Ice Dams on Older Roofs: Why Philadelphia-Area Rowhouses and Twins Leak

How does an ice dam form on a roof?

Heat from the house escapes into the attic and warms the roof deck, so snow on the upper roof melts. The meltwater runs down to the eave, which is colder because it hangs past the warm walls, and freezes there. Layer by layer, the ice builds a dam that holds back more meltwater.

A roof is meant to shed water downhill. With an ice dam, the lower edge of the roof is blocked by a ridge of ice, and the water behind it has nowhere to go. It rises until it finds a gap, and shingles are designed to shed water running down, not to stop water pushed up and back under them.

Three things have to happen together: there has to be snow on the roof, a warm upper roof and a cold eave. That is why ice dams are a winter problem, most common in January through March in this area, and why they follow freeze-thaw cycles, when a cold night follows a day just above freezing.

Winters here are milder than in the snow belts, but National Weather Service climate normals give about 23 inches of seasonal snowfall and a January average low near 25.6 degrees Fahrenheit. That is enough, in a house that leaks heat, to build an ice dam. The early 2026 winter brought snow at levels not seen in about ten years, and it is the kind of winter that exposes weak attics.

Why do older houses get ice dams more often?

Older houses typically have little attic insulation, gaps where warm air leaks into the attic, and few or blocked vents. That sends more heat to the roof deck and keeps the whole roof warm. Many older Philadelphia-area rowhouses are described as poorly insulated, which makes the effect worse.

The pattern is general, not measured for every house. A rowhouse or twin from the late 1800s through the early 1900s was built before modern insulation and air sealing. The attic floor may be bare plaster and lath with a thin layer of fill, or no insulation at all, and it was never designed to be sealed from the rooms below.

Heat escapes through the places no one thinks about: the gap around a chimney, the top of a wall where it meets the attic floor, a pull-down stair, a bath fan that vents into the attic and old recessed lights. Each of these warms the roof deck from below.

Ventilation matters because it keeps the roof deck as cold as the outside air. In many older houses the eave vents have been painted shut, covered by insulation or never existed. Without airflow, a warm attic stays warm.

Older houses also share roofs and walls. In a row, the neighbor's attic heat and the party wall can affect your roof. The rowhouse and twin water damage page explains how one house's problem can become the next house's.

How does an ice dam turn into a wet ceiling?

Water backs up under the shingles, runs down through the roof deck and soaks the attic insulation, then spreads across the top-floor ceiling and down inside the exterior wall. Stains, a sagging ceiling and peeling paint are the usual signs. Plaster ceilings can hold the water for a long time before a stain appears.

Water follows gravity and the paths of least resistance. It enters through nail holes, shingle joints and gaps around vents and chimneys, then follows the rafters or the deck until it reaches a ceiling. In older houses with plaster over lath, the water can spread above the ceiling and show up far from where it came in.

A ceiling that has taken on water may bulge or sag, because plaster and drywall are heavy when wet. Keep out from under a bulging ceiling, because it can drop without warning. If water is dripping from a light fixture, do not touch the fixture, and turn off power to that circuit at the panel only if you can reach it safely.

Wet attic insulation loses its value, and the wet wood and plaster around it can grow mold if they stay damp. The EPA's guidance is to dry water-damaged materials within 24 to 48 hours, which is hard to do in a cold attic with the snow still on the roof. The ceiling water damage page explains how a wet ceiling is opened, dried and repaired.

What should you not do about an ice dam?

Do not chip at the ice with a hammer, ax or pick, and do not put rock salt on the shingles. Chipping can crack shingles and gouge the roof and can cause injury, and salt can corrode metal flashing and gutters and kill plants below. Do not climb onto an icy roof either.

The urge to do something is strong, because the damage is visible. But most quick fixes make the roof worse. The table below sets out the common ones.

Raking snow off the lower roof from the ground, with a roof rake that has a long handle and wheels or a smooth edge, is the one commonly suggested step, and only if you can do it standing safely on the ground. Never stand on a ladder under a roof edge heavy with ice, since sheets of ice can slide off without warning.

What can you do inside while the snow is still on the roof?

Move belongings out from under the stain, put a bucket under any drip, photograph the damage and keep out from under a bulging ceiling. Do not turn on a ceiling light that is wet. Call a roofer about the cause, and call a restoration crew about the wet ceiling and insulation.

The aim is to keep the damage from spreading until someone can open the ceiling and dry it. Take photographs before anything is moved, and keep notes on the date and what you saw. Whether any of it is covered is for your insurer to decide, and the crew can supply moisture readings and photos for a claim.

If you can get into the attic safely, look for wet insulation, dark patches on the deck and daylight at the eave, but do not stay if the air feels damp and cold, and wear a mask in an old attic. Do not disturb old insulation, pipe wrap or plaster dust, since houses built before the 1980s may have asbestos in them and houses built before 1978 may have lead paint.

Frozen gutters are not the same problem, although they often go with an ice dam. If a gutter is full of ice, do not hack at it. A roofer can tell whether the gutter or the roof edge needs work.

What does the restoration crew do, and what does a roofer do?

A restoration crew dries the interior: it removes wet insulation, opens the ceiling where needed, dries the structure with air movers and dehumidifiers, and can tarp or board up an opening in an emergency. A roofer repairs or replaces the roof, and an insulation or ventilation contractor fixes the cause.

The ice dam roof leak page explains what the crew does inside. Roof replacement is not part of this service. If the roof must be repaired or replaced, a registered roofing contractor does it.

Working estimates for the crew's side of the job, confirmed on site, run roughly $250 to $1,200 for an emergency tarp or board-up and $500 to $1,800 for repairing one section of a water-stained ceiling, including paint. Those are estimates, not quotes, and the price depends on the area and the materials.

Repairs after drying are quoted separately, and the cause needs its own fix. If the attic is not sealed, insulated and ventilated, the same ice dam can form again next winter. Ask the contractor what will be done about the air leaks, and get that in writing.

How do you prevent ice dams in an older house?

Seal the air leaks between the living space and the attic, add insulation to the attic floor and make sure the eaves can breathe. The aim is a cold roof deck, so snow melts only from the sun and not from house heat. Check gutters and downspouts in the fall, before the first snow.

Air sealing comes first, because insulation does not stop air that moves through gaps. Common leak points are around chimneys and pipes, at the tops of walls, around the attic hatch and at recessed lights. An energy auditor or insulation contractor can find them with a blower door test or an infrared camera.

Insulation is the second step. How much a house needs depends on the climate zone and the current guidance, which is published by the U.S. Department of Energy. A contractor can tell you the figure for your attic.

Ventilation is the third. Eave vents let cold air in, and a ridge or roof vent lets warm air out. In a rowhouse, the options may be limited by the roof shape, and the contractor should say what is possible.

Fall is the time to act. Clear gutters and downspouts, and check flashing around chimneys and vents. After a freeze, check the ceiling under the roof edge in daylight, and look for new stains. September is a good month for a roof and gutter check, before the leaves and the cold arrive.