University of Wisconsin - Extension

Estimating load on a roof

The following table and example equations can be used to estimate the load on a roof.

Calculated Roof Loading (lb/ft2) = Depth (ft) x Density (lb/ft2 /ft depth)

Example: Two (2) foot depth of packed snow

Calculated Roof Loading (lb/ft2) = 2 ft x 30 lb/ft2 / ft = 60 lb/ft2

A more accurate method is to weigh the snow/ice on a known area, dividing the weight (lb) by the area (ft2) to obtain the loading (lb/ft2).

For two buildings constructed with the same strength, the one with the largest snow load should be of most concern to the owner, according to Holmes and Kammel. In addition to estimating the roof loading, it’s important to know the loading the roof can resist.

Wisconsin’s Uniform Dwelling Code requires 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.

Holmes and Kammel said, "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 the minimum snow loading to be used in the design of any new structure."

Roof failures can occur in buildings at snow loadings lower than those for which they were designed because of one or more of the following:

• 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.

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.

Heating air in an uninsulated building can help to melt snow/ice in contact with the roof surface allowing snow/ice to slide from a sloped roof. Unvented heaters consume oxygen in the building and can contribute carbon monoxide to the building space. Both conditions are poisonous to people and livestock. People and animals should not be allowed in the building when using unvented heaters. Before starting an unvented heater, make plans for turning off the heater from outside the building.

Snow slide from the building roof can cause crushing/suffocating injuries or death. Keep people and animals away from the fall zone when snow/ice slides from the roof.

Roof snow can also be removed by shoveling. This is a dangerous process as falling from ladders and the roof is a real possibility. Consider hiring experts who own boom trucks or other specialized equipment for safely performing this task.

The following apply to removing snow from the roof:

• 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.
• Use fall protection equipment when workers are on the roof. Tie workers off so they don’t fall from the roof.
• 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.
• Generally remove snow from the most heavily loaded areas first.
• Remove snow in narrow strips instead of large areas to help keep loading somewhat uniform.
• Don’t pile removed snow onto snow-covered areas increasing the load in those areas.
• 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.
• Do not pick or chip at ice near the roof surface to avoid damaging roofing material.
• Do not use snow blowers as they can damage the roof.