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Ice Dams and Winter Roof Care for New Hampshire Homes

Updated 2026-10-10 · 5 min read

Why ice dams form on NH roofs, what to do when you see one, roof snow safety, and how air sealing, insulation and the eave ice barrier rule help.

How ice dams form

An ice dam is a ridge of ice along the lower edge of a roof. According to University of Minnesota Extension, it forms when the upper part of the roof is above freezing while the edge is below freezing. Snow melts higher up, runs down and refreezes at the cold eave. Water then backs up behind the ice and can work its way under shingles into the attic, walls and ceilings.

The warm upper roof is usually caused by heat escaping from the house. Extension names warm air leaking through ceiling openings like light fixtures as a major cause, along with thin insulation, chimneys and leaky ductwork in the attic. ENERGY STAR lists ice dams as a common sign that an attic needs air sealing.

What to do when you see one

University of Minnesota Extension suggests these short-term steps:

Remove snow from the roof with a roof rake or push broom. Extension cautions that this can damage roofing materials, so work gently.
In an emergency, when water is flowing into the house, make channels through the ice dam with tap water on a warm day so water can drain. Extension says this is only a temporary fix and stops working within days.
If water gets inside, dry wet areas right away to prevent mold and mildew, and hold off on repairing ceilings and walls until they're dry.
Stay off a snowy or icy roof. Extension warns that winter roof work, even from below, risks injury and roof damage, and recommends hiring professionals. Find a roofer for rooftop work.

Roof snow: safety first

FEMA's snow load safety guidance says roof snow removal is dangerous and best left to qualified people. If you do clear snow from the ground, its tips include:

Use a long-handled non-metallic snow rake from the ground. Metal rakes can damage roofing and pose an electrocution risk.
Stand back from the eave. Roof snow can slide at any moment, and falling icicles can injure people.
Leave at least a couple of inches of snow on the roof. Scraping down to the shingles can damage them and lead to leaks.
Don't use picks or other sharp tools, and inspect the roofing for damage afterward.
Watch for warning signs of an overloaded roof: sagging ceilings or roof lines, popping or cracking sounds, doors or windows that suddenly stick, new cracks in walls, or severe leaks. FEMA says to leave the building promptly and contact your local building official or a qualified design professional.

Keep exhaust vents clear

Snow piled against the outside vents of fuel-burning appliances such as furnaces can let carbon monoxide build up in the house, and the U.S. Consumer Product Safety Commission advises clearing snow away from them. The NH Department of Energy likewise advises keeping exhaust vents on your home clear of snow, ice and debris. The NH State Fire Marshal's office recommends CO alarms outside each sleeping area and on every level. If a CO alarm goes off, get everyone outside and call 911.

Long-term prevention: keep heat out of the attic

The lasting fix is to keep the roof deck cold. Extension's guidance is to air seal the ceiling first so warm, moist air can't flow into the attic, then consider adding insulation. ENERGY STAR notes that air sealing combined with attic insulation can help prevent ice dams, and that attic ventilation helps keep the attic cold in winter, as long as insulation doesn't cover the soffit vents (ENERGY STAR on attic ventilation). A weatherization professional can find leaks with a blower door test or infrared camera.

Tightening a house has side effects. ENERGY STAR warns that sealing can, in some cases, cause naturally venting combustion appliances to backdraft, and recommends having a contractor test oil or gas appliances before and after sealing. Extension also notes that cutting heat loss means more snow stays on the roof. A roof built to current code should handle expected loads, but if you have doubts, ask your building department or an engineer.

NHSaves offers home energy audits and incentives for air sealing and insulation, plus low-interest financing for program participants while funding lasts. See our energy rebate guide and the NHSaves weatherization page. Insulation contractors and HVAC contractors who do weatherization can help.

What the building code requires at the eaves

Where there's a history of ice forming along the eaves, the residential code requires an ice barrier under shingles and several other roof coverings. It can be two layers of underlayment cemented together or a self-adhering membrane, running from the roof edge to at least 24 inches inside the exterior wall line. On steep roofs (8 in 12 or steeper), it must also run at least 36 inches up the slope from the eave. That's the rule in both the 2024 International Residential Code and the 2021 NH Residential Code. The NH State Building Code moved to the 2024 codes on July 1, 2026, and permit applicants can use either version through December 31, 2026. Whether the ice barrier rule applies depends on how the code's climate table is filled in for your area, so ask your building department.

Towns may describe this in their own terms. Nashua, for example, tells homeowners who strip a roof to use water and ice shield on the first 3 feet. Nashua doesn't require a permit to re-roof a one- or two-family home, but the work must still follow the building code. Other towns may require a permit, so check your town's rules and see our building permit guide.

Plan roof work for the right season

Shingle adhesive strips need warmth to seal. Owens Corning's installation instructions, for example, call for hand-sealing shingles around the roof perimeter in cold weather, so many homeowners schedule roof replacements for warmer months (see our seasonal planning guide). Fall is a good time to have a roofer look at flashing, gutters and attic ventilation before snow arrives.

This guide is general information. For leaks, structural concerns or heavy snow loads, get a qualified roofer or engineer to look at your roof, and confirm code and permit questions with your town.