What is an Ice Dam?
An ice dam is a ridge of ice that forms at the lower edge of a roof and prevents water produced by melting snow from draining off the roof. The water that backs up behind the dam can leak into a building and cause damage to walls, ceilings, insulation and other areas.
What Causes Ice Dams?
Non-uniform roof surface temperatures lead to ice dams. Heat loss from a house, snow-covered roofs and outside temperature interact and lead to ice dam formation. For ice dams to form, there must be snow on the roof, and, at the same time, the upper portions of the roof’s surface must be above 32 degrees F, while lower surfaces are below 32 degrees F. For a portion of the roof to be below 32 degrees F, outside temperatures must also be below 32 degrees F. (When we say temperatures above or below 32 degrees F, we are talking about average temperatures over sustained periods of time.)
The snow on a roof surface that is above 32 degrees F will melt. As water flows down the roof, it reaches the portion of the roof that is below 32 degrees F and freezes. This creates an ice dam.
The dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that are, on the average, below 32 degrees F. So the water above backs up behind the ice dam and remains a liquid. This water finds cracks and openings in the exterior roof covering and flows into the attic space. In the most common case, the water simply backs up and flows under the shingles. From the attic, it could move into exterior walls or through the ceiling insulation and stain the ceiling finish.
For a roof to last, your Roofing System must include protection from ice dams and wind-driven rain. This is accomplished by installing waterproofing underlayment directly beneath the shingles.
In addition to protecting the roof deck from damage from ice dams and wind-driven rain, Ice and Water Shield should be used on areas of the roof where water has a tendency to collect or flow. Valleys, vents, chimneys and skylights are examples of spots where waterproofing underlayment should be considered.