channel open through a bad winter and buy time until the attic work gets done, but they run on electricity all season and do nothing about the underlying cause. Raking the bottom few feet of snow off a roof after a storm helps too, done safely from the ground with a long-handled rake rather than climbing up. Neither replaces the insulation and ventilation work described above.