Preventing roof collapses from snow on agricultural buildings

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Editor's note: Larry D. Jacobson and Kevin A. Janni are agricultural engineers with University of Minnesota Extension.

The recent snowstorm left large amounts of snow and ice on agricultural building roofs. The extended cold temperatures prevent snow and ice from sliding off, which increases the total weight that trusses and rafters must support. Even well-designed roofs cannot take excessive amounts of snow, greater than four to six feet.

What is a "safe" amount of snow to have on your roof over an extended period of time, such as several weeks? It is extremely difficult to say, but an estimate for the upper Midwest, where snow loads are typically at least 20 pounds per square foot, would be four feet of dry snow or two feet of wet, heavy snow and ice.

Factors that affect the amount of snow that can accumulate on a roof:

  • roof pitch--snow will not easily slide off of flatter roofs (3/12 pitch or less)
  • drifting--wind blowing snow around other buildings and trees can create huge snow drifts and uneven or unbalanced snow loads
  • a roof on other lower buildings--also known as a "lean-to," these receive snow or ice
    sliding off another roof above it
  • shingled or roof decks--snow and ice are not shed as easily as with metal roofs
  • roof valleys--or other roof areas that collect a lot of snow

What should you do if your building exceeds "safe" snow depths? The simple answer is to remove the deepest snow as soon as possible. There is generally time to get that done before a possible structural failure.

Physically getting on the roof to shovel the snow off is one approach, but falling is an obvious safety concern. Use ladders and safety ropes and take necessary precautions. Hire a professional if possible.

Alternatively, snow rakes or specialty tools can be used from the ground or from portable scaffolding. When using a snow rake or similar specialty tools, exercise extreme caution when working near overhead electrical power lines. Also, avoid excessive scraping or trying to chip ice off, which can damage the roof and cause it to leak.

Warming the inside of the building sufficiently with large heaters to melt the ice layer, and then waiting for the snow and ice to slide off the roof is another method. A lot of heat is necessary for even a moderately sized building. The building must be an open-trussed structure (no flat ceiling) and have an uninsulated metal roof. Caution is necessary to prevent large chunks of ice and snow from falling on people, animals or equipment. Putting heaters in an attic of buildings with flat ceilings is not recommended because of the fire and carbon monoxide danger and the possibility of creating ice dams along the building eaves.

Effective snow fences and/or tree windbreaks or shelterbelts for farmsteads and agricultural buildings can help prevent excessive snow on building roofs in future years. Some of the failed roofs in the past either had no protection for the buildings or were located too close to shelterbelts/windbreaks. In either case, it resulted in large snow drifts on top of these buildings. It would seem that producers, builders and farmstead planners are forgetting how important it is to protect farm sites from blowing and drifting snow.

When placing a snow fence or tree windbreak, remember the protected area downwind will generally be 10 to 15 times the height of the shelterbelt or fence. Research in Canada has shown that an 80 percent solid fence (if 1 x 10-inch boards are used you would vertically space them 2.5 inches apart or with 1 x 8-inch boards the spacing would be 2 inches) distributes the snow more evenly and gives better protection downwind than a solid fence. As an example, a 12-foot high windbreak fence (80 percent solid) will protect from 120 to 180 feet downwind. Also, most of the snow will drop within the first 30 to 40 feet immediately downwind of the fence/trees or roughly within the first 25 percent of the protected area. When locating a machine shed or livestock building downwind from a shelterbelt, leaving an area or space for snow to accumulate is very important. If the building is too close, it will be within this snow drop area, and if too far from the windbreak, it will be outside of the wind protection zone.

Hopefully, March 2014 will provide some opportunity for accumulated ice and snow on roofs to melt or slide off. But if we receive even normal amounts of snow during the remainder of this winter accompanied with cold temperatures, you should monitor the snow load situation on agricultural buildings and take appropriate action. Check high-risk areas, and be extremely careful if you need to remove snow.



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Bill    
Central PA  |  March, 05, 2014 at 09:54 AM

A certified engineering design would have checked load design and bracing requirements to help reduce the chance of overload. Bracing is not just the individual truss, but how those trusses are braced to each other. An excess load can cause a truss to twist thus putting extra stress on the other trusses with failure. Most insurance companies have checked this in large facilities before they insured the owner's property. Of course this winter's extraordinary snowfall has tested the design specs, but may be a heads up for future winters snowfall. Check with your Extension Ag Engineer if you need help in pro-active actions.


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