Ice Dams on Long Island: Why They Form and How to Stop Them for Good

An ice dam is a ridge of ice along the edge of your roof that stops melting snow from draining. The water backs up under the shingles and comes into the house, usually along the top of an exterior wall or around a window. Ice dams are not caused by snow or by your gutters. They are caused by heat leaking from the house into the attic. Fix the heat leak and the dams stop, which is why the real repair is done in the attic in September, not on the roof in January.

This is how they form on Long Island houses, what to do when one is leaking right now, and what actually prevents them.

How an ice dam forms

1. Snow lands on the roof and sits.
2. Heat from the living space escapes into the attic through gaps around light fixtures, the attic hatch, plumbing stacks, chimney chases and thin insulation, and warms the roof deck from below.
3. The snow on the warm part of the roof melts and runs down toward the eave.
4. The eave sits past the heated walls, so it stays cold. The water refreezes there.
5. The ridge of ice grows. Water pools behind it, finds the gaps between shingle courses, and runs into the soffit, the wall or the ceiling.

The tell is a roof with a bare strip up the middle and ice at the edge: the middle melted because it is warm, the edge froze because it is not. Icicles alone are not an ice dam, but big icicles at the eave with a bare roof above them almost always are.

Why Long Island gets them

Our winters are the kind ice dams like: a few inches of snow, a week around freezing, sun during the day, hard freezes at night. That cycle melts and refreezes a roof a dozen times a season. Add the housing stock. Capes and splits in Commack, Stony Brook and Wading River were built with shallow attics, a few inches of loose insulation, and soffits that were sealed off when the house was re-sided decades ago. Cape Cod houses with finished upstairs rooms and knee walls are the worst, because the sloped ceiling sits right against the roof deck with almost nothing between them.

What to do if one is leaking today

The goal is to get water moving again without damaging the roof.

• Do not chip at it. A hammer, an ice pick or a shovel on a frozen shingle takes the granules off and often the shingle with it. We repair a lot of roofs in March that were fine before someone attacked the ice.
• Do not use rock salt. It corrodes aluminum gutters and flashing and kills whatever is planted below.
• Pull the snow off the lower roof with a roof rake from the ground. Less snow above the dam means less meltwater feeding it.
• Melt a channel. Fill a leg of pantyhose with calcium chloride (not sodium chloride) and lay it across the dam so it melts a path for the water to drain. It is ugly and it works.
• Catch the water inside. Move furniture, put a bucket under the drip, poke a small hole in a sagging ceiling bubble so the water drains where you want it.
• Call for steaming if it is bad. Low-pressure steam removal takes the ice off without touching the shingles. It is the only removal method we recommend on a roof under warranty.

What stops ice dams for good

In order of how much difference each one makes.

1. Air sealing the attic floor. This is the fix nobody sees and the one that matters most. Warm air gets into the attic through the attic hatch, recessed lights, the top plates of interior walls, plumbing and wiring penetrations, and bathroom fans that vent into the attic instead of outside. Sealing those with foam and caulk, and boxing or replacing non-rated recessed lights, cuts the heat that melts the snow.

2. Insulation to the right depth. Long Island attics should carry roughly R-49 of insulation, which is about 14 to 16 inches of blown fiberglass or cellulose. Most 1960s and 70s houses have four to six. Insulation goes on after air sealing, not instead of it; blown insulation over an unsealed attic floor still lets warm air through.

3. Ventilation that actually moves air. Intake at the soffit, exhaust at the ridge, and a clear path between them. On a lot of Long Island homes the soffit vents were covered by solid aluminum or vinyl soffit years ago, so the ridge vent has nothing to pull. We check for baffles at the eave that keep insulation from blocking the intake, and we replace solid soffit with vented panels when we re-side.

4. Ice-and-water shield at the eaves. New York's residential code requires a self-adhering ice barrier from the eave to at least 24 inches inside the exterior wall line. On a re-roof, we run it wider than code on north-facing slopes and at every valley and penetration. This does not prevent the dam, but it means a dam that forms anyway does not leak.

5. Heat cables, as a last resort. Self-regulating heat cables zig-zagged along the eave keep channels open. They use electricity all winter and they treat the symptom. We install them on roofs where the attic cannot be fixed, such as a cathedral ceiling with no access, and we tell homeowners that is what they are.

What does not work

Bigger gutters, gutter guards and cleaning the gutters more often. Gutters fill with ice because the roof is melting snow, not the other way around. A house with no gutters at all gets ice dams.

The right time to fix it

September and October. Attic work is miserable in August and impossible to inspect in January under snow. If you had an ice dam last winter, have the attic looked at before the first storm. If you are planning a roof replacement, this is the time to fix the ventilation and extend the ice barrier, because both are easiest with the deck open.

Have us look before winter

We inspect the attic for air leaks, insulation depth and blocked soffits, and the roof for ice barrier coverage and ventilation, and we write up what would stop the dams on your house in order of impact. The inspection is free. Call (631) 972-0873 or request an inspection.