Ice Dams in New Jersey: Why They Form and How to Stop Them

What is an ice dam?

An ice dam is a ridge of ice that forms along the lower edge of a roof, usually right above the gutter. It builds in a set order, and once you know the order, the damage makes sense.

Heat from the house gets into the attic. The attic warms the underside of the roof boards, and the snow on top starts melting from below, even when it is below freezing outside. The water runs down the roof under the snow until it reaches the eaves, the part of the roof that hangs out past the wall. There is no warm attic under the eaves, so the water freezes there.

One cold night leaves a small lip of ice. A week of the same pattern builds a ridge. Now the next melt has nowhere to go. It pools behind the ice and works its way up under the shingles. Shingles are laid to shed water running downhill. They are not built to hold back a puddle.

That is why the stain usually shows up near an outside wall, at the top of a window, or where the ceiling meets the wall. The water gets in right where the roof crosses over the wall.

Why do New Jersey roofs get ice dams?

Ice dams need snow on the roof, freezing temperatures at the eaves, and heat getting into the attic. New Jersey has all of it, in different amounts depending on where the house is.

The north gets the most snow. The Office of the New Jersey State Climatologist puts average snowfall at 40 to 50 inches a year in the northern part of the state, against 10 to 15 inches in the far south. In the highlands, snow can fall from about the middle of October to the end of April. That is a long season for snow to sit on a roof and melt from underneath.

Sussex County gets its own rule. The state building code names it as the one county where the average daily temperature in January is 25°F or lower.

The shore gets ice dams less often. The ocean keeps more nights above freezing there: about 217 freeze free days a year along the coast, against 163 in the northern highlands. But every weather station in the state has recorded 0°F or below, and a heavy snow followed by a cold week can build a dam on any roof in any of the 14 counties we work in.

The houses matter as much as the weather. Almost half of New Jersey homes were built before 1970, according to the Census Bureau. Warm air gets into an attic through gaps around plumbing vents, electrical wires and chimneys. An older house has usually had more of those holes cut through its ceilings over the years.

How do you know you have an ice dam?

A few icicles on a cold morning are normal. These are the signs that point to a dam:

• A thick ridge of ice along the edge of the roof, with icicles hanging from it.
• Icicles that keep dripping on a day when it is below freezing. Something is melting that snow, and it is coming from inside the house.
• A brown stain or bubbled paint where the ceiling meets an outside wall, or along the top of a window frame.
• Gutters bending or pulling away from the house under the weight of the ice.
• In the attic, damp or frosty insulation near the eaves.

If you see the ice and the stain together, water is already getting in.

What should you do if water is coming in now?

Put a bucket under the drip and move anything that can get ruined. If the water is near a light fixture or an outlet, turn that circuit off at the panel. Then:

1. Stay off the roof. Ice on a pitched roof is how people get hurt, and it is not worth it for a leak.
2. Do not chip at the ice. A hammer or an ice pick breaks the shingles before it breaks the ice. Minnesota’s Commerce Department warns that chipping, chemicals or heat can damage shingles and gutters.
3. Do not throw rock salt on the roof. Chemicals can damage the shingles and the gutters, and they will not stop the next dam from forming.
4. Use a roof rake only from the ground. If you can reach the lower edge of the roof from the ground, pulling the bottom few feet of snow off takes away what feeds the dam. Watch your footing, and keep the rake away from the wires where power comes into the house.

Then call us. We come out, and when it is safe, we put on a temporary fix to slow the water down. The permanent repair comes when the weather lets us do it right. If the leak cannot wait, see emergency roof repair.

What stops ice dams for good?

Getting rid of the ice this week does not stop it from coming back next week. What stops it is cutting off the heat that melts the snow.

Keep warm air out of the attic

The U.S. Department of Energy calls this the most important step: sealing the air leaks between the house and the attic, so warm air never gets up there. Insulation comes after that and slows down the heat that passes through the ceiling. Minnesota’s Commerce Department says insulation on its own is not enough. Warm air carries moisture, and it moves right through gaps that insulation does not cover.

Ventilation helps, up to a point

Attic vents carry off the heat that does get in, which keeps the roof boards closer to the outside temperature. The Department of Energy calls venting critical in cold climates. Minnesota’s Commerce Department adds a warning: more vents will not eliminate ice dams on their own, and in a leaky attic they often make things worse. So we find where the warm air is coming from before we add any vent. Here is how we check attic ventilation.

An ice barrier under the shingles at the eaves

An ice barrier is a waterproof layer under the shingles along the bottom edge of the roof, usually a self-adhering rubberized sheet. It does not stop ice from forming. It keeps the water that backs up behind the ice from getting into the house.

The New Jersey code spells out where it goes: from the lowest edge of the roof to at least 24 inches inside the exterior wall line. That is the stretch where the water gets in. But the code only requires it in Sussex County. Everywhere else in the state it is the roofer’s call, so whether your roof has one depends on who put it on.

Heat cables and gutters

Heat cables melt a channel through the ice. They do nothing about the warm air causing it, and Minnesota’s Commerce Department says they shorten the life of a roof and cost money to run.

Gutters do not cause ice dams. A roof with no gutters at all gets them too, because the dam forms on the cold eaves. Gutters do take the damage. A gutter packed with ice is heavy, and it can pull right off the fascia. If yours came down this winter, see gutter replacement.

Is it the roof or the attic?

Usually the attic. If your shingles are in good shape and you still get ice every winter, a new roof will not change that on its own. A new roof over the same warm attic gets the same ice.

That is worth knowing before anyone sells you a roof. When we inspect for ice dam damage, we go into the attic, look at the underside of the roof boards and the insulation, and photograph what we find. Sometimes the right answer is a repair at the eaves and air sealing in the attic, not new shingles.

If the roof is near the end of its life anyway, a roof replacement is the time to deal with the eaves and check the ventilation in the same job, while the roof is open.

Does homeowners insurance cover ice dam damage?

Sometimes. It depends on your policy and on what the adjuster decides caused the damage. Call your insurance company and ask before you assume either way.

What we can do is photograph what we find: the ice, the edge of the roof, the attic and the ceiling. That gives you something to hand your adjuster. We do not decide the claim, and we will not tell you a claim is a sure thing.