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Learning how to prevent ice dams is mostly an attic job, and fall is the time to do it. An ice dam is a ridge of ice along the edge of your roof that traps melting snow behind it. That water backs up under the shingles and leaks into your ceilings and walls, often weeks before you notice a stain.
This guide covers what ice dams are, the signs to watch for, what causes them, the 5 steps to prevent ice dams before winter, how to remove one safely if it forms, and when to call a pro.
WHAT’S IN THIS GUIDE
What Is an Ice Dam?
Ice dams form when the upper part of your roof is above freezing and the lower edge is below freezing. Snow on the warm upper roof melts, the water runs down to the cold edge over the eaves, and it freezes again. As that ridge of ice grows, it blocks the next round of meltwater. The trapped water pools behind the dam and finds its way under the shingles, into the roof deck, and eventually into your attic, ceilings, and walls. Source: U.S. Department of Energy — Attic Air Sealing, Insulating, and Ventilating for Ice Dam Prevention
Signs of Ice Dams on Your Roof
Watch for these after a snowfall followed by a few cold days:
- Thick icicles hanging from the gutters and roof edge. A few small icicles are normal, but large ones can be a sign that ice dams are building. Don’t stand under them, since they can fall.
- A ridge of ice along the edge of the roof, right above the gutters.
- Water stains or damp spots on ceilings near outside walls, or on the underside of the roof deck in the attic.
- Snow that melts off the upper roof while it stays put on the lower edge. That pattern means heat is escaping through the roof.
Source: Travelers — How to Prevent and Remove Ice Dams
If you find a stain, our guide on how to check for water damage walks through what to look for next.
What Causes Ice Dams?
Ice dams need three things: snow on the roof, freezing temperatures outside, and an attic that’s leaking heat. The U.S. Department of Energy’s Building America program points to a poorly air-sealed or poorly insulated attic as the cause. Warm air from your living space rises into the attic, warms the underside of the roof, and melts the snow from below. Source: U.S. Department of Energy — Attic Air Sealing, Insulating, and Ventilating for Ice Dam Prevention
You can’t control the snow or the weather, but you can control the heat. That’s why learning how to prevent ice dams comes down to the attic, not the roof.
How to Prevent Ice Dams in 5 Steps
Step 1: Seal air leaks into the attic
This is the most important step. The Department of Energy calls air sealing the top priority, because it stops warm, moist air from reaching the roof in the first place. Adding insulation over leaks often won’t fix the problem. Grab a flashlight and look in the attic for these common leaks:
- Holes where electrical wiring and plumbing pipes come up through the ceiling
- Recessed can lights and bathroom exhaust fans
- Gaps around the furnace flue and chimney
- Dropped soffits over kitchen cabinets that open into the attic
- The attic hatch or pull-down stairs
Source: U.S. Department of Energy — Attic Air Sealing, Insulating, and Ventilating for Ice Dam Prevention
ENERGY STAR recommends caulk for gaps 1/4 inch or smaller and expanding foam for gaps from 1/4 inch to 3 inches. Don’t use spray foam around the furnace flue, since it gets very hot. That gap needs a metal flashing and special high-temperature caulk. For the attic hatch, an attic stairway insulator cover seals and insulates the opening in one step. Source: ENERGY STAR — Attic Air Sealing Project
A few safety notes from ENERGY STAR before you go up there: walk only on the joists, not the drywall or insulation between them, or you can fall through the ceiling. Wear a dust mask around insulation. If you see shiny, pebble-like vermiculite insulation, don’t disturb it until it’s been tested for asbestos, and call an electrician if you find old knob-and-tube wiring.
Step 2: Add attic insulation
Once the leaks are sealed, insulation keeps the heat that’s left from reaching the roof. ENERGY STAR recommends R-60 for an uninsulated attic in Zones 4 through 6, or adding up to R-49 if you already have 3 to 4 inches. Central Indiana falls in Zone 5, so those are the targets here. If you can see the tops of your ceiling joists, you don’t have enough. Source: ENERGY STAR — Recommended Home Insulation R-Values
The same insulation that prevents ice dams in winter also keeps the upstairs cooler in summer. Our guide on whether attic insulation helps cool your house covers the summer side.
Step 3: Keep attic vents open
A cold roof doesn’t melt snow, and attic ventilation helps keep it cold. Outside air should flow in through the soffit vents under the eaves and out through vents near the ridge. The Department of Energy says it’s critical to vent the roof in cold climates, and to keep insulation from blocking the soffit vents. Install rafter vent baffles at the eaves to hold insulation back and leave an air channel of at least 2 inches under the roof deck. Source: U.S. Department of Energy — Attic Air Sealing, Insulating, and Ventilating for Ice Dam Prevention
Step 4: Clean the gutters
Clean gutters won’t stop ice dams by themselves, since the problem starts in the attic. But gutters packed with leaves hold water and ice right at the roof edge, where an ice dam starts, and the weight of the ice can pull them loose. Clear them out after the leaves are down. Our guide on how to clean gutters walks through it.
