Preventing roof collapses from snow and ice on agricultural buildings
Winter storms in Minnesota and other sections of the upper Midwest can dump large amounts of snow and ice on agricultural building roofs which can lead to a roof collapse. Building owners can do several things to reduce the chances of a roof collapse.
Build structure that can withstand load
One preventive measure is to build a structure that is properly designed and constructed to meet or exceed expected snow loads from winter storms. Buildings can be designed based on the amount of snow expected from storms once in either 25, 50 or more years. The roof snow load for residential buildings in Minnesota is set by state statutes and is 42 pounds per square foot (psf) in northern Minnesota and 35 psf in southern Minnesota. Agricultural buildings can be built with lower snow load design values because the building importance for agricultural buildings can be set lower than for residential buildings. Many agricultural buildings are built using a 20 psf snow load which would be expected to handle four feet of dry snow or two feet of wet, heavy snow and ice. Some people combine the snow load with the building dead load (i.e., weight of the roofing and trusses). Be clear when talking with your building designer.
Snow fences or tree shelterbelts
Another preventive measure to avoid excessive snow on building roofs is to have effective snow fences and or tree shelterbelts upwind of farmsteads and/or agricultural buildings. Proper snow fence design and location is important for protecting a building or farmstead. Some buildings roofs’ have failed in the past because the buildings were located too close to shelterbelts and/or windbreaks which resulted in large snow drifts on top of these buildings. Remember when placing a 50% solid snow fence or tree windbreak that snow will be deposited downwind a distance of up to 10 times the shelter belt or snow fence height. An 85% solid fence deposits the snow within a distance of about four times the fence height. Porous snow fences distribute the snow more evenly and give better protection downwind than a solid fence.
Leaving an area 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. For more information on living snow fences and snow fence design go to "Living snow fences: Functions and benefits."
Numerous 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, trees and roof structures (ex. cupolas) can create huge snow drifts and uneven or unbalanced snow loads of a roof
- Roofs on other lower buildings or “lean tos” 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 to do if your building has excessive snow depth
So, what can you do if your building has an excessive snow depth? The simple answer is to get the deepest snow off as soon as possible. Generally, one has some time between a large snowfall event and possible structural failure. Before beginning removing snow or even entering a building with excessive snow on the roof, look for indications of building damage and the beginning of failure. Look at the sidewalls to see if there are any bulges or indications that knee braces have failed. Look at the roof line to see if it is still straight. When entering a building with excessive snow, look at the ceiling, open trusses and walls for indications of damage or failure. If there are indications of building damage or failure, do not climb onto the roof or enter the building while the snow is on the roof.
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. One should use ladders, safety ropes and take necessary precautions. Hire a professional if possible. Other alternatives are to use snow rakes or specialty tools that can be used from the ground or from portable scaffolding. When using a snow rake or specialty tools use extreme caution when working near overhead electrical power lines. Also, avoid excessive scraping on the roof or trying to chip off the ice. These practices can damage the roof and lead to a leaky roof.
There are other methods of removing snow and ice from roofs. If the weather is not too cold, hot water or some other heat source can be used to melt snow and ice. Another method 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 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 that slide off the roof 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's eaves.
It is difficult to say how much snow or ice is safe because it depends on the building design and the snow or ice weight. In most cases, agricultural buildings will have an excessive snow load if there are more than four to six feet of snow on the roof.
Excessive snow and ice followed with cold temperatures can create excessive snow loads. You should monitor the snow load situation on your agricultural buildings and take appropriate action. Check high risks areas and if you need to remove snow please be extremely careful.