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Heavy Snow Threatens Farm Buildings

Heavy Snow Threatens Farm Buildings
Each foot of snow on a roof weighs 15-20 pounds.

Some farm buildings have already collapsed under the deep snow that has accumulated on roofs.

Is there too much snow on the roofs of your buildings?

Dick Nicolai and Steve Pohl, South Dakota State University agricultural engineers, provide the following guidelines:

Most post-frame ag building are designed to withstand a 20-40 pound snow load per square foot.

If the snow is dry, one foot of snow on the roof can equal 3 pounds per square foot. The figure increases to 6.5 pounds for two feet of snow, 9.5 pounds for three feet of snow, 12.5 pounds for four feet of snow, and 15.5 pounds for five feet of snow. 

Snows that are "in between" dry and wet tend to weigh about four times as much as dry snow, while truly wet snow accumulations can weigh as much as seven times as dry snow loads on a roof. For example, five feet of wet snow on a roof can weigh as much as 104 pounds per square foot.

The snow that accumulates on roofs may be two to three times denser than snow that falls on the ground. "Normally, snow that is 12 inches deep would probably weigh about 6 pounds per square foot, but on a roof, a person can figure it weighs up to 15-20 pounds for that same 12 inches of snow depth. This is in part due to the settling that takes place on the roof and warming underneath that melts the snow somewhat." Nicolai says.

The length of time deep snow is on the roof makes a difference. All building materials have a fatigue factor, and most roofs may be able to support a specific snow load for several days or a few weeks, but probably not for 30 days or more. "Engineers typically design wood structural components to resist the designed snow load for 60 days," Pohl says. "The 60-day load duration factor is cumulative for the life of the structure, not for just one winter. For example, a building intended for 30 years use should, on average, have a maximum design snow load on it for two days per year. The longer that a snow load that near or in excess of design stays on the structure, the more likely it is to fail under a heavy load in the future." 

Source: SDSU AgBio Communications

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