What Causes an Ice Dam on a Roof?

How Ice Dams Form

• When outside temperatures are below freezing, heat from inside a house can keep parts of the roof warm enough that snow melts in those areas and flows down the slope until it reaches a lower area, where the exterior roof temperature is below 32 degrees Fahrenheit. The water freezes on that part of the roof, but continues to melt on the warmer areas of the roof. The ice dam eventually keeps water on the warm parts of the roof from draining away, just like a dam across a river.

Insufficient Insulation

• The key to preventing ice dams is to keep the whole surface of the roof below freezing during extended periods of cold weather and snow. Insufficient insulation is one of the main causes of roof heating. Heat from inside the house rises into the attic space, and from there, into the material of the roof itself. Attic insulation acts as a barrier to keep heat inside the main part of the home and away from the roof.

Air Leakage

• Another major cause of roof heating is air leakage from the home's living areas into the attic. Any place where a fixture, duct or pipe goes through the ceiling and into the attic space, air may also go through. Because hot air is lighter than cold air, it rises and seeks out such openings in a process called convection. The hot air from inside the home heats the roof directly. To reduce air leakage, seal all ceiling fixtures with caulk or other insulation.

Chimneys and Exhaust Systems

• Bathroom or kitchen exhaust systems may heat areas of the roof if they terminate too close to it. Extending the outlet of the exhaust system or re-routing it away from the roof altogether overcomes the problem. Chimneys can be particularly problematic if the homeowners use the fireplace or wood stove frequently during cold weather, since heat from the chimney escapes into the attic and the area of the roof where it protrudes.