The following is from Larry Jacobson and Kevin Janni, agricultural engineers with the University of Minnesota's Extension Service:
Recent weather conditions in southern and western Minnesota may result in excessive roof loads on agricultural buildings due to the accumulation of snow and ice. The November ice storms that hit the area left one to three inches of ice on some roofs. The recent large snowfalls have added to the total weight that trusses and rafters must support. Because of this ice layer, snow that has recently fallen on the roof is not sliding off like it normally does in the winter. Since it's likely that the ice layer will not melt until spring, each additional snowfall this winter will intensify the problem.
How much weight can most roofs take? Snow loads for agricultural buildings in southern and western Minnesota are generally around 20 pounds per square foot. This does not include the weight of the wood members (dead load) that make up the truss or rafter, a ceiling if one is added to the lower cord of a truss, a wind load, or any equipment hung from the trusses or rafters. Many roofs for livestock barns and machine sheds are designed for a "total" (sometimes confused with snow load) load of 25 to 30 pounds per square foot. This assumes that the building was properly designed by a registered Professional Engineer (PE), and constructed by a qualified builder according to the design.
So how much does snow weigh? It depends. Start by asking how much water, or ice in the solid form, weighs. A one-inch layer of water or ice weighs approximately five pounds per square foot. So a roof designed for a 20-pounds-per-square-foot snow load could theoretically hold up four inches of ice. How much snow is that? Meteorologists often estimate that about 12 inches of snow is equivalent to one inch of water. Using that "rule of thumb," a roof should hold up to four feet of snow. Wet and packed snow weigh more per inch of depth, meaning that a roof may only be able to hold up to three feet of snow.
This level of loading, 20 pounds per square foot, is not intended to last all winter; there is a fatigue factor. A roof may be able to support this snow load for several days or a few weeks, probably no more than 30 days.
What is a "safe" amount of snow to have on your roof over an extended period of time? An educated guess would be about half of the designed load (20 pounds per square foot), or about two feet of snow or 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:
So what do you do if you have too much snow on your roof? 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. Unfortunately, one good way to remove snow from a roof is to physically get up on the roof and push the snow off with a shovel and/or broom. There obviously is the safety concern of falling off when working on a snow-covered and icy roof. It's important to use ladders, safety ropes and take necessary precautions. Snow rakes also can be used to remove snow. When using a snow rake, 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 a leaky roof.
There are other, more "innovative" 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 hoping the snow and ice 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), plus 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. That's because of the fire danger, the difficulty of being able to provide enough heat to get the job done, and the possibility of creating ice dams along the building's eaves.
the remaining winter will provide some opportunity for the
existing ice and snow on roofs to melt or slide off. However,
if we continue to receive average or above normal snowfalls
you may want to monitor the snow load situation on your agricultural
buildings and take appropriate action. Check high risks areas,
and if you need to remove snow, be extremely careful.
Larry Jacobson, (612) 625-9733; Kevin Janni, (612) 625-3108
Editor: Joseph Kurtz, EDS, (612) 625-3168
Disclaimer and Reproduction Information: Information in NASD does not represent NIOSH policy. Information included in NASD appears by permission of the author and/or copyright holder. More