Ice Dams: Why They Form on Pennsylvania Roofs and How to Stop Them

It comes down to one thing: an uneven roof temperature. Heat escaping from the living space into the attic warms the upper part of the roof deck, enough to melt the underside of the snow sitting on it. That meltwater runs down the slope until it reaches the eaves and the overhang, which hang out past the warm walls and stay at the outdoor temperature. There it refreezes.

Do that over a few cold days and the refrozen ice builds into a ridge, the dam, along the eaves. Now every bit of new meltwater coming down the roof backs up behind that dam and pools. Shingles are designed to shed water flowing downhill, not to hold a standing pond, so the trapped water works its way under the shingles, through the deck, and into the walls and ceilings below. That is the brown stain that shows up on an upstairs ceiling in February.

If the roof is the same temperature top to bottom, the snow either stays frozen or melts evenly and drains, and no dam forms. So the houses that get ice dams are almost always the ones leaking heat into the attic unevenly: thin or gapped insulation, air leaks around recessed lights and the attic hatch, ductwork or a bath fan dumping warm air into the attic, and poor ventilation that lets that heat pool against the deck instead of flushing out.

This is why two identical-looking houses on the same street can have completely different winters. It is not luck. It is insulation and ventilation.

Roof heat cables and de-icing tape treat the symptom. They can carve a drainage channel through the ice in a trouble spot, but they run up your electric bill, they do not last forever, and they do nothing about the heat loss that caused the dam in the first place. We are not against them as a stopgap on a stubborn eave, but they are a bandage, not a cure.

The lasting fix is keeping the roof deck cold and even. That means air-sealing and insulating the attic floor properly so household heat stays in the house, and balanced ventilation so whatever heat does get up there is carried away before it can warm the deck. Get those two right and the ice dam usually stops forming at all.

An ice-and-water shield membrane along the eaves is the backstop we install on replacements. It does not stop the dam from forming, but it seals the deck at the most vulnerable few feet so that when water does back up, it cannot get into the house. On our winters, that membrane at the eaves and in the valleys is not optional in our book.

When a dam is already up and a stain is spreading, the instinct is to get out there and smash the ice off. Please do not. Chipping at ice with a hammer or a shovel gouges shingles and dents gutters, and doing it from a ladder on frozen ground is a real way to get seriously hurt. Clearing snow off the lower few feet of roof with a roof rake, from the ground, is the safer move, because it removes the snow that feeds the dam. If water is actively coming in, that is the time to call, not to climb.

If you are seeing large ice dams every winter, or water stains showing up after a thaw, the roof, the insulation, or the ventilation needs a look, and often it is a combination. We inspect all three, from the attic as well as the roof, and tell you which is the actual culprit instead of guessing or selling you the fix that is easiest for us. Sometimes it is as simple as sealing air leaks and adding insulation. Sometimes ventilation is the missing piece. Either way, fixing the cause beats fighting the ice every January.