What Are Ice Dams? Causes and Warning Signs in New England

What is an ice dam on a roof?

An ice dam is a band of ice that forms along the eaves of a snow-covered roof and blocks melting snow from draining off. Behind that band, water collects in a shallow pool on the roof surface. Most roofs are designed to shed water downhill, not to hold standing water, so that pool looks for any gap it can find: shingle joints, nail holes, flashing seams and the line where the roof meets a wall. Ice dams usually appear after a good snowfall followed by several days of cold weather. They tend to be thickest above heated rooms and are often thinner or missing over unheated spaces such as a garage or a porch roof. That pattern is a strong clue to what is really going on: the dam is a symptom of heat moving from the house into the roof.

What causes ice dams?

Ice dams are caused by uneven roof temperatures, and those come from heat leaking out of the house. Warm indoor air rises through gaps in the top-floor ceiling, such as recessed lights, attic hatches, plumbing and wiring holes, chimney chases and bath fans that exhaust into the attic. Thin or settled insulation adds to the problem by letting heat conduct straight through. That heat warms the roof deck above the attic past 32°F even when the outdoor air is well below freezing. Snow sitting on the warm section melts from underneath, and the water trickles downhill. When it reaches the overhang at the eave, there is no heated space beneath it, so the deck there is as cold as the outside air and the water freezes. Each cycle adds another layer, and the dam grows taller and wider. Sunshine can add some daytime melting, but the steady driver is heat from inside.

Why are ice dams so common in New England?

Ice dams are common across New England because the region combines heavy snow, long cold spells and a large stock of older homes. Nor'easters can drop deep, heavy wet snow on Massachusetts, Rhode Island and Connecticut, and inland parts of New Hampshire, Vermont and Maine often keep snow on roofs for weeks. A roof that stays covered while temperatures sit in the teens and twenties gives the melt-and-refreeze cycle plenty of time to work. The housing matters too. Many Capes have finished upper rooms with sloped ceilings tucked right under the roof, leaving little room for insulation and airflow. Older Colonials, Victorians and triple-deckers often have balloon framing, attic floors pierced by decades of wiring and plumbing changes, and limited soffit venting. Additions and dormers create complicated roof lines with valleys where snow piles up. All of these make it harder to keep the roof deck evenly cold.

What are the warning signs of an ice dam?

The most visible warning sign is a thick ridge of ice along the roof edge, often with long icicles hanging from the gutters or drip edge. A few small icicles are normal; heavy icicles above heated rooms usually mean heat loss. Other outdoor clues include gutters frozen solid, ice forming on siding or behind the gutter, and bare streaks or melted patches on an otherwise snow-covered roof, which show where heat is escaping. Inside, watch for water stains or damp spots on ceilings near exterior walls, peeling paint or bubbling drywall along the tops of walls, wet or compressed insulation in the attic, and frost on the underside of the roof sheathing or on nail tips. Water dripping from window or door trim during a cold snap is another sign. Act early if you see any of these, because leaks behind an ice dam can spread inside walls before they show.

What damage can an ice dam cause?

An ice dam can cause water damage to the roof structure, insulation, walls and interior finishes. Water forced under roofing soaks the roof sheathing, and repeated wetting can lead to rot and delamination of plywood or board decking. It drips onto attic insulation, which loses much of its value when wet and can stay damp long after the thaw. From there, water travels down inside exterior walls, staining ceilings, loosening paint and wallpaper, and wetting drywall and plaster. Damp framing and insulation invite mold if they don't dry out. Outside, the weight of a large dam and its icicles can bend gutters or pull them off the fascia, and falling ice is a hazard to people, pets and anything parked below. Several winters of small leaks can be as costly as one big event, because the damage hides inside the structure.

How do you stop ice dams for good?

You stop ice dams for good by keeping the whole roof deck close to the outdoor temperature, which means fixing the heat loss that melts the snow. The order of work matters. First, air-seal the attic floor and top-floor ceiling so warm air stops rising into the attic. Second, bring insulation up to a level suited to a cold climate. Third, make sure the attic is ventilated with intake at the soffits and exhaust near the ridge, so any heat that gets through is carried away. At the roof itself, an ice barrier membrane at the eaves gives a second line of defense if water does back up; building codes in cold-climate areas generally require one. Roofing material on its own does not prevent ice dams. Our separate articles on prevention, safe removal, gutters and insurance go into each of these topics in more detail.

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