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Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 34   December 1998


Determining Snow Loads

by Jeff Quell, P.E.

It's that time of year again, when people start thinking about the holidays, shopping and that first big snowfall. Those with roofing responsibilities should also be thinking about the accumulation of white stuff on their roofs. Every winter a building's roof structure and drainage system are "tested" by the build up of snow and ice. In this article we will discuss the key factors that determine snow loads, and the ability a building has to safely sustain these loads.

Design snow loads for roof structures are based on a number of factors. These parameters include geographical location, roof exposure to wind, thermal building conditions, roof slope, and building configuration. In addition, snow drifts, sliding snow, unbalanced snow loads, ponding instability and building modifications must be considered. The methodology for calculating design snow loads is defined in the model building codes, or may be specified by the local building code. It is the application of these building codes by a design engineer or architect that determines the design snow load for each building.

Snow Drifts

Adjacent buildings, terrain features, rooftop projections, penthouses and parapet walls can all cause snow drifts to occur on roof surfaces. Extra snow can also collect in the valleys of sawtooth and barrel vaulted roofs. Roofs equipped with satellite dishes or solar collectors are also subjected to additional snow loads. Since most structural failures associated with snow loads are caused by localized overloads, the proper design of the roof, and in some cases, the removal of snow drifts is key to the integrity of the roof structure.

Sliding Snow

Additional snow load can occur when snow slides off a sloped roof onto a lower roof, resulting in exceedingly high snow loads. Under the right conditions, roofs sloped as low as 1 inch per foot can shed snow loads by sliding. The probability that a sloped roof will develop sliding snow is related to the number of rooftop obstructions, the roof surface temperature, and the slipperiness of the roof surface. Slippery roof surfaces typically include metal, slate, bituminous, EPDM and thermoplastic membranes with smooth surfaces. Any roof sloped to an unobstructed eave is a potential source of sliding snow.

Partial and Unbalanced Snow Loads

Unbalanced snow loads usually result from sunlight, wind scour, and snow removal operations. Winds tend to reduce snow loads on the windward side of a roof and increase snow loads on the leeward side. Case studies indicate that significant unbalanced loads can occur when the slope of adjacent gable roofs is as low as 1/2 inch per foot.

In addition, reduction in snow load from one area of the roof may result in greater stresses in the roof structure than typically occurs when the entire roof is loaded. For example, removing half the snow load from the cantilevered portion of roof joists will actually increase the stresses and deflection of the adjacent continuous span. In other situations, adverse stress reversals may result. Low-slope, internally drained membrane roofs typically do not have unbalanced snow load problems.

Ponding Instability

When heavy rains or melting snow water are added to a snow-filled roof, progressive deflection of the roof structure may occur. Intermittent heating of buildings may cause snow on the roof to thaw and subsequently refreeze in lower areas. Exhaust fans and other mechanical equipment on roofs may also melt snow and develop icings. Drainage systems may become clogged with ice, and the additional loads in the drain sumps or low roof areas may result in increased deflection of the roof structure. This condition, known as ponding instability, may result in total loads that exceed the capacity of the roof structure. Therefore, roof slope is critical, since it decreases the potential for drain blockage and for freezing snow meltwater.

Building Modifications and Additions

The design snow load on an existing roof structure may be affected by building additions and alterations, or by the construction of a new building adjacent to the existing structure. When a new structure is built within 20 feet of an existing structure, snow drifting potential should be investigated for the existing structure. When these conditions occur, it behooves the owner to have a structural analysis of the existing roof structure conducted by a registered structural engineer. In some cases, structural modifications to the existing structure will be required to withstand the additional snow loads that result from changes in the building configuration.

Conclusions

Fall is one of the best times to get out and check your roof for debris and other roof defects. Any debris contained on the roof should be removed and disposed of. Interior roof drains, scuppers, gutters and downspouts should be inspected to ensure that the drainage system is clear and working properly. If drains or scuppers are blocked by ice, do not physically break or chip away the ice. Instead, use heat tapes or deicing salts to prevent drains from freezing closed.

In addition, if snow removal is required, use tools that will not easily damage the roof such as plastic snow shovels or snow blowers with plastic blades or guards. Take care not to gouge or puncture the roof membrane during any rooftop activity. As always, preplanning and preventive maintenance will go a long ways towards ensuring your roof performs as designed during the harsh winter months.