While all of the recent snow fall has caused major problems with roof leaks from ice dams, I've also started hearing from a lot of homeowners who are concerned about how much snow their roof can hold. The concern is that with all of this snow, some roofs might collapse under the weight. The video clip below from a 2011 Allstate commercial gives a nice image of what this might look like.
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. The chart below, courtesy of Paul Schimnowski, P.E., gives some examples of snow loads.
Last Wednesday, just before the most recently dump of snow that we received, I checked a section of undisturbed snow in my back yard to see what it weighed; 1/2 of a cubic foot was about 10 pounds. The depth of the snow varied between 14" and 20", so to make the math easy, lets say it was 18". That would make the snow weigh about 30 lbs / sf.
If you have a properly constructed roof, you shouldn't have to worry about your roof collapsing because the snow on a sloped roof won't be as deep as snow on the ground. Snow drifts are the result of light, fluffy snow blowing around; not heavy wet snow. Also, the 35 pounds per square foot requirement is only a minimum requirement.
On the other hand, if you know you have structural problems with your roof and you get some huge snow drifts, it wouldn't hurt to have some snow removed from your roof. Here's a short video clip where I spoke with WCCO news earlier this year about roof snow loads, saying not to worry (yet).
Author: Reuben Saltzman, Structure Tech Home Inspections