Ice Dams: Why They Leak Into Ceilings and How to Stop Them

An ice dam is a ridge of ice along the eaves that holds back meltwater, which then backs up under the shingles and leaks into the attic, walls and ceilings. Research by the U.S. Army Corps of Engineers found that heat from the building, not the sun, is the primary cause. The lasting fix is keeping the roof deck cold with air sealing, insulation and ventilation, and the building code adds a waterproof layer at the eaves as a backstop.

How an ice dam forms

It takes a warm roof over a cold eave. The Department of Energy’s Building America program describes the sequence: warmth from the attic melts snow on the roof, the meltwater runs down to the eaves, and it refreezes at the colder overhang, building up ice. That happens when the roof deck is above freezing while the outdoor air is below freezing.

The Army Corps’ Cold Regions Research and Engineering Laboratory (CRREL) studied the problem on buildings at Fort Drum in upstate New York. Its 2001 guidelines open with photographs of two identically built buildings near Watertown, New York, taken minutes apart: the heated one had large ice dams and icicles, and the unheated one had none. They also point out why ice dams leak. Most steep roofs are built to shed water, not to hold back standing water, so water that ponds behind an ice dam may leak into the building.

CRREL’s guidelines size attic ventilation to keep the underside of the roof below freezing when it is 22°F outside. When it is colder than that, outdoor air removes heat more easily. When it is warmer, meltwater seldom refreezes at the eaves. According to CRREL, 57 buildings at Fort Drum modified to these guidelines essentially eliminated their chronic icing problems.

Signs an ice dam is leaking into the house

Water from an ice dam often shows up away from where it entered. The Energy Department lists these signs of possible water intrusion into an attic:

• wet insulation in the attic, especially near the eaves,
• warped or rotted soffit panels or fascia boards,
• bubbled paint on the fascia or soffits,
• worn or warped shingles at the roof edges,
• mold or mildew on attic surfaces.

Inside the house, look for stains or peeling paint on ceilings and on the upper parts of exterior walls. The National Park Service notes that water from melting ice dams can travel great distances down walls and along pipes and conduits, so a stain can appear well below or beside the leak.

Why heat reaches the roof

Air leaks are a major route. The Building America guide calls heat lost into the attic through air leakage, which warms the roof deck, a major source of ice dams in houses. Warm air escapes through gaps around light fixtures, ducts, pipes, wiring and the attic hatch.

Insulation and ventilation are the other two parts. The Building America guide recommends:

• Insulation. R-60 or more for these roofs in cold climates, Climate Zone 5 and colder, with the insulation over the tops of the exterior walls at least matching the walls’ own R-value.
• Ventilation. Every rafter or truss bay vented at the eave and at the ridge.
• Clearance. At least 2 inches of vent space above the insulation, double the 1-inch minimum in the International Residential Code (IRC).

CRREL also acknowledges two of the same points: insulation has to be installed so that it works and does not block the air pathways, and there has to be a continuous air barrier between the living space and the attic. Roofs with dormers, valleys and other complications are prone to ice dams near those features, and CRREL says their ventilation details deserve extra attention.

What the building code requires

The 2024 IRC, Section R905.1.2, requires an ice barrier in areas where there has been a history of ice forming along the eaves and backing up water. Your local building department designates whether your area is one of them, in its version of Table R301.2. Where the requirement applies:

• It covers roofs of asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes.
• It is either two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment.
• It runs from the eave edge to a point at least 24 inches inside the exterior wall line of the building.
• On roofs with a slope of 8 in 12 or steeper, it extends at least 36 inches along the roof slope from the eave edge.
• Detached accessory buildings with no conditioned floor area are exempt.

An ice barrier limits leaks; it does not stop ice from forming. The same code, in Section R806, requires attic ventilation and at least 1 inch of space between insulation and roof sheathing at the vents. Many jurisdictions still use an earlier edition of the IRC or local amendments, so ask your building department which rules apply to your roof.

What to do while an ice dam is leaking

Deal with the water inside first. The EPA advises drying water-damaged areas and items within 24 to 48 hours to prevent mold growth. Catch drips, move belongings out of the way, and photograph the stains and the roof edge.

Removing ice from a roof is roof work on a frozen, sloped surface. It is a job for a roofing contractor with the right equipment.

When to call a professional, and what to ask

You may need three kinds of help. A roofer deals with the ice and the roof covering. An insulation or home performance contractor deals with the heat loss that causes the dams. A water damage contractor dries wet ceilings, walls and insulation. The Building America guide puts sealing the air leaks from the house into the attic ahead of every other step, and says that in existing homes an infrared camera may help locate air leaks or missing insulation.

Questions worth asking:

1. Where is heat getting into the attic, and how will you find the leaks: blower door, infrared camera, or both?
2. What insulation level will the attic have when you finish, and will it cover the tops of the exterior walls without blocking the eave vents?
3. Is every rafter bay vented at the eave, with a clear channel to the ridge?
4. Does the roof have an ice barrier, and how far past the exterior wall line does it extend?
5. Were the insulation at the eaves and the top of the exterior wall below the leak checked for moisture, and what reading must they reach before they are covered again?
6. Does this work need a permit here?

Heat cables and ice removal treat the ice. Keeping the roof deck cold treats the reason it formed, and that is what the ventilation changes at Fort Drum did.