Step 5: Rake snow off the roof edge after big storms
If a heavy snow is followed by a cold stretch, clearing the lower edge of the roof takes away the snow that would melt and refreeze there. Use a roof rake from the ground, and clear about 3 to 4 feet up from the edge. Never climb onto a snowy or icy roof, and stay out from under the snow as it slides off. Source: Travelers — How to Prevent and Remove Ice Dams
How to Remove an Ice Dam Safely
If ice dams form anyway, the goal is to give the trapped water a way off the roof without damaging the shingles:
- Rake the snow off the lower 3 to 4 feet of the roof from the ground first.
- Fill a nylon stocking or the leg of old pantyhose with calcium chloride ice melt and lay it on the roof so it runs vertically across the ice dam, with the end hanging just past the gutter. It slowly melts a channel through the ice so water can drain.
- Cover the shrubs below with a light tarp to protect them from the runoff.
- Don’t use rock salt (sodium chloride), which can damage the roof.
Source: Travelers — How to Prevent and Remove Ice Dams
Never chip at an ice dam with a hammer, ice pick, or shovel, and never use a torch. You’ll tear up the shingles and create the very leaks you’re trying to stop. If you can’t reach the roof safely from the ground, hire a contractor. Many roofers remove ice dams with low-pressure steam, which melts the ice without damaging the roof.
Calcium chloride is also the best ice melt for your driveway on the coldest nights. Here’s how it compares to the others.
Do Heat Cables Work?
Heat cables, the zigzag cables clipped along the roof edge, can keep a drainage path open, but they treat the symptom instead of the cause. University of Minnesota Extension notes that their melting effect only reaches a few inches from the cable, and they use a lot of electricity. They’re a reasonable fix for a trouble spot you can’t otherwise solve, like a low roof over a porch, but sealing and insulating the attic is the real cure. Source: University of Minnesota Extension — Preventing Ice-Dams on Minnesota Homes
When to Call a Pro
Call a professional if:
- Water is already leaking into the house
- The ice dam is on a second-story roof or any roof you can’t reach from the ground
- You get ice dams every winter even after sealing and insulating
- You find vermiculite insulation, knob-and-tube wiring, or signs of mold in the attic
An energy auditor can run a blower door test to find hidden air leaks, which is often the fastest way to fix a roof that dams up every year. Sealing the attic also cuts your heating bill. See how much a lower bill adds up with our thermostat savings calculator, and work through the rest of our winter home maintenance checklist while you’re at it.
Frequently Asked Questions About Ice Dams
What causes ice dams on a roof?
Heat escaping from the house into the attic warms the roof and melts the snow on top. The water runs down to the cold roof edge and refreezes, forming a ridge of ice that traps more water behind it.
Does homeowners insurance cover ice dam damage?
Many policies cover water damage inside the house from an ice dam, but not always the ice removal or the roof repair itself. Coverage varies, so check your policy or call your agent before winter.
Are icicles a sign of ice dams?
They can be. A few small icicles are normal, but large icicles along the roof edge, especially with a ridge of ice behind them, often mean ice dams are forming. Watch for water stains on ceilings near outside walls.
Do gutters cause ice dams?
No. Ice dams start with heat loss through the attic, and they can form on roofs without any gutters. Clogged gutters can make the ice buildup at the roof edge worse, though, so keep them clean.
What is the fastest way to remove an ice dam?
Rake the snow off the roof edge from the ground, then lay a nylon stocking filled with calcium chloride ice melt across the dam to melt a drainage channel. For large ice dams or high roofs, a pro with a steamer is fastest and safest.
When should I prevent ice dams?
In the fall, before the first snow. The work involved in how to prevent ice dams, sealing attic air leaks and adding insulation, is much easier and safer in mild weather, and it needs to be done before snow sits on the roof.
Sources & Further Reading
- Source: U.S. Department of Energy — Attic Air Sealing, Insulating, and Ventilating for Ice Dam Prevention
- Source: ENERGY STAR — Attic Air Sealing Project
- Source: ENERGY STAR — Recommended Home Insulation R-Values
- Source: University of Minnesota Extension — Preventing Ice-Dams on Minnesota Homes
- Source: Travelers — How to Prevent and Remove Ice Dams