ice dam
A mass of ice formed at the transition from a warm to a cold roof surface, frequently formed by refreezing meltwater at the overhang of a steep roof, causing ice and water to back up under roofing materials.
ice dam: frozen meltwater barrier that backs water into walls
An ice dam forms where warm roof air melts snow, sending water down the slope until it hits a cold zone (usually above an unheated overhang or soffit) and refreezes into a ridge of solid ice. As more meltwater arrives and freezes, the dam grows higher and wider, eventually trapping liquid water against the roofline. That backed-up water seeks the path of least resistance, which is under the shingles and into the attic, walls, or interior ceiling.
The root cause is thermal imbalance. Heat from a warm attic, poor attic insulation, or direct solar gain melts snow from underneath while outside air temperatures stay below freezing. The water runs down until it reaches the uninsulated overhang, where it refreezes. Sloped roofs steeper than about 6 inches per 12 inches of horizontal run are most vulnerable because meltwater travels faster and pools more readily at the cold edge.
Winter ice dams cause hundreds of millions in water damage annually in cold climates. Water wicks backward through felt underlayment and wood sheathing, staining ceilings and rotting framing within days. Gutter systems often fail under the weight and pressure of a mature dam, which can exceed 10 pounds per linear foot. Once damage begins, it may not show until weeks later when water migrates into insulation and dries slowly.
Prevention focuses on temperature and drainage
The engineering solution is to keep the entire roof surface cold or to manage meltwater. This means attic ventilation, sealing air leaks from heated spaces, raising insulation levels above the attic floor, and creating continuous soffit-to-ridge airflow. Some climates use heated cable or tape running along the eave in a serpentine pattern to keep the dam zone above freezing, though this is energy-intensive and temporary. Standing seam or metal roofing with low friction can shed snow and ice before dams form, but at significant cost.
Gutters and downspouts do not prevent dams and may make them worse by trapping water. In high-risk situations, synthetic underlayments rated for ice and water shield (typically butyl-based membranes) are installed in a 3- to 6-foot band above the eave to redirect water that does back up. These materials are not a cure, only damage mitigation.