Your Roof Should Be Built To Handle Normal Snow Load
Ken Hellevang, Agricultural Engineer
Don't be too hasty to shovel the snow off your roof, says a North Dakota State University agricultural engineer. Most roofs are designed to handle the snow load of a typical winter.
"Just because one roof has gone down, doesn't mean every roof in the area is in danger," says Ken Hellevang of the NDSU Extension Service. "The collapse may have been a case of poor workmanship, design, unusual amounts of snow or some other special circumstance."
Hellevang says most house roofs in the eastern and northern parts of North Dakota should be built to hold 30 to 40 pounds of snow per square foot. In the southwestern part of the state where snowfall is typically lighter, roofs are built to hold less; about 30 pounds per square foot.
"Agricultural buildings usually aren't designed to those same standards," Hellevang says. "That's because risk of damage or injury from collapse is considered to be lower." Agricultural buildings should be built to carry from 24 to 34 pounds of snow per square foot depending on location. Check your insurance policy, Hellevang recommends. Sometimes failure due to snow is not covered on agricultural buildings without a purchased rider.
Snow load standards may not have been in place when older homes and buildings were erected, but if those buildings have withstood the test of time, they'll probably withstand a normal winter's snow load.
How can you tell if the snow load on your roof is excessive? That's a tough call, Hellevang says.
"The weight of snow varies greatly. Light fluffy snow may only weigh about seven pounds per cubic foot. More average snow may weigh 15 pounds per cubic foot and drifted compacted snow may weigh 20 pounds or more," he notes. Ice buildup adds weight rapidly, he adds.
"The decision to shovel off a roof is a case-by-case decision," he says.
The engineer says to watch for very deep drifts caused by surrounding buildings or trees. Roofs that have more than one level often accumulate deep snow drifts, but those roofs should have been built to carry that added load, Hellevang says. You can inspect the rafters and trusses and if they are bending downward or flexing to the side, the roof is in danger, he says.
Unless you can get a recommendation from a friendly engineer or a building official in your city, the decision to shovel off a roof will be based on an educated guess, Hellevang says.
If you do decide to shovel off your roof, remember that snow and ice can make the job dangerous. Agricultural buildings with their metal roofs can be especially treacherous. And power lines can be an added hazard.
"You can also damage your roof," Hellevang says. "Cold temperatures make shingles brittle so they break easier. You're also more likely to remove many of the little pebbles from the surface of the shingles, shortening the life of your roof."