Ice dam prevention

What actually forms a dam

Heat leaking from the house warms the underside of the deck over the heated part of the roof. Snow on that section melts, the water runs down, and at the eave — which overhangs unheated space — it refreezes. Each cycle builds the ice a little further up the slope, and behind the dam standing water backs up under the shingles, which were never designed to hold water, only to shed it.

The tell is a clear melted band above the wall line with snow still lying below it. That is the heated part of the roof announcing itself.

The three fixes, in order

One: ventilation. A cold roof deck cannot melt snow. Intake at the soffit and exhaust at the ridge, sized to the code’s 1/150 net free area (or 1/300 where the exception applies) and split roughly half low, half high. Check yours on the ventilation calculator.

Two: air sealing. Ventilation removes heat that has already arrived; sealing stops it arriving. Recessed lights, attic hatches, plumbing stacks, top plates and duct penetrations are the routes. This is the fix that most reliably ends a dam and the one most often skipped, because it happens in an attic rather than on a roof.

Three: insulation. Depth at the eave matters more than depth in the middle. Baffles keep the soffit intake open while insulation runs right out over the top plate, which is exactly where most attics are thin.

The ice barrier is insurance, not a cure

Where eave ice has a history, the code requires an ice barrier: two layers of underlayment cemented together, or a self-adhering polymer modified bitumen sheet, running from the lowest edge of the roof to at least 24 inches inside the exterior wall line. That membrane seals around nails and holds standing water — which is precisely what a dam creates.

When one has already formed

Steam, not an axe. A professional ice-dam service melts channels through the ice with low-pressure steam, which removes the dam without removing the shingles. Chipping with hand tools takes granules, tabs and sometimes flashing with it, and turns a winter nuisance into a spring repair bill. Calcium chloride in a stocking laid across the dam opens a drainage channel and is the safe amateur option; rock salt corrodes metal and kills planting below.