What causes ice dams?
Uneven or inconsistent roof surface temperatures will lead to ice dams.
Heat escape from a house, snow or ice cover and cold outside temperatures collaborate to form ice dams. For ice dams to form there must be snow covering portions of the roof and, at the same time, large portions of the roof’s outside surface temperatures must be above 32 degrees F (freezing) while lower surfaces have temperatures that are below 32F. These are average temperatures over consistent long periods of time. For a portion of the roof to be below freezing, outside temperatures must also be below freezing for a length of time.
As the temperature increase ever so slightly above freezing snow on a roof surface that is above freezing will start to melt. As water flows down the pitch of the roof it reaches the section of the roof that is below 32F and freezes. This is what causes an ice dam.
The dam will grow as it bolstered the melting snow above it, but it will stay limited to the sections of the roof that temperatures stay below 32F. So the water from stemming from above or the higher elevation of the roof backs up behind the ice dam and remains a liquid. The water will find cracks and openings in the exterior roof covering and will flow into the attic space. From the attic space, it could flow down into the exterior walls or through the ceiling insulation onto the drywall and stain the ceiling finish.
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