How to prevent ice dams
Ice dams form when heat escaping into the attic melts snow on the roof, and that water refreezes at the cold edge. The ridge of ice traps more meltwater behind it, which can back up under shingles and leak into walls and ceilings. To prevent ice dams, you need to keep heat out of the attic so the whole roof stays cold. In the meantime, you can safely remove snow from the ground and melt channels through a dam that's already formed. Ice dams are an attic problem that shows up on the roof. Keep the attic cold and they stop forming.
How ice dams form
On a roof with an ice dam, the upper part of the roof sits over a warm attic, so snow there melts even when it's below freezing outside. The water runs down until it reaches the eaves, which hang out past the warm walls and stay cold, and it freezes there. Over days, that ice builds into a ridge.
Signs to watch for include thick icicles along the gutters, ice building up on the roof edge, and water stains or peeling paint on ceilings or walls near outside walls. The root cause is almost always warm air leaking into the attic, too little insulation, or poor attic ventilation.
What causes ice dams
Gaps around fixtures, pipes and the hatch heat the roof deck.
Heat from living spaces warms the roof.
Insulation or debris covers the soffit vents.
Water sits at the roof edge and freezes.
| You notice | Likely cause | Start at |
|---|---|---|
| Big icicles along the gutters | Heat loss melting roof snow | Step 1 |
| Deep snow on the roof edge | Snow feeding meltwater | Step 2 |
| Ridge of ice with water behind it | Active ice dam | Step 3 |
| Gutters full of ice | Gutters clogged before winter | Step 4 |
| Bare patches on a snowy roof | Attic air leaks | Step 5 |
| Frost or dark spots on attic sheathing | Poor ventilation or vented moisture | Step 6 |
| Bath fan duct ends in the attic | Moist air venting into the attic | Step 7 |
Let's fix it
- Look for the warning signsWalk around the house after a snowfall. Big icicles, a thick band of ice along the roof edge, or bare patches of roof in the middle of snow point to heat escaping. Inside, check ceilings and upper walls near outside walls for stains or damp spots. Catching it early keeps a small problem from becoming a leak.
- Rake snow off the roof edgeAfter each heavy snow, use a roof rake from the ground to pull snow off the bottom three to four feet of the roof. Less snow near the edge means less meltwater feeding the dam. Work from the side so snow doesn't fall on you, pull gently so you don't damage shingles, and stay well clear of power lines. Never climb onto a snowy or icy roof.
- Melt channels through an existing damFill a nylon stocking or long sock with calcium chloride ice melt and tie it off. Using a roof rake from the ground, push it into place across the ice dam so it runs from the roof edge up past the dam. It slowly melts a channel that lets trapped water drain. Don't use rock salt, which can damage shingles, gutters and plants, and don't hack at the ice with a hammer or shovel.
- Keep gutters and downspouts clearClean gutters in the fall after the leaves drop. Clogged gutters hold water that freezes and gives an ice dam a head start. Make sure downspouts are clear and drain away from the house so meltwater has somewhere to go.
- Seal attic air leaksMost heat reaches the attic through gaps, not through the insulation itself. In the attic, look for dirty or dark insulation, which shows where air has been moving. Seal around plumbing vents, wiring holes, the attic hatch and light fixtures with caulk or spray foam. Around chimneys and flues, use metal flashing and fire-rated caulk instead of regular foam, and use only insulation-contact (IC) rated fixtures where insulation will touch them.
- Add insulation and check ventilationOnce leaks are sealed, make sure the attic has enough insulation for your climate; your local building department or utility can tell you the recommended level. Keep insulation from blocking the soffit vents, using baffles at the eaves so air can flow from the soffits to the ridge or roof vents. Good ventilation keeps the underside of the roof close to the outdoor temperature.
- Make sure fans vent outsideCheck that bathroom exhaust fans, the kitchen hood and the dryer vent run all the way outside, not into the attic. Warm, moist air dumped into the attic adds heat and causes frost and mold. Reconnect loose ducts and run them to a proper roof or wall cap.
Watch it done
- No ice ridge forming at the roof edge
- Snow melts evenly across the roof
- No stains or drips inside after a thaw
If ice dams keep forming
If ice dams keep forming after you've raked and kept gutters clear, the attic is still losing heat. An energy auditor can use a blower door and infrared camera to show exactly where.
If you see water stains after an ice dam melts, check the attic and ceiling for damp insulation and wet drywall. There's a separate guide for a ceiling water stain, and have the roof checked for damaged shingles or underlayment.
What a pro typically charges (a rough estimate; prices vary by area): Professional ice dam removal with steam typically runs $400–$1,000 or more depending on roof size and access. Attic air sealing and added insulation usually costs $1,500–$4,000, a home energy audit $200–$600, and installed heat cables $400–$1,200.
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Ice dam questions
What causes ice dams?
Heat escaping into the attic melts roof snow, and the water refreezes at the colder roof edge.
Can I use rock salt to melt an ice dam?
No. Use calcium chloride in a stocking; rock salt can damage roofing, gutters and plants.
Do heat cables prevent ice dams?
They can keep drainage paths open, but sealing and insulating the attic is the real fix.
Should I get on the roof to remove snow?
No. Use a roof rake from the ground, and call a pro for large ice dams.
Why is water leaking from my roof in winter?
It can be an ice dam. Meltwater trapped behind a ridge of ice at the roof edge can back up under the shingles and leak into walls and ceilings.