Southern and western Minnesota experienced some unusual weather this fall and winter which might be causing excessive roof loads on agricultural buildings due to the accumulation of snow and ice. First, ice storms left from one to three inches of ice on some roofs. Then, large snowfalls added to the total weight that trusses and rafters must support. Because of the ice layer, snow is not sliding off as it usually does each winter. Since it is likely that the ice layer will not melt until spring, each additional snowfall this year will intensify the weight problem.
How much weight can most roofs take? Snow loads for agricultural buildings in southern and western Minnesota are generally around 20 lb per square foot. This does not include the weight of the wood that makes up the truss or rafters, a ceiling if one is added to the lower cord of a truss, any equipment hung from the trusses or rafters, or wind loads. Many roofs for livestock barns and machine sheds are designed for a total load of 25 to 30 lb per square foot. Roofs should be able to handle this load if the building was properly designed by a registered professional engineer and constructed by a qualified builder who followed the design.
How much does snow weigh? A one inch layer of water or ice weighs approximately five lb per square foot. A roof designed for a 20 lb per square foot snow load could theoretically hold up to four inches of ice. How much snow is that? Meteorologists estimate that about 12 inches of snow is equivalent to one inch of water, which means a roof should hold up to four feet of snow. Wet and packed snow weighs more, so a roof with a load of wet or packed snow might only be able to hold up to three feet of snow. A roof might be able to support the maximum snow load for several days or a few weeks, but probably no more than 30 days because wood can fail due to fatigue under prolonged loading.
What is a safe amount of snow to have on your roof over an extended period of time? A good estimate would be about half of the 20 lb per square foot design load, or about two feet of snow, or approximately one inch of ice and one foot of snow.
Several factors affect the amount of snow that can build up on a roof. They include:
What do you do if you have too much snow on your roof? The simple answer is to remove it as soon as possible. Generally, there is some amount of time between a large snowfall and possible structural failure. One good way to remove snow from a roof is to physically get on the roof and push the snow off with a shovel or broom. Use ladders, safety ropes, and take necessary precautions, as walking on a roof can be dangerous. Snow rakes can also be used, but be very careful when handling a snow rake near overhead electrical power lines. Also avoid excessive scraping or chipping of ice because this can damage the roof and lead to leaks.
Other more innovative methods of removing snow and ice from roofs are possible. One method involves warming the inside of the building with large heaters in an attempt to melt the ice layer and induce the snow and ice to slide off. Obviously, a lot of heat is needed for even a moderately-sized building and the building must be an open-trussed structure (no flat ceiling) that has an uninsulated metal roof. For flat-ceiling buildings, placing heaters in the attic is generally not recommended because of the fire danger, the amount of heat needed to melt the snow and ice, and the possibility of creating ice dams along the eaves.
There is a chance that the weather will get warm enough this winter for the existing ice and snow on roofs to melt or slide off. But if we continue to receive average or above-average snowfalls, monitor the snow-load situation on your agricultural buildings and be prepared to take appropriate action. Check high risks areas and please be extremely careful if you need to remove snow or ice.
This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/