After the recent collapse of the Metrodome roof, the collapse of a hardware store roof in Glenwood, and a recent Super bowl commercial from Allstate, some Minnesota homeowners are starting to wonder if they should be worried about their own roofs. In December, I even received a question on the Structure Tech Facebook page asking how much snow a roof should be able to hold if built to code.
The video below shows the Super bowl commercial; this is what finally inspired me to write a blog on this topic.
The required roof snow loads for Minnesota aren't clearly spelled out anywhere, but the numbers can be found by using Table R301.2(1) of the Minnesota Administrative Rules. This table says that roof snow loads equal .7 times the ground snow load. To find the ground snow load, we use section 1303.1700 of the Minnesota Administrative Rules. The southern portion of Minnesota, which includes the Twin Cities metro area, uses a ground snow load of 50 pounds per square foot.
For the Twin Cities metro area, the ground snow load equals 35 pounds per square foot, or .7 x 50.
So how much snow does this equal? It depends. As everyone knows, cold fluffy snow is very light, while wet snow can be extremely heavy. I've heard that very wet snow could weigh up to 20 lbs per cubic foot, but we haven't received much of that this year. I just checked a section of undisturbed snow in my back yard, where the snow is about 12" deep. For the record, we've received over 60" of snow this winter. According to my calculations, the snow weighs about 10 lbs per cubic foot, which conveniently equals 10 lbs per square foot.
If you have a properly constructed roof, you shouldn't have to worry about your roof collapsing, even if you get some big snow drifts on your roof. On the other hand, if you know you have structural problems with your roof, this would be a good year to have some of the snow removed from your roof.
Reuben Saltzman, Structure Tech Home Inspections