Due to the recent heavy snowfall there is reason to have concern over heavy snow loads on farm buildings. There have been several reports of farm buildings going down in western Wisconsin. In addition, many have concerns for buildings that still have significant amounts of snow on them especially if more snow falls before the current snow melts or slides off.
“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 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:
- Structure was not designed, just built.
- Trusses/rafters/purlins/ with reduced quality materials or smaller dimensions than specified in design were used.
- Trusses/rafters/purlins installed at a wider spacing than specified.
- Critical bracing not installed or improperly installed
- Moisture condensed on or leaked onto structural members can cause decay/corrosion weakening the structure. Top chords of trusses, rafters, purlins and truss plates are particularly susceptible.
- Loads added to the roof which were not considered in the original design. Examples include: ceiling, roof surface overlay, equipment installed on roof or hung from trusses.
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).
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