How ice dams form, and how to stop them
An ice dam starts in the attic. Heat from the house warms the roof, the snow melts from below, and the water freezes again at the cold edge. Set the weather and the attic, run a week of winter on the roof edge, and try the fixes.
The roof over the attic is above freezing, so the bottom of the snow is melting.
Right now
- Attic air
- 38°F, 18° above outside
- Shingles under the snow
- 32°F, melting
- Melt reaching the edge
- About 29 gallons a day along a 40 ft roof
- Ice dam
- None
- Behind the dam
- Dry
- Inside
- Dry
A warm roof over a cold edge.
Ice dams need three things at once: snow on the roof, air below freezing, and a roof that is warmer over the attic than at the edge. The first two are New Hampshire in January. The third comes from the house, which is why two houses on the same street can look so different after the same storm.
- Attic in the 40s
Heat leaks up
Warm house air gets into the attic through gaps in the ceiling and through thin insulation.
- Shingles at 32°F
The roof warms
The roof deck sits on that warm air, and the snow on top holds the heat in.
- Under the snow
Snow melts from below
The bottom of the snow turns to water and runs down the roof where nobody sees it.
- Over the overhang
It refreezes at the edge
The overhang has no attic under it and is as cold as the air. The water freezes there and the dam grows.
Get the water off the roof safely.
These deal with the dam in front of you. Stay off an icy roof, and stay out from under icicles: big ones come down with the gutter attached.
Rake from the ground
A roof rake with an extension pole takes the snow off the bottom few feet after each storm. Stay on the ground, and keep a metal rake away from the power line to the house.
Calcium chloride, in a sock
Fill a nylon stocking with calcium chloride ice melt and lay it across the dam, hanging over the edge. It melts a channel so the water behind it drains. Rock salt damages shingles, gutters and plants.
Steam, by a roofer
Roofers take dams off with low-pressure steam, which melts the ice without hurting the shingles. Chipping with a hammer, an ax or a shovel breaks shingles and people.
If water is coming in
Put out buckets and move things away. Stay out from under a ceiling that sags. If water is near a light fixture, turn that circuit off at the panel.
Keep the roof cold.
A cold roof doesn't make dams, because the snow on it only melts from the top on a warm day. The work is in the attic, in this order.
- 1
Seal the attic floor
Foam and caulk the holes for wiring, plumbing and the chimney, the tops of the walls, and the attic hatch. Put airtight covers over recessed lights. This is usually the biggest single fix, and an energy audit with a blower door finds the leaks.
- 2
Insulate to R-49 or more
About 15 inches of blown cellulose or fiberglass on the attic floor, after the sealing. Thin insulation over the top of the outside wall is where most dams start.
- 3
Keep the vents open
A baffle in every rafter bay holds the insulation back from the soffit vents, so outside air runs from the soffit to the ridge and keeps the attic cold.
- 4
Shield at the next roof
Ice and water shield from the edge to at least 24 inches inside the wall keeps the water out if a dam forms anyway. It goes on when the roof is replaced.
Heat cables along the edge melt a path for the water and can help on one problem spot, but they run on electricity all winter and leave the warm attic as it is. Taking the gutters off doesn't stop dams either: the ice forms on the shingles over the overhang.
Same storm, five different roofs.
Each of these houses gets a foot of snow and a week in the 20s. What happens next depends on the attic. Pick one to run it on the drawing.
A cold roof
"Nobody calls."
A sealed ceiling, R-49 on the attic floor, baffles at every rafter bay and open soffit and ridge vents. After a foot of snow and a cold week, the attic is a few degrees above outside and the shingles under the snow stay below freezing.
Air leaks through the ceiling
"We have plenty of insulation and we still get ice dams every year."
There's a fair amount of insulation, but warm air goes straight around it: recessed lights, the attic hatch, plumbing and wire holes, the tops of interior walls. Air carries far more heat than the insulation lets through, so the attic warms up anyway.
Thin attic insulation
"It's an older house. We get icicles all along the front every winter."
A few inches of old insulation, with the tops of the joists showing. Enough house heat comes through it to warm the attic, and after a deep snow the roof deck goes above freezing.
Insulation over the soffit vents
"We had insulation blown in a couple of years ago and the ice dams got worse."
Insulation was blown in without baffles, and it filled the eaves and buried the soffit vents. With no outside air moving through, the attic holds the heat that does get up there.
A leaky attic, an old roof
"There's water coming through the ceiling by the window, and there's ice all along the gutter."
Everything at once: a leaky ceiling, thin insulation, buried vents, and a roof with only felt paper under the shingles. A big dam forms in a few days, and the pond behind it gets into the house.
Where AI helps in ice dam season.
Ice dam calls come in waves, a few days after a big storm, and most of them come from houses whose real problem is in the attic. Answering the wave and following up later are both office work that software handles well.
The morning after the storm
After a big storm and a cold week, every house with a warm attic calls at once. A phone agent asks whether water is coming in now, takes the address and asks for photos by text, and puts active leaks at the top of the list.
AI voice agentsThis winter's calls, next summer's work
Each ice dam call is a house that needs air sealing and insulation. An agent keeps the photos and notes from the call and follows up in the spring with an energy audit offer, when the crew has time for attics.
Custom AI agentsEstimates from the photos
The office learns to turn a homeowner's photos and the attic notes into a written estimate and a plain explanation of why the dams happen, so the customer understands why the fix is in the attic.
AI Cowork training
This page shows a typical roof edge on a New Hampshire house with a 6/12 roof, simplified for learning. Attic temperatures, melt and ice amounts come from a simple heat balance with typical values and leave out sun and wind; they are not measurements of any house. It is not a repair guide: stay off icy roofs, and have a roofer or an insulation contractor look at your house.
