How to Prevent Ice Dams on Your Roof

An ice dam is a ridge of ice at the edge of a roof. It traps melting snow behind it, and that water can back up under shingles and leak into walls and ceilings. They are common in Colorado, the Midwest and the Northeast.

How ice dams form

1. Heat leaks from the house into the attic.
2. The warm roof deck melts the snow on top of it.
3. Meltwater runs down to the cold roof edge, which hangs past the warm walls, and refreezes.
4. The ice ridge grows and water pools behind it.

The root cause is a warm roof. The fix is keeping the roof deck cold.

Long term prevention

1. Seal air leaks into the attic

Warm air escaping through gaps around light fixtures, attic hatches, plumbing stacks and chimneys is often the biggest heat source. Sealing them is usually the most effective step.

2. Add insulation

Bring attic insulation up to the level recommended for your climate zone. More insulation keeps heat in the living space.

3. Improve ventilation

Soffit vents at the eaves and ridge or roof vents at the top let cold outside air flow under the roof deck. Make sure insulation is not blocking the soffit vents.

4. Use ice and water shield when reroofing

This waterproof membrane under the shingles at the eaves is required by code in many cold areas. It stops water that gets past an ice dam from leaking in.

Quick fixes this winter

• Roof rake: pull snow off the first few feet of the roof from the ground after each storm.
• Heat cables: zigzag cables along the eaves can create channels for water. They treat the symptom, not the cause.
• Calcium chloride: a stocking filled with it laid across the dam can melt a channel. Do not use rock salt, which damages roofs and plants.

What not to do

• Do not chip ice with a hammer or shovel; it damages shingles.
• Do not climb on an icy roof.
• Do not use open flame or a pressure washer.

For a big dam or an active leak, hire a pro who uses low pressure steam to remove ice safely.

Frequently asked questions

Are icicles a sign of ice dams?

Large icicles along the gutters often are. They show water is melting on the roof and refreezing at the edge.

Do gutters cause ice dams?

No, but clogged gutters full of ice can make them worse.

Post your roofing or insulation job and compare bids from local pros.