Ice Dams: How to Spot the Risk Before Winter

Big icicles look picturesque. On a home inspector's checklist, they're a warning sign. They often mean an ice dam is forming, and ice dams can push water under your shingles and into ceilings and walls.

01How ice dams form

1. Heat from the house leaks into the attic and warms the roof deck.
2. Snow on the upper roof melts, even when it's below freezing outside.
3. The water runs down to the cold eaves (the overhang), where it freezes.
4. The ice builds a ridge, a "dam." More meltwater backs up behind it, under the shingles, and leaks into the house.

The root cause is almost always heat escaping into the attic, plus snow and cold weather.

02Signs your house is at risk

• Big icicles along the eaves, year after year
• Stained ceilings or walls near exterior walls, or peeling paint, after winter
• Snow melting off the upper roof while the eaves stay loaded
• Thin attic insulation or obvious air leaks. See inspecting your attic
• Recessed lights, bathroom fans or ducts in the ceiling below the attic
• Low-slope roofs and complex roofs with valleys

03Prevention (the real fix)

• Air-seal the attic floor so warm air can't get into the attic.
• Add insulation to cold-climate levels.
• Make sure the attic is ventilated from soffit to ridge.
• When re-roofing, an ice and water shield membrane along the eaves is required by code in many cold areas and helps a lot.

04During winter

• A roof rake from the ground can pull snow off the first few feet of roof after storms. Stay off ladders on icy days.
• Don't chip or hack at ice with tools; you'll damage the shingles.
• Heat cables can help in trouble spots but don't fix the cause.
• Ice dam removal with steam is a job for pros.

05Gutters aren't the cause, but they matter

Clogged gutters don't cause ice dams, but they make icy buildup worse. Clean them after the leaves fall. See gutters and downspouts.

Fix the heat leaks this fall and you stop paying to heat the outdoors, and stop the ice dams too.

Photo: Icicles, Downpatrick, December 2010 (03).JPG) by Ardfern, CC BY-SA 3.0.

Related: Inspect Your Roof From the Ground: What Binoculars and the Attic Tell You