Due to the recent heavy snowfall there is reason to have concern over heavy snow loads on farm buildings. There have been several farm buildings that have collapsed in Waupaca County and throughout northeast Wisconsin. In addition, many have concerns for buildings that still have significant amounts of snow on them.
“Snow and ice accumulations on roofs cause a loading which can cause roof collapse when the roof is not strong enough to resist the load,” said Brian Holmes, University of Wisconsin-Extension emeritus agricultural engineer.
He added, “The more dense the snow and ice, the greater the load for a given depth. Wind blown-off and snow slide-off can reduce snow load on a roof. However snow drifting into leeward or lower roofs and valleys and snow slide onto lower roofs can add significant loads from accumulated snow.”
In addition to estimating the roof loading, it’s important to know the loading the roof can resist. Wisconsin’s Uniform Dwelling Code requires most homes to have a minimum snow load rating of 30-40 pounds per square foot (lbs/ft2 ), with the greater requirement for Northern Wisconsin. Agricultural structures are exempt from this requirement. Furthermore, structural failures can occur at snow loads less than the building was designed for if:
At snow loads greater than recommended or if the structure is showing stress from the snow (sagging, trusses out of alignment or bowed, creaking sounds etc.), you may need to remove some snow.
If you are unsure of the snow load on your roof, a ballpark estimate can be made using the formula:
Calculated Roof Loading (lb/ft2) = Depth (ft) x Density (lb/ft2 /ft depth). The approximate density (lb/ft2 /ft depth) for light snow is 5-20, packed snow 20-40, packed snow with ice 40-58, and ice 58. So for example, a roof with three feet of light snow has a estimated roof loading of 60 lb/ft2 (3 ft depth X 20 lb/ft2/ft depth density = 60 lb/ft2).
Removing Snow – Use Caution
If you need to remove snow from a roof, use caution. Falls from roofs or from ladders going to the roof can easily occur. Removing snow can allow the snow up slope to suddenly slide down, burying people or animals below. Using a roof rake from a safe distance away can reduce some of this risk to the person removing the snow.
This information was taken from a 2010 article written by Brian Holmes, Professor Emeritus Biological Systems Engineering Department University of Wisconsin-Madison, regarding Heavy Snow Loads on Farm Buildings.