Snow Loads on Roofs: take care of it

How much snow can a roof support? It depends on the way you design it.

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780 CMR Commercial Mass State Building Code Table 1604.11 provides snow load for roofs. The Ground Snow Load ranges from 45 lbs to 65 lbs per horizontal square foot depending on each community and the zone in which it is located. The Residential Code 5301.2(5) for 1 and 2 families’ ground snow ranges offer different values again depending on the community and zone of 25 lbs. to 50 lbs. per horizontal square foot. To determine the snow load for your specific community and zone refer to the chart.

A flat roof, gable roofs and roofs with multiple valleys and snow gathering points will vary the weight differential factors as will the number and type of roof coverings effect the value. First we have to presume that the structure has been constructed to sustain the minimum snow load requirements of the code.

The weight of snow/ice, not the depth, is critical in assessing a roof’s vulnerability. The water content of snow may range from 3% for very dry snow to 33% for wet, heavy snow to nearly 100% for ice. An inch of water depth weighs 5.2 psf. Thus, a roof designed to a carry a snow load of 20 lbs per horizontal square foot is expected to support nearly 12 inches of wet, heavy snow. If using the heaviest wet snow value for roof designed to sustain 40 psf of snow load then it can be viewed that it should be capable of supporting nearly 24″ inches of wet heavy snow. If the snow moisture content is less, the depth of snow for sustaining can be greater.

Please use this information as a tool to assist you . If you need more precise information please contact a structural engineer to measure your roof loads. Roof failures may not be solely attributed to built up snow, there may be other actions that occur that affect the roof structural stability and sustainability.


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