Ice Dams in New Jersey: Causes and Prevention

Icicles along the gutters look pretty. The water stains they leave on your ceiling don't.

Most winters in Union County bring at least a few snowstorms, followed by days where temperatures bounce above and below freezing. That combination is exactly what creates ice dams, and they're one of the most common causes of winter roof leaks in older homes around Linden.

What an ice dam is

An ice dam is a ridge of ice that builds up along the lower edge of the roof. Behind it, melting snow forms a pool of water that can't drain off. Shingles are designed to shed water running downhill, not to hold back standing water, so that pool works its way under the shingles and into the house.

What causes them

1. Heat from the house escapes into the attic.
2. The warm attic heats the roof deck, and snow on the upper roof melts even when it's below freezing outside.
3. The meltwater runs down to the eaves, which hang out past the warm walls and stay cold.
4. There it refreezes. Each day the ridge of ice grows.
5. More meltwater backs up behind it and gets under the shingles.

So the root cause isn't really the roof. It's heat leaking into the attic, plus not enough ventilation to carry it away.

Signs you have a problem

• Thick icicles hanging from the gutters
• A ridge of ice along the roof edge
• Water stains or peeling paint on ceilings near outside walls, or at the top of walls
• Damp spots that appear on sunny days after a snowstorm
• Snow melting on the upper roof while the edges stay frozen

If you have an ice dam right now

• Don't chip at it with a hammer, axe or shovel. You'll damage the shingles and gutters, and it's dangerous on a ladder.
• Use a roof rake from the ground to pull snow off the lower few feet of the roof. Less snow means less meltwater.
• Calcium chloride (not rock salt) in a nylon stocking, laid across the dam, can melt a channel for water to drain. Rock salt damages shingles and plants.
• Catch water inside and photograph any damage for your insurer.
• For serious dams, a roofing or snow removal pro can clear them with low-pressure steam, which doesn't damage shingles.

How to prevent ice dams for good

1. Seal air leaks into the attic. This is the most important step and the one most often skipped. Gaps around attic hatches, recessed lights, plumbing stacks, chimneys and top-floor wall plates let warm air pour into the attic. Sealing them keeps the heat in the house.

2. Add insulation. Many older homes have far less attic insulation than is recommended today. More insulation means less heat reaching the roof.

3. Ventilate the attic. Cold outside air should come in at the soffits (under the eaves) and leave at the ridge. That keeps the roof deck cold and evenly frozen. Make sure insulation isn't blocking the soffit vents.

4. Vent bathroom fans outside. Fans that dump warm, moist air into the attic cause both ice dams and frost on the underside of the roof.

5. Ice and water shield. When a roof is replaced, a self-adhering waterproof membrane installed along the eaves and in the valleys acts as a second barrier if water does get under the shingles. Most roofers in New Jersey install it as standard. See roof replacement.

6. Keep gutters clear before winter, so meltwater has somewhere to go.

Heat cables along the roof edge can help in problem spots, but they treat the symptom rather than the cause.