How much snow was on the roof when it collapsed?
Well, the weight of ice and snow is affected by many factors, such as, depth of snow, density of the snow, consolidation, or rainfall on the snow. This article will discuss information helpful in calculating the actual snow load.
Of course the best method to determine the weight of ice and snow on a roof is to take a cylindrical sample with a tube of known diameter, weigh the sample, and using ratios calculate the weight of the snow per square foot. However, this method is available only on rare occasions.
Furthermore, the interest in the weight of the snow often arises long after the snow has melted. Nevertheless, there is often other information available to calculate a reasonably accurate estimate of the weight.
Eyewitness reports of the depth may be helpful in quantifying the depth, especially of a drift. An eyewitness may be able to provide information regarding what the depth of the snow was compared to surrounding building component such as windows, wall or roof fixtures or equipment, etc. Photographs or security videos may also be available for examination. The proliferation of photo capable cell phones has made this source of information more readily available. Measuring the building components and taking photographs from the same location as those taken with the snow accumulation is helpful to determine the depth of the snow.
Review of weather records from multiple weather stations and airports provides information regarding the amount of the snowfall, the ground accumulations, and sometimes the water equivalent of the snow fall. These may help determine density of the snow and the weight of undrifted snow.
Calculation of the design snow loads and the design drifting loads for the configuration, exposure, and orientation of the subject roof helps to understand the pattern (the shape, the extent, and the depth) of the snow accumulations and the drifting common to the subject roof.
Lastly, there is information available that relates the weight of ground snow to the depths of ground snow. The information is used to determine the design snow loads for a roof, including drifting. This equation takes into account the consolidation of the snow from the weight of the overlying snow. This equation, which is used for the determination of ground snow loads, when applied in conjunction with the previous considerations provides understanding regarding the effects of the depth on the density, and thus, the weight of the snow and drifting.
Thus, although the snow has often times been long gone, there is usually information and engineering principles available that assist in rendering a professional opinion regarding the weight of ice and snow on the roof at the time of the failure or the collapse.