Larry D. Jacobson and Kevin A. Janni, University of Minnesota Extension, Published December 19 2012
Avoid roof collapses from snow on agricultural buildingsST. PAUL —It looks like 2012-13 could be a year for the Midwest to get hit by heavy snow and ice storms, which can leave large amounts of snow and ice on roofs of agricultural buildings.
The recent large snowfall left over a foot of snow and ice on roofs in central Minnesota, but fortunately most of that snow came off roofs during the warmer temperatures that followed. With colder temperatures expected, we cannot count on future snowfalls sliding off. Trusses and rafters must support the increased weight, or roofs may collapse.
University of Minnesota Extension advises that even agricultural buildings with well-designed roofs cannot take excessive (greater than 4 to 6 ft.) amounts of snow. What is a “safe” amount of snow to have on your roof over an extended period of time? An educated guess for the upper Midwest (where snow loads are typically at least 20 lbs. per square ft.) would be 4 feet of dry snow or 2 feet of wet heavy snow and ice.
The following factors affect the amount of snow that can build up on a roof. They include:
• Roof pitch - snow will not easily slide off flatter roofs (3/12 pitch or less).
• Drifting - wind blowing snow around other buildings and trees can create huge snow drifts and uneven snow loads.
• ”Lean-to” 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 and ice as easily as metal roofs.
• roof valleys or roof areas which collect a lot of snow.
What should be done if your building exceeds the above mentioned “safe” snow depths? The simple answer is to get it off as soon as possible. Generally, there is some time between a large snowfall event and possible structural failure. One way to remove snow from a roof is to physically get up on the roof and shovel off the snow. There obviously is a human safety concern of falling off the roof when working on a snow covered and icy roof. Use ladders, safety ropes and take necessary precautions. Hire a professional if possible.
Other alternatives are the use of snow rakes or specialty tools that can be used from the ground or from portable scaffolding. Use extreme caution when working near overhead electrical power lines. Also avoid excessive scraping on the roof or trying to chip off ice. These practices can damage the roof and lead to leaks.
There are other methods of removing snow and ice from roofs. One involves 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. Obviously, a lot of heat is necessary for even a moderately-sized building, and it 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 that slide off the roof from falling on people, animals or equipment. For flat-ceiling buildings, putting heaters in the attic is generally not recommended because of the fire danger and the possibility of creating ice dams along the building's eaves.
A preventive measure to avoid excessive snow on building roofs in future years is to have effective snow fences and or tree (shelterbelts) windbreak for farmsteads and/or agricultural buildings. Some of the buildings’ roofs that have failed in past years were located either too close to shelterbelts and/or windbreaks or there were no protections for the buildings at all.
When placing a snow fence or tree windbreak, remember that 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 distributes the snow more evenly and gives better protection downwind than a 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.) As an example, a 12-foot high windbreak fence (80 percent solid) will protect from 120 to 180 feet downwind. 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.
Leaving an area or space for snow to accumulate is very important when locating a machine shed or livestock building downwind from a shelterbelt. 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, the 2012-13 winter will provide some opportunity for the accumulated ice and snow on roofs to melt or slide off, but if we receive above normal snowfalls with cold temperatures, monitor the snow load situation on agricultural buildings and take appropriate action. Check high risks areas and please be extremely careful.
For more information about protecting your home and landscape from winter damage, visit www.extension.umn.edu/extreme-weather and click on “Winter Damage.”
Larry D. Jacobson and Kevin A. Janni are agricultural engineers with University of Minnesota Extension.