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Snow Covered Roofs Collapse


Snow loads are influenced by weather and moisture conditions, roof configuration, wind direction, slope direction and geographic location. For the Edmonton area (the Calgary area is similar), the Alberta Building Code states that the design snow load for a home or small building is106kg/m2 or 22 lb/ft2. While the Alberta Building Code provides a minimum design requirement, a properly constructed building should be able to withstand significant snowfall, often in excess of the code design. It is often other factors in combination with age, dramatic weather changes and heavy snow loads that can bring a roof to the point of collapse.

Here are some common factors that contribute to roof collapse:

Construction deficiencies
Improper modifications and inadequate maintenance can contribute to roof collapse under significant snow loads. Modifications can include: insulation without adequate roof deck ventilation, re-roofing over three or more layers of shingles, and/or modifications to the structure causing weakening of the roof’s load tolerances. A roof in an older building may also be “under-built” and more vulnerable to structural problems. These deficiencies, combined with excessive loading, can lead to roof collapse.

Weight and distribution of loading
The weight of the snow can be unevenly distributed with concentrated accumulation of snow in a particular area. This can create increased snow loading on parts of the roof. Roof pitch (3/12 pitch or less) can affect accumulation as snow is less likely to slide off flatter roofs. Also, drifting snow can accumulate against protrusions from the roof or uneven roof lines, such as rooftop equipment or walls between roof levels, in turn increasing the loading on the roof in a particular area. In addition, rain falling on accumulated snow will add more weight. Excessive loads, uneven weight, and drifting snowfall can contribute to increased snow loads and possible roof collapse.

Ice damming
Significant snow accumulation, combined with warming weather, can result in ice damming. An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. Insufficient attic insulation and/or attic ventilation can cause roof snow to melt and slide down to the eaves, where the roof surface is cooler and the melting snow freezes to the roof. Subsequent melting causes water to pool behind this dam and can lead to water seeping between the shingle layers, wetting the roof deck and ingressing into the ceilings and walls. This creates favourable conditions for deterioration of a roof’s structural members, decreasing roof fastener strength, and possible roof collapse.

Every structure is unique and the cause of a collapse can result from multiple contributing factors. The weight of the snow on a roof may not be the only factor. Design, material, weather conditions, and modifications all need to be examined and considered. A structural engineer is uniquely positioned to assess each individual case, determine which factors contributed to the collapse, and provide technical insight to assist in the assessment of liability.


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