Avoid a roof collapse this winter

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Heavy snowfalls in many parts of the country this winter have raised new questions about snow load capacity and the potential for roof collapse.

According to Dan Meyer, agricultural engineering field specialist with Iowa State University, you should try to learn what each of your barns can handle in terms of snow load. Usually, this information is noted on the building plans. But, if you are still unsure, have a structural engineer do an inspection.

Older buildings may have weakened capacity due to weathering, rotting of wood, or other factors. Be careful of rough roof surfaces that do not allow snow to slide off. And, an unbalanced snow load can be far more likely to cause a failure than an evenly-balanced load, Meyer says.

A typical snow load for buildings in the Upper Midwest varies from 20 to 30 pounds per square foot. That is equivalent to 4 to 6 feet of snow. (As a general rule, figure 1 foot of snow for every 5 pounds per square foot of load capacity.)

“This level of loading — 20 to 30 pounds per square foot — is not intended to stay on the roof all winter,” says Larry Jacobson, extension engineer at the University of Minnesota. “A roof may be able to support this snow load for several days or a few weeks, probably no more than 30 days.”

Jacobson says an educated guess regarding a “safe” amount of snow to have on a roof over an extended period of time would be about half of the designed load. If that load is 20 pounds per square foot, then about 2 feet of snow would be the maximum.

“If you have too much snow on a roof, it’s a good idea to get it off as soon as possible,” Jacobson says. Obviously, one way to remove snow is to physically get up on the roof and push snow off with a shovel or broom. But, this involves a safety risk. If you decide to go up on a snow-covered and icy roof, it’s important to use ladders, safety ropes and other precautions, Jacobson points out.



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