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.