An ice dam is a ridge of ice at the eave that holds back melting snow, so water backs up under the shingles and into the house. It forms when heat from the house melts the snow on a warm roof and the water refreezes at the cold edge. Winter ice dams are a named roof risk for Long Island, and the fix lies mostly in the attic: air sealing, insulation and ventilation.
Key takeaways
- An ice dam forms when warm air from the house melts snow on the roof, and the water refreezes at the cold eave.
- Water that backs up behind a dam runs under the shingles and can reach the walls and ceilings below.
- Removing the ice is a job for a roofer from a safe position, not for the homeowner on a ladder.
- The attic is the root of the cycle: air sealing, insulation and ventilation reduce the melt that forms the dam.
- A check each winter, after a snowfall and a thaw, finds a forming dam before it does damage.
How does an ice dam form on a Long Island roof?
Heat escaping from the house warms the roof, the snow melts, the water runs down to the cold eave and freezes into a ridge. The ridge holds back the next melt and the cycle repeats.
Snow on a roof melts when the roof surface is warm, and the warmth usually comes from the living space below. Heat escapes through gaps in the ceiling and the attic hatch, warms the underside of the roof boards and melts the snow above them. The meltwater runs down the slope toward the eave.
The eave overhangs the wall and is not heated from below, so it is much colder than the rest of the roof. The water freezes there, and the ice builds up with each melt. Over a few days a ridge of ice can form along the eave, and the next melt pools behind it.
Why does the water go under the shingles?
Because the pooled water has nowhere else to go. It is pushed up under the shingles by the weight of the water behind the dam, and it finds its way under the covering and into the roof.
Shingles overlap, and they shed water that runs down them. Water that pools behind an ice dam is pushed up under the lower courses of shingles, and it can work through the nail holes and the seams. The underlayment at the eave is the last barrier, and it is only a barrier if it is sound and extends far enough up the roof.
Once the water is under the covering, it runs along the decking and down into the wall or the ceiling. The stain appears at the place the water finally comes through, which can be far from the dam itself.
What damage does an ice dam leave behind?
Stains and soft plaster at the ceiling and the exterior wall, peeling paint, damp insulation and rot in the decking and the fascia. The damage is often slow and easy to miss.
The first sign is usually a stain on a ceiling or an exterior wall after a thaw. Paint on the outside wall under the eave may bubble or peel. Inside, plaster can soften and the insulation can become damp, which reduces its value and holds moisture against the decking.
Over several winters the decking and the fascia can rot where water has run. The rot weakens the fixings for the gutters and the shingles, and a repair that treats only the stain can leave the rot in place.
Why is the attic the main place to fix the problem?
Because the warm air from the house that reaches the attic is what melts the snow on the roof. Air sealing, insulation and ventilation reduce that heat, and ventilation keeps the roof close to the outside temperature.
Warm, moist air from the house rises through gaps around the light fixtures, the pipes, the attic hatch and the top plates of the walls. It warms the attic and the roof boards, and the snow above melts. Sealing those gaps stops the warm air reaching the attic, and it is often the single most useful change.
Insulation keeps the heat in the living space rather than letting it into the attic. Ventilation at the eave and the ridge carries away any warm air that does reach the attic and keeps the roof cold. The three steps work together, and a roof that is repaired but not ventilated will see the dam return.
What about heated cable and why it is not the whole answer?
Heated cable at the eave can melt a channel through the ice so the water drains. It helps in some cases, but it uses power and does not fix the heat loss that forms the dam.
Heated cable is laid along the eave and in the gutter, and it melts a path through the ice so the water can run off. It can reduce the damage for a time, and some homeowners use it as a support measure. It needs power, it needs to be installed correctly and it does not stop the melt on the roof.
The honest position is that cable is a support measure, and the attic is the cause. Air sealing, insulation and ventilation are the work that addresses the cause, and they should come first.
What should a homeowner never do?
Do not climb onto an iced roof, do not chip the ice from a ladder with a metal tool and do not stand below a line of hanging icicles. Those are the most common ways a homeowner is hurt.
An iced roof is slippery, and the eave is a fall hazard. A ladder placed against a frozen gutter is unstable, and a metal tool chipped at the ice can damage the shingles, the gutters and the flashing. Icicles and ice sheets can fall without warning, so the area below the eave should be kept clear.
Call a roofer for removal. A crew works from a safe position with fall protection and the tools to clear the ice without damaging the roof.
- Do not climb onto an iced roof
- Do not chip ice from a ladder with a metal tool
- Keep children and pets away from the eave and the icicles
- Call a roofer for removal from a safe position
How does a homeowner check for a forming dam?
After a snowfall and a thaw, look at the eave and the gutters from the ground, and check the ceilings and the walls near the outside. A growing ridge of ice is the sign to call.
From the ground, look for a ridge of ice along the eave, icicles that are thick and long, and snow that has melted off the upper roof while the eave stays covered. Those are the signs that a dam is forming, and they are the moment to call.
Inside, check the ceilings and the walls near the exterior for new stains after a thaw. Check the attic on a cold morning for frost on the roof boards, since frost is a sign of warm moist air escaping into the attic.
- A ridge of ice along the eave
- Thick, long icicles hanging from the gutter
- Snow melted off the upper roof while the eave stays covered
- A stain on the ceiling or wall after a thaw
What does the attic check involve?
The air sealing around the hatch, the light fixtures and the pipes, the depth and evenness of the insulation and the balance of the soffit and ridge ventilation. The roofer notes each finding and the change that would help.
The roofer checks the gaps that let warm air into the attic, the insulation depth and whether it is even, and the vents at the eave and the ridge for blockage. The decking is checked for damp and staining, and the findings are recorded in the note.
The note lists the changes in order of priority. Air sealing at the hatch and the fixtures is often the first, followed by insulation and ventilation. The homeowner decides which to have done and when.
Preventing ice dams before winter
The steps are air sealing at the attic hatch and the fixtures, adding or evening out insulation, and clearing the soffit vents so the air can move. Those steps are done before the cold season, not in the middle of a thaw.
Before winter, a roofer or a homeowner can seal the gaps around the attic hatch, the light fixtures and the pipes that pass into the attic. Insulation can be checked for depth and evenness, and the soffit vents can be cleared of insulation and debris so the air can move from the eave to the ridge.
Those steps reduce the heat that reaches the roof, and they keep the snow on the roof from melting unevenly. The roofer's pre-winter check records what was done and what remains, so the homeowner knows where the work stands.
Ice dams and the insurance question
Ice dam damage is documented in the same way as other storm damage, and the insurer decides coverage under the policy. A dated photograph and a report of the damage are the evidence.
When an ice dam causes damage inside the house, the homeowner should photograph the stain and the wet area before cleaning, and should ask the roofer for a written report. Whether the damage is covered depends on the policy and the cause, and the insurer decides.
A homeowner who has had ice damage before should keep the record of the repair and the attic work, since the insurer may ask whether the cause has been addressed.
Frequently asked questions
Will removing the ice fix the leak?
Often the leak stops once the dam is cleared, but the cause is still there. The attic check is what stops the dam forming again.
Can I use a heated cable instead of fixing the attic?
Cable can help in some cases, but it does not fix the heat loss. The attic work is the part that addresses the cause, and the roofer can explain the order.
Is ice dam damage covered by insurance?
Your insurer and your policy decide what is covered. The roofer's report documents the damage for the insurer.
How quickly can a roofer come to a dam?
Phones are answered 24/7. An active leak from a dam is booked for the first available crew, and the crew removes the ice from a safe position.
Sources
This guide is general information for Nassau County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.