Roofs are designed to support a limited load coming from snow, wind, and building materials, according to U of M Extension engineer Larry Jacobson. Snow loads for agricultural buildings in Minnesota vary from 20 to 30 pounds per square foot, says Jacobson. A roof may be able to support this snow load for several days or a few weeks, probably no more than 30 days. This assumes the building was properly designed by a registered professional engineer, and constructed by a qualified builder according to the design, he adds. If that load is 20 pounds per square foot, about two feet of snow or one inch of ice and one foot of snow would be the maximum.
Several factors affect the amount of snow that can build up on a roof.
They include the pitch of the roof, drifting, and shingles that don't shed snow easily. Roofs on a lower building may collect snow sliding off a higher roof, and roof valleys and other areas may collect snow. Generally, there is some time between a large snowfall and possible structural failure. He says one way to remove snow is to physically get up on the roof and push snow off with a shovel or broom. This obviously involves a safety risk.