What is an Ice Dam?

An ice dam is a ridge of ice that forms at the edge of a roof.

Icicles often form with an ice dam.

The dam prevents melting snow (water) from draining off the roof.  The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. 

Ice dams can also damage gutters and loosen shingles.

 

What causes an ice dam?

Ice dams usually occur after a heavy snowfall followed by sunny days or above-freezing temperatures. 

Heat from the sun or indoor heat loss causes snow to melt.

There are 2 primary causes:

1. Often caused by poor attic insulation or ventilation, an ice dam is formed when the warm air inside your home leaks into the attic and warms the underside of the roof causing snow and ice on the roof to melt.
2. Ice dams can also form if the radiant heat from the sun melts the snow on your roof, despite the ambient temperature being below freezing.