WRITTEN BY Shannon Pierce ON May 9, 2019
The old saying “just be happy you’ve got a roof over your head!” is especially poignant in winter climates, where roofs protect us from a snowy and unforgiving winter. However, the