It looks like 2012-13 could be a year for some areas of 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 ft. of dry snow or 2 ft. 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.
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