Near record amounts of snow this winter have raised concerns about roof collapses. 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. The deeper the snow, the higher the loading.

Brian Holmes and David Kammel, University of Wisconsin-Madison/Extension professors and specialists note that the more dense the snow and ice, the greater the load for a given depth. Rain held in snow can add about 5 pounds per square foot per for each inch of rain.

Wind blow-off and snow slide-off can reduce snow load on a roof. However, snow drifting onto leeward or lower roofs and valleys and snow slide onto lower roofs can add significant loads from accumulated snow.

If you have two buildings constructed with the same strength, the one with the largest snow load should be of the most concern. In addition to estimating the roof loading, it’s important to know the loading the roof can resist.

Wisconsin’s Uniform Dwelling Code requires that one- and two-family residences in southern Wisconsin be designed to withstand a uniform snow load of 30 pounds per square foot and those in the north to support a 40-pound per square foot load. Required design snow loads for the state’s commercial buildings are calculated on a case-by-case basis, with design snow loads for those in the far north often exceeding 60 pounds per square foot.

Building plan approval and subsequent construction inspection do not always guarantee a building has been designed and constructed in accordance with state codes. Agricultural buildings are exempt from the state’s building codes, which means farmers must work with their builders to establish and require the minimum snow loading to be used in the design of any new structure.

Roof failures can still occur in buildings at snow loadings lower than those for which they were designed.

Owners should have their roofs checked by a qualified builder and/or engineer to assure their structures are best prepared for the snow loadings they will experience.

If an owner suspects a snow loading is in excess of the design loading or if the roof is showing signs of impending collapse such as buckled or broken members, plans to remove the snow should be implemented.

Holmes and Kammel recommend the following if you will be removing snow from your roof:

n Use a snow roof rake if at all possible. This allows you to stand on the ground in a safe place. Check the local hardware store or building supply store. Removing snow from the edge of the roof could allow snow above the edge to avalanche. Make sure you are out of the fall zone when scraping snow from a smooth roof surface.

n Use fall protection equipment when workers are on the roof. Tie workers off so they don’t fall from the roof.

n If ladders are used, locate and secure them so they do not fall while workers are standing on them. Also, locate ladders so they do not fall if snow slides off the roof knocking workers off the ladder or leaving them stranded on the roof.

n Generally, remove snow from the most heavily loaded areas first.

n Remove snow in narrow strips instead of large areas to help keep loading somewhat uniform.

n Use plastic shovels or wooden roof rakes to avoid damaging roofing material. Don’t feel as if all snow must be removed. A layer of snow next to the roof surface can protect the surface from damage during the snow removal process.

n Do not pick or chip at ice near the roof surface to avoid damaging roofing material.

n Do not use snow blowers as they can damage the roof.

For more information or to make comments on the information contained in Coulee Consumer, please contact the La Crosse County UW-Extension, 400 North 4th Street, La Crosse, WI 54601, or call (608) 785-9593.