Prevent roof collapses due to heavy snow

Posted at 3 p.m. on February 13, 2019

This story originally appeared in April of 2018 after a blizzard that caused a number of barn roofs to collapse. Circumstances have repeated themselves in the February 12 snowstorm, and so we are re-publishing this article as a public service.

By Ed Byrne

The Brillion News

The Great April Blizzard of 2018 was costly for many farmers in the area. The storm produced up to 33 inches of heavy, wet snow, and the thick layer of “frosting” turned out to be more than the roofs of many barns, industrial and commercial buildings could support.

Rural fire departments were dispatched at a half-dozen farms in the area to help prevent roof collapses, mostly on free-stall barns, the barns that dominate on large dairy farms in the area – the confined animal feeding operations (CAFOs).

Unlike the barns of old, the new free-stall barns are wide and have roofs without significant pitch. The roofs are supported by long trusses, and many were never designed to support the load created by two to three feet of heavy, wet snow – the kind of snow typical in the front and back ends of winter in the Upper Midwest.

A professor with the University of Minnesota-Extension, Kevin A. Janni, has written extensively about the danger of barn roof collapses.

“Winter storms in Minnesota and other sections of the upper Midwest can dump large amounts of snow and ice on agricultural building roofs which can lead to a roof collapse,” Janni wrote. “Building owners can do several things to reduce the chances of a roof collapse.”

When a barn roof begins to fail, it is a sign that it was not designed for the reality of winter in state like North Dakota, Minnesota, Wisconsin, Michigan, Vermont, New Hampshire or Maine.

In Minnesota, the roof snow load for residential buildings is set by state code and is 42 pounds per square foot (psf) in northern Minnesota and 35 psf in southern Minnesota.

Because barns do not house people, a lower standard is used for them.

“Many agricultural buildings are built using a 20 psf snow load which would be expected to handle four feet of dry snow or two feet of wet, heavy snow and ice,” Janni said.

He said that some people combine the snow load with the building dead load (i.e., weight of the roofing and trusses). When a farmer is talking with his building designer, he needs to make it clear that the total load is the target: the structure must be designed to support both the weight of the roofing and trusses, plus the weight of the worst snow you expect to see.

Janni said that numerous factors affect the amount of snow that can build up on a roof, including:

  1. Roof pitch – Snow will not easily slide off flatter roofs (40 degree pitch or less).

  2. Drifting – Wind blowing snow around can create drifts producing uneven or unbalanced snow loads of a roof.

  3. Roof material – Shingled roof decks do not shed snow and ice as easily as metal roofs.

  4. Roof valleys – Roof areas which collect a lot of snow produce significant load on those areas.

Metal structures often don’t give much warning before collapse, whereas structures with wooden posts, beams and trusses often do.

Here are some tell-tale warning signs cited by the The Bullvine dairy newsletter: Sagging roof; severe roof leak; doors popping open; sudden emergence of cracks in walls or masonry; bends or ripples in supports; visible bowing of rafters; cracks or splits in beams and posts; screws in steel frames shearing off; creaking, cracking or popping sounds; doors or windows that suddenly stick or