The short version
Snow on the roof. Heat from the house warms the roof deck above the living space. Snow there melts, even on a cold day. The water runs down to the eave, which hangs out past the warm walls and stays cold. It freezes. Over days and weeks, a ridge of ice builds. Meltwater pools behind it and works its way under the shingles, which are designed to shed water running down, not water sitting still.
Signs you have one
- A thick band of ice along the roof edge, often over the gutter
- Icicles more than a foot long, especially with water dripping in daytime
- Stains or peeling paint at the top of outside walls or around upstairs windows
- Damp or frosty insulation and dark nail tips in the attic
During an ice dam
- Catch the water inside and move anything valuable.
- Use a roof rake from the ground to pull snow off the bottom three or four feet of roof. That removes the meltwater supply.
- Don't chip, hammer or climb. Chipping breaks shingles and tears gutters; climbing an icy edge is how people get hurt.
- Call for steam removal if water is getting in.
- Photograph the inside damage for your insurer.
Fixing it for good
The goal is a cold roof: the roof deck should stay close to the outdoor temperature. That comes from three things done together. Air sealing the attic floor so warm air from the house can't get up there. Insulation deep enough to keep the heat below. Ventilation that pulls outside air in at the soffits and out at the ridge. Older Syracuse houses with finished half-story rooms under the roof are the hardest, because there's little space in the slopes; they need careful work at the knee walls and the narrow channel behind the ceiling.
On the roof itself, ice and water shield along the eaves is the safety net if water does back up. It goes on every replacement we do.
Questions readers ask
Do heated cables fix ice dams?
They keep a path open in specific spots. They don't fix the cause and they use power all winter.
Do gutters cause ice dams?
Not by themselves. Clogged or frozen gutters can make them worse, but the dam forms at the cold eave either way.
Is calcium chloride safe to use?
Filled into a sock and laid across the dam, it can melt a channel. Never use rock salt, which damages shingles, metal and plants.
Keep reading: Ice dam repair · Winter roof care · Gutters and ice guards