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Look at roof design, snow load when assessing collapse risk

By Larry D. Jacobson and Kevin A. Janni, University of Minnesota Extension

ST. PAUL, Minn. (2/15/2010) —After the many recent roof collapses of agricultural buildings caused by excessive snow loads, we have been monitoring conditions in a research effort to relieve or reduce the amount of snow that builds up on a roof. Although people are rightfully concerned about their other buildings once they've had one collapse, buildings handle snow loads differently. The collapse of one building doesn't necessarily mean all the buildings on the farm are in danger.

More information will be available when our data gathering and analysis is complete. Currently, indications are that the following factors affect the amount of snow that can build up on a roof:

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

As a general rule of thumb, remove snow if there are more than four feet of dry snow, or more than two feet of heavy, wet snow and ice on the roof.  For information on removing excessive snow loads, and important safety recommendations, see our column "Watch farm buildings for excessive snow loads; remove with extreme caution" on the University of Minnesota Extension website at www.extension.umn.edu/news. Look for the Dec. 29 column in the short list.

A preventive measure to avoid excessive snow on building roofs in future years is to have effective snow fences and or a tree (shelterbelt) windbreak for farmsteads and/or agricultural buildings. Some of the buildings that have failed this year were located either too close to shelterbelts and/or windbreaks or there was no protection for the buildings at all, resulting in large snow drifts on top of these buildings.

For more information on shelter belts and snow fences, see University of Minnesota Extension's online publication "Living Snow Fences." Go to the Extension website at www.extension.umn.edu and enter "living snow fences" in the search bar. For more detail on snow fences in relation to agricultural buildings, see North Dakota State University's online publication "Farmstead Windbreak Design" at www.ag.ndsu.edu and enter "farmstead windbreak design" in the search bar.

If we continue to receive average or above normal snowfalls, monitor the snow load situation on agricultural buildings and take appropriate action. Check high risks areas, and if you need to remove snow please be extremely careful.


Any use of this article must include the byline or following credit line:
Larry Jacobson and Kevin Janni are agricultural engineers and professors with University of Minnesota Extension.

Media Contact: Catherine Dehdashti, U of M Extension, (612) 625-0237, ced@umn.edu

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