Ice Dams on Long Island Roofs: Prevention Starts in the Attic

Long Island is not the coldest place in the country, which is part of why ice dams show up here. In places where it stays cold all winter, snow sits on a roof in a stable state. On Long Island, winter temperatures swing above and below freezing again and again, and that freeze-thaw pattern is harder on a roof than steady cold.

How an ice dam forms

Attic heat escapes through an under-insulated or poorly ventilated roof deck and warms the upper part of the roof. Snow melts from the top down and runs toward the eave. The eave overhangs the wall and is not heated from below, so the water meets cold air and refreezes. Each cycle adds ice. By the time you see thick ice in the gutter and hanging off the fascia, water may already have been working under the roof covering for some time.

The gutter is not the problem

People see ice hanging from the gutter and conclude the gutter is the issue. It is not. The gutter catches ice that would otherwise slide off. The cause is a temperature difference at the roof deck, with the upper roof warm and the eave cold, and that difference exists because heat is escaping from the house into the attic. The gutter is where you see the problem. The attic is where it lives.

What the damage looks like

Ice in the gutter is visible. The damage it signals often is not.

When water backs up behind an ice dam, it sits. A roof is designed to shed water over shingles, not hold it. Water can work its way under shingles, through aging underlayment and into the deck, then saturate the insulation below. Wet insulation insulates less, which increases heat loss and the chance of further ice dams, so the problem compounds.

A stain on a ceiling in February can trace back to water that got in weeks earlier, since wet insulation moves water slowly.

After a freeze-thaw event, signs to look for in the attic include dark staining on the roof deck at the eave, wet or compressed insulation near the exterior wall, and daylight along the soffit framing.

Ice and water shield

Building codes call for a self-adhering ice barrier membrane at the eaves where ice dams have a history of forming, extending up the roof from the edge. Check with your local building department for what applies to your house. This membrane is a last line of defense: water can back up against it without reaching the deck.

Older roofs may not have it. When an older roof is replaced, ask for the ice and water shield coverage to be written into the contract explicitly so you know what you are getting, and ask who is pulling the permit.

The real fix: a cold, ventilated attic

The lasting fix is in the attic. Insulation goes at the attic floor, blocking heat from rising out of the living space, and the attic itself is ventilated from soffit to ridge so that the air in it stays near outdoor temperature. When the whole roof deck is about the same temperature as the air outside, the melt-and-refreeze difference disappears.

Soffit-to-ridge ventilation means unobstructed airflow from the soffit vents up through baffles in each rafter bay to the ridge vent. If insulation has been blown in over the soffit vents, which is common in older attics, ventilation is not working however much insulation is up there.

Insulation levels and ventilation details are governed by code and vary by house, so have an insulation and ventilation professional assess your attic. This fix addresses the cause. Everything else manages a symptom.

Heated cables

Electric heating cables along the eave and in gutters are sold as ice dam solutions. They are a management tool, and a limited one. A cable can keep a channel open in the gutter while the dam continues to grow on the roof above it, and water can still back up under the roof covering. Cables need to be maintained and do not last forever.

They make sense in one situation: an attic that cannot practically be corrected, such as one with a finished ceiling directly below and no accessible attic space. There they can limit damage. They are a response to a constraint, not a substitute for a fix.

Timing

Attic insulation and ventilation are best addressed in the fall, before the first significant snow. Gutter replacement and roofing work are best done in settled weather, when the deck is dry and the schedule is not driven by an emergency.

Work after a damaging winter is necessary when damage has already occurred, but it is reactive, and the attic conditions that caused the problem are still in place, sometimes with saturated insulation that needs to be replaced.

If you have ice dam damage at the roof edge, or want your roof and gutters assessed before winter, get a quote.