Ice Dams and How to Avoid Them
What is an ice dam?
An ice dam is a situation of home and building maintenance on sloped roofs in cold climates. Ice dams form when snow that accumulates on a sloped roof melts and flows down the roof, under the blanket of snow, until it reaches the air that is below the freezing, typically at the eaves. When the melted water reaches the freezing air, ice accumulates forming a dam, and snow that melts later cannot drain properly through the dam and puddles behind the dam. Ice dams may result in leaks through the roofing material, resulting in damaged ceilings, walls, roof structure and insulation. Damage or injury can also be prevalent when the ice dam falls off or from attempts to remove ice dams.
The snow melt can come from two reasons: Heat from inadequate roof insulation and heat leaks, and inadequate ventilation called a warm roof. A warm roof and the heat conducted through the roof melts snow on those areas of the roof that are above heated living spaces. The heat is not lost into roof overhangs so this area does not have melting. A small amount of ice damming can occur from the normal freeze-thaw cycle as the snow melts on warm or sunny days. This melting continues to run down the roof on cold nights and will freeze when it contacts the cold air.
Various methods are taken to prevent damage from ice damming with varying success and risk:
In new construction, ice dams are mostly prevented through building designs with adequate insulation and roof ventilation.
Existing roofs may allow the addition of more insulation and mechanical ventilation. Research into ice damming conditions determined that ice damming occurs when the attic temperature is above 30 °F (−1 °C) and the outdoor air temperature is below 22 °F (−6 °C). Attic/roof temperatures can be controlled by installing sufficient insulation and providing natural or mechanical ventilation to produce a "cold roof" to keep the roof temperature below 30 °F (−1 °C). A cold roof is vented so that the rooftop stays cold and the snow melts evenly over the entire roof due to outside temperatures only. A cold roof can also help end any problems that have been experienced with condensation inside the home or attic. By having an installation of a roof that is elevated above the existing surface the ambient temperature is relatively close above and below the new roof surface.
Removal of snow from a roof with a special tool called a roof rake. To be successful, the entire roof must be shoveled. Shoveling part way up a roof will cause an ice dam to form at the location where the snow was left, because the meltwater will freeze when it hits the freezing air. Many roofs are too tall to reach from the ground and must be shoveled from a ladder or by walking on the roof, which increases the risk of personal injury or roof damage.
Use a roofing material which sheds snow, such as a metal roof, with adequate roof pitch and minimal valleys. Care should be noted of possible “avalanching” of snow build-up that releases.
Removal of ice buildup on a roof can be completed by trained professionals that use special steam equipment to ensure quick and safe removal without causing damage to the roof.
A roofing membrane required by building codes in the United States, installed under the roofing material helps prevent leakage from ice damming, but has no effect on the formation of ice dams.
An ice belt, a band of metal roofing, installed at the eaves helps prevent the formation of ices dams by placing a low-friction surface where ice dams are likely to form. If an ice dam does form, an ice belt will reduce the penetration of standing water.
Heat tape installed at the eaves melts channels for the water to flow through. However, there are many criticisms of heat tapes including expense, wasted energy, long-term damage to asphalt shingles, and risk of fire. Some insurance companies will not allow the use of heat tapes due to the fire danger. Ice dams can also form just above the heat tape. Heat tapes are a common way to keep gutters and downspouts from icing.
Ice melt socks: A manufactured, permeable material (the sock) filled with ice melt placed on the ice dam will lower the freezing point of the ice, thus causing it to melt.