If you've seen thick ice and icicles building along your roof's edge in winter, that's an ice dam — and it's one of the most common causes of winter roof leaks in New Jersey.
What causes an ice dam
It's a heat problem, not just a weather problem. Warm air escaping into the attic melts snow on the upper roof. That meltwater runs down to the cold eaves (which overhang the unheated exterior), where it refreezes into a dam. Water then pools behind the dam and backs up under the shingles — into your ceilings and walls.
The three real fixes
- Air sealing & insulation: stop warm air from leaking into the attic in the first place.
- Ventilation: a balanced ridge-and-soffit system keeps the roof deck cold so snow doesn't melt unevenly (see our ventilation guide).
- Ice-and-water shield: a self-sealing membrane installed at the eaves and valleys that blocks backed-up water from reaching the deck. New Jersey code and best practice call for it, and Lightning installs it on every roof.
What NOT to do
Don't chip at ice with hammers or axes (you'll damage shingles), and skip the salt-filled pantyhose tricks. If you have an active leak, a roofer can safely steam the dam and address the source.
Prevent it for good
The permanent fix is a properly insulated, ventilated roof with ice-and-water shield — exactly how we build them. If you fight ice dams every winter, a new or corrected roof solves it. Get a free inspection.
How an ice dam forms
An ice dam is a heat problem wearing a weather costume. Warm air escaping into the attic melts snow on the upper roof; the meltwater runs down to the eave, which is cold because there is no heated space beneath it, and refreezes. The ridge of ice that builds up there holds the next round of meltwater against the roof, and water standing on a shingle roof gets underneath it. The leak that appears on the ceiling is almost never a hole — it is water that went uphill under the shingles.
What actually prevents them
- Air-seal the attic floor. Recessed lights, the attic hatch, plumbing and wiring penetrations, and the top plates of interior walls. This is the real fix and the one that gets skipped.
- Insulate to a proper depth after sealing, not instead of it.
- Ventilate so the deck stays cold — balanced intake and exhaust.
- Ice-and-water shield well past the interior wall line at the eaves. This does not prevent the dam; it prevents the dam from becoming a leak.
What does not work
Heat cables melt a channel and often move the problem a few feet up the roof; they also fail quietly and cost money to run. Chipping ice off with anything hard damages the shingles you are trying to protect. Salt tablets in stockings stain the roof and corrode the gutters. A roof rake used carefully from the ground is the only safe in-the-moment measure, and it treats the symptom.
Where ice dams actually happen in New Jersey
| County | Design ground snow load | Municipalities | What it means |
|---|---|---|---|
| Sussex | 50 psf | 22 | Yes — ice barrier strongly recommended at every eave |
| Passaic | 40 psf | 16 | Yes — ice barrier strongly recommended at every eave |
| Warren | 35 psf | 21 | Yes — ice barrier strongly recommended at every eave |
| Hunterdon | 35 psf | 23 | Yes — ice barrier strongly recommended at every eave |
| Morris | 35 psf | 34 | Yes — ice barrier strongly recommended at every eave |
| Somerset | 30 psf | 21 | Yes — ice barrier strongly recommended at every eave |
| All other counties | Site-specific (typically low-to-mid 20s) | 407 | Ice barrier still required by code at the eaves |
Ground snow loads are from New Jersey Department of Community Affairs Bulletin 19-1, which supersedes Bulletin 94-8. Counties not listed generate a site-specific value from the ASCE hazard tool.