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, WeatherLock 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.
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.