When heat from the interior of a house with a sloped roof escapes into the attic space, it warms the underside of the roof. Meanwhile, the roof eave outside the heated space remains a colder temperature. As snow accumulates on the rooftop, it melts over the warmer portion of the attic and the melt water runs down the roof. When it encounters the cold edge of the roof it refreezes. The refrozen water along the roof edge creates an “ice dam” and consequently, the melted snow running down the roof begins to back up underneath the roof covering. This water will soak the roof sheathing and leak into the attic unless there is a barrier above the sheathing. Sealing the roof deck is an effective way to prevent the water from entering your home and causing damage.
A two-step approach is the most effective way to reduce the size of ice dams. First, keep the attic floor well insulated to minimize the amount of heat from within the house that rises into the attic. Second, keep the attic well ventilated so that the cold air outside can circulate through it and reduce the temperature of the roof system. The colder the attic, the less thawing and refreezing on the roof. These two measures are the best ways to keep ice dams from increasing in size.
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