Midwest Snowstorm Warning
A blizzard warning was in effect Sunday for Chicago and much of northern Illinois, all of Iowa, large sections of southern Minnesota and Wisconsin, and smaller areas in North Dakota, South Dakota, Missouri and Michigan, according to the National Weather Service. The storm left one and a half feet of snow in areas and caused the collapse of the Metrodome roof in Minneapolis. Additionally, weather experts are forecasting additional lake effect snow with the blast of arctic air early this week.
Our experts have seen and reported on massive snowstorms throughout the United States. While roof codes require roof design to withstand these types of typical snow loads, we have seen damage from storms consistent with this weekend’s many times before.
- The majority of commercial and residential structural roof collapses from snow loads observed and analyzed in the past by Pie Forensic Consultants failed as a result of insufficient design and/or insufficient construction, which could include missing or damaged structural components.
- In fact, during a major blizzard in 2003 in Colorado, Pie found that upon structural analysis, many roofs in Denver County, on paper, failed between 10 to 20 pounds per square foot (psf). Today Denver requires between 25 to 30 psf, depending on the category.
- A large percentage of the residential failures observed in this 2003 blizzard were related to “rafter”-type structures. Many older homes are constructed with rafter-type systems in lieu of pre-engineered wood truss systems or post-and-beam construction. With a rafter-type system, the rafters rely on the exterior walls to be sufficiently rigid. As the roof structure is loaded, the rafters compress the ridge board at the top and exert outward pressure on the bearing walls. Based on our observations, these rafter-type roofs typically failed when the bottoms of the rafters were not adequately tied together by means of collar ties, or the bearing walls were not sufficiently rigid.
- A large number of claims occurred due to excessive deflection and cosmetic damage in the structures. Many residential structures were subject to large deflection and “serviceability failures,” including drywall cracking, sagging roof members, bowed bearing walls, and cracked doors. In these cases, a serviceability failure has occurred, which is more difficult to assess than a catastrophic collapse.
- In many cases, failures may have occurred that have not yet exhibited symptoms or become apparent. These failures may not be perceived until a more minor event occurs or over time. As in the 2003, a number of claims may be seen over following years that relate back to snow load damage from this weekend’s storms.
- Test Your Claims Knowledge: Live Loads vs. Dead Loads With Roof Claims (pieglobal.com)
- Snowstorm hits North Dakota, Minnesota, dropping up to 14 inches in some areas (usnews.nbcnews.com)