An ice dam is a length of ice along the edge of the roof.
Icicles are usually present below ice dams, but not always. Ice dams are formed when the snow at the peak of the roof melts and then refreezes along the edge of the roof. The peak of the roof is warm due to the home leaking warm interior air into the attic which then rises to the peak of the roof and heats up the shingles, thus melting the snow at the peak.
That melted snow then drains from the roof, as designed. However, as it approaches the edge of the roof, that part of the roof matches the outdoor air temperature since there is no heat along the edge of a roof. Therefore, instead of the melted snow exiting the roof, it freezes along the edge of the roof. As the winter goes on that cycle repeats and makes the ice dam bigger and bigger. At some point that ice dam has grown so large that the melting snow pools behind the ice dam, similar to a river dam.
The water behind the ice dam then begins to back up under the shingles then goes past the ice shield and through the tar paper into the home causing water damage to ceilings, walls and floors and setting up the home for future mold. Ice dams can cause tens of thousands of dollars in damage. Click here to learn how to prevent ice dams.
More simply, here are the steps.
- Indoor heat escapes into the attic warming the roof shingles at the peak and melting the snow
- The melted snow at the peak runs down the roof to where the temperature of the roof is below 32 degrees and refreezes which forms an ice dam
- The ice dam grows larger and larger to where the melted snow behind the ice dam doesn't refreeze but pools behind the ice dam
- The pooling water behind the ice dam starts to leak under the shingles and past the ice shield into the home potentially causing tens of thousands of dollars in damage
Keep in mind how roofs are designed. Roofs aren't designed to be waterproof. They are designed to shed rain and snow down and off the roof. That is why residential roofs are pitched. An ice dam prevents the melted snow from exiting the roof. Since the roof is not waterproof, the melted snow has no place else to go but inside the home.