What Is an Ice Dam?
An ice dam is a ridge of ice that forms along the edge of a roof, preventing melting snow from draining properly and forcing water back under roofing materials.
Definition
An ice dam occurs when snow on a roof melts, flows downward, and refreezes at colder roof edges or eaves.
The resulting ice buildup blocks normal drainage paths, causing meltwater to back up beneath roofing layers.
How Ice Dams Form
Ice dams are the result of temperature differences across the roof surface.
- Heat loss warms the upper roof area
- Snow melts and flows downward
- Colder eaves cause refreezing
- Ice accumulates and thickens over time
Why Ice Dams Cause Damage
When meltwater is trapped behind an ice dam, it can infiltrate roofing systems in ways they are not designed to handle.
- Water forced beneath shingles or panels
- Moisture intrusion into insulation and sheathing
- Freeze–thaw expansion beneath roof layers
Contributing Factors
- Inadequate roof or attic insulation
- Unbalanced roof ventilation
- Complex roof geometry and valleys
- Extended snow accumulation periods
Ice Dams and Long-Term Roof Performance
Repeated ice dam formation accelerates roof deterioration by increasing moisture exposure and freeze–thaw cycling.
Roof systems intended for cold climates must be evaluated based on their ability to manage snow melt, drainage, and temperature balance.