As many are aware, New England has just come through a very unusual series of snowstorms that led to over 150 reported incidents of roof collapse or other snow related property damage in Massachusetts alone! While the amount of snow was staggering (totaling over 70 inches in some areas), the most unusual factor in all this was that there were no warm-ups or melting periods between the storms.
One of the first storms after Christmas left a layer of heavy, wet snow about 12-18 inches deep over the entire region. The first several inches were so wet that they froze into a blanket of ice overnight. This layer of ice remained throughout most of January as the temperatures stayed cold and more and more snow continued to fall. As a result, many roofs in the region quickly became covered with several feet of snow.
After the third heavy snowfall, with more storms in the forecast, our office began to receive calls from building owners and property managers asking whether they should be concerned about the weight of the snow on their roofs. To address these concerns, you have to first estimate how much the snow weighs, and, secondly, how much weight the roof structure can support.
A quick internet search finds that snow typically weighs anywhere from 5 to 25 pounds per cubic foot, depending on how wet it is. For example, three feet of wet snow with a layer of ice underneath could easily place a load of 50-60 pounds per square foot (psf) on a flat roof. However, if the snow remains dry and powdery, the same three feet of snow might also only weigh around 20-30 psf.
Historically, structures built in New England over the past several decades have been designed and constructed to support snow loads of around 30-45psf (depending on geographic location). The most recent Code editions have increased these requirements significantly to 45-65 psf in most areas. If they were properly designed, most commercial buildings with flat roofs in New England should be able to safely carry two feet of fairly wet snow. Buildings with pitched roofs, such as townhouse condominiums or residential houses, should be able to support significantly more than this due to the fact that most of the roof loading is transferred to the building walls.
Given the amount of snowfall that occurred in the Greater Boston area, and the fact that one of the later storms included a period of rain that made the snow heavier, we ended up advising most of our clients with flat roofs to shovel them as a precaution. When dealing with life safety issues, it is better to err on the side of caution.
Out of curiosity, after the heaviest storms had passed, we performed a simple field test by cutting a 12 inch cube of undisturbed, naturally compacted snow from a nearby field. The cube weighed in at 20.2 pounds, just as expected!