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Snow on Roofs Jenson Article
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Record February snowfall brings with it concerns about snow loads
Tom Jensen, Fire Code Specialist

Fire departments should always approach roof operations with safety in mind. That is even more important as we wrap up the snowiest February on record — a month that saw very little melting. Add the weight of fire personnel in a concentrated area, and the potential for partial collapse is real, especially on roofs of buildings never designed to Minnesota building codes such as agricultural barns.

Residential roofs are to be constructed to an anticipated design depth of 42 pounds per square foot in northern Minnesota and 35 pounds in the southern part of the state, according to the Minnesota Residential Code.  

Snow weighs between 7 and 20 pounds per cubic foot depending upon how much moisture exists. This translates to a range of 0.58 pounds per square foot for every inch of snow depth for light powder snow to 1.67 pounds per square foot for every inch of heavy wet snow.

How can we use this information practically? If you multiply the depth of snow on your roof by a median snow density of 1.12 pounds per square foot of snow per inch, the result will be an approximate load on the roof. For example: 20 inches of snow x 1.12 pounds per square foot = 22.4 pounds per square foot, well under the 42 pounds per square foot strength required in the northern part of the state. But add to the weight of the snow the weight of fire crews on roofs of buildings and you could have a big problem.

Another worst-case scenario with heavy wet snow: The roof on a house in the southern part of the state, designed to the state residential code, can support up to 21 inches of the heaviest, wettest snow we typically see. In the northern part of the state, roofs can support up to 25 inches of the heavy wet stuff.

Please use extra caution if you and your firefighters find yourselves on a rooftop.  

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