Why ice dams form and how to stop them

An ice dam starts when snow covers a roof and the roof deck under it rises above freezing. Heat escaping from the house usually warms it. The snow touching the deck melts and runs down to the eaves, where the overhang is colder, and it freezes again. As the cycle repeats, the ice builds into a ridge that holds back more meltwater. That water backs up under the shingles and leaks into the attic and the walls.

Why some roofs get them

Ice dams need lasting snow and long stretches below freezing, so they're most common in cold, snowy climates. Snow itself insulates, at roughly R-1 to R-2 an inch, so a deep snowfall traps more heat against the roof. Complicated roofs, with several levels, valleys and dormers, are harder to seal and insulate. They get ice dams more easily than a simple roof over a vented attic.

What stops them

The goal is a roof deck that stays as cold as the air outside. The work happens in the attic, and it's work to have done by a professional.

• Seal the air leaks into the attic. This is the most important step. Warm air leaking up from the house is one of the biggest sources of ice dams.
• Insulate the attic floor. In the coldest climate zones, a vented attic needs R-60 or more, carried out over the tops of the outside walls.
• Ventilate the attic, with air flowing in at the soffits and out at the ridge. Baffles keep the insulation out of the soffit vents and leave at least 2 inches of air space under the roof deck.
• Keep heat sources out of the attic. A furnace or ducts in the attic warm the roof deck from below.

In snowy country, a new roof usually gets a self-adhering waterproof membrane at the eaves and in the valleys. It protects against the water that does back up.

An annotated example

What to do next

After a snowfall, look along the eaves for a thick ridge of ice and long icicles. Check the ceilings and walls under the roof for stains too. Have any ice on the roof removed by a roofer rather than climbing up yourself. Then book an assessment of the attic's air sealing, insulation and ventilation before next winter.