Ice dams: why the ceiling leaks in February, and what actually fixes it

Every February, roofers get calls from homeowners with water dripping through the ceiling by an outside wall, on a day when it has not rained. The roof is fine. What has happened is an ice dam, and it is one of the most misunderstood problems in the house because the fix is not on the roof at all.

How it forms

Snow lands on the roof. Heat escaping from the house into the attic warms the roof deck from below and melts the snow from underneath. The meltwater runs down the slope until it reaches the eaves, which hang out past the warm house and stay cold. There it refreezes. Over days a ridge of ice builds along the roof edge, and water backing up behind it has nowhere to go but under the shingles and into the wall.

Shingles are designed to shed water running downhill. They are not designed to hold back a pond.

What to do this week

• Do not climb up and chip at it. It takes the shingles and the gutters with it, and people fall.
• A roof rake from the ground, pulling the snow off the lowest three or four feet of the roof, removes the water supply. Do it after every snowfall.
• For a dam that is already leaking, a Pro with steam equipment melts a channel through it without damaging the roof. Some homeowners fill a leg of pantyhose with calcium chloride ice melt and lay it across the dam to melt a channel; it works, slowly, and it is hard on the gutters and the plants below.
• Inside, catch the water and pull back wet insulation so the ceiling can dry. If the drywall is sagging, poke a small hole to let the water out into a bucket.

What fixes it for good

The cause is heat in the attic, so the fix is keeping the heat in the house.

• Air sealing. Most of the warm air in an attic got there through gaps: the attic hatch, recessed lights, plumbing and wiring penetrations, the top of interior walls, bath fans venting into the attic instead of outside. Sealing them is the highest-return job in the house and it costs less than the insulation.
• Insulation, to the level for your climate, which for most of the country is far more than older houses have. Blown cellulose or fiberglass over the sealed attic floor.
• Ventilation. Soffit vents that let cold outside air in at the eaves, and a ridge vent that lets it out at the top, keep the roof deck close to the outside temperature so the snow does not melt from below in the first place. Insulation stuffed into the eaves blocks the soffit vents; baffles keep the path open.

A roofer, an insulation Pro or an energy auditor with a blower door and an infrared camera can show you exactly where the heat is leaving. Do the sealing and insulation first. Heat cables along the eaves are a last resort that manages the symptom for a monthly electric bill.

When you replace the roof

Make sure the estimate includes ice and water shield, a self-adhered membrane, from the eave up at least two feet past the inside wall line, and in every valley. Code requires it in cold climates. It does not stop the dam, but it keeps the water out of the house when one forms.