Drive down any street on the North Shore, the Seacoast or the Maine coast after a storm and you'll see it: icicles on the Cape, a clean edge on the Colonial next door. Same snow, same weather, different houses. Here's why.
Cape Cod (1½ story)
Where it leaks: Upstairs rooms are tucked under the roof, with sloped ceilings, short "knee walls" and small attic spaces behind them. The sloped sections often have little insulation and many air gaps. That's the classic ice dam house.
The fix: Seal and insulate the knee walls and the floor behind them, or insulate the whole roofline. Then deal with the sloped bays, often with dense-pack cellulose or spray foam.
Colonial (2 story, open attic)
Where it leaks: Usually only at specific points: the attic hatch, bath fans, recessed lights and the chimney.
The fix: The easiest of all. Air seal the attic floor, then blow in insulation.
Split-level & raised ranch
Where it leaks: Several roof planes run into walls, and low roofs sit over heated rooms. Melt runs onto lower roofs and refreezes at the joints.
The fix: Air seal and insulate each separate attic, and check the flashing where roofs meet walls.
Cathedral ceilings & skylights
Where it leaks: There's no attic as a buffer, so any air or heat leak goes straight into the roof deck. Skylight wells are often poorly insulated.
The fix: Fill rafter bays carefully (dense-pack or spray foam, depending on the assembly and ventilation), and insulate and seal the skylight wells. Sometimes the real fix is exterior insulation at the next re-roof.
Additions & porches
Where it leaks: New roofs tied into old ones create valleys and low-slope sections where melt collects. A heated addition under a shallow roof often has very little room for insulation.
The fix: Insulate and seal the addition's roof, check how the valley and flashing were detailed, and keep drainage paths clear.
Lake camps & cottages
Where it leaks: Built for summer, with open rafters, thin insulation and no air barrier. Heat it for winter and the roof melts everywhere.
The fix: A whole-envelope plan. See our camp-to-year-round guide.
Fix difficulty at a glance
- Easier: Colonial, or a ranch with an open attic.
- Moderate: Split-level and raised ranch, or additions with accessible roofs.
- Harder: Capes, cathedral ceilings and skylights, where the space is tight and needs careful detailing.
Find your fingerprint
Match your house type above, then check the roof after the next snow with our icicle field guide. If it's leaking now, follow our first-24-hours plan. For the full explanation of how heat creates ice, read Follow the Heat.
Smooth works on all of these house types across the North Shore, Southern New Hampshire and Southern Maine, and Capes with plaster walls are our specialty.
Quick answers
Why do Cape Cod houses get so many ice dams?
Their upstairs rooms sit directly under the roof, with sloped ceilings, knee walls and small attic spaces that are often poorly insulated and leaky, so heat reaches the roof deck easily.
Will more attic vents stop ice dams?
Not on their own. Air sealing comes first, then insulation, then ventilation. Ventilation can't overcome heat pouring into the attic through air leaks.
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