Although we get closer to spring every day, the recent and ongoing chilly and precipitation-filled weather reminds us that it is still indeed winter. If the pesky groundhog has it right, this wintery weather will carry on into mid-March, along with its effects on structures – particularly those of snow.
The weight of snow on a structure’s roof can be significant and can, in extreme conditions, lead to overload and failure. Unsurprisingly, failures from the weight of snow are generally more frequent where the expected snow loads are most severe – namely the northeast (see Figure 1), followed by the Midwest and Western states. In these areas, snow loading is often the controlling load for the design of building roof framing.
Figure 1. Heavy snow accumulation on a roof in the northeast United States.
While collapses from snow loading are relatively rare, such failures (and other snow-related failures) tend to occur during particularly heavy snowfall events or from an accumulation of several snow events. Figures 2 and 3 are from a roof collapse at a barn in northern Wisconsin. The collapse occurred due to the weight of snow on the roof. This portion of the barn was about 50 years old at the time of the collapse. In this case, although the roof had existed for half a century, the snow loads from the contemporary building codes (at the time of the collapse) were found to exceed the capacity of the roof, as constructed. An older section of the barn (97 years of age at the time) had a steeper roof slope and did not experience damage.
Figure 2. Collapse of a barn roof due to the weight of snow. This occurred in northern Wisconsin, close to the south shore of Lake Superior.
Figure 3. Fractured roof framing at the barn from Figure 1.
Figure 4 shows a fractured rafter in a residential structure in northern Minnesota. Several rafters had fractured at this structure due to the cumulative weight of snow and, as a result, the roof had areas with permanent deflection. Such conditions were identified as unsafe due to the potential for more significant failure or even collapse if subjected to another significant snow event. Fortunately, the excessive deflection served as a “warning” of the potential for a more catastrophic failure.
Figure 4. Fractured and deflected rafter from the effects of snow load on a residence in Minnesota.
Nelson’s team of professionals is well versed in assessing structural damage, including collapses and failures from snow loading. We stand ready to assist our business partners at a moment’s notice to determine the cause, extent, and repairability of damage caused by snow and other weather events.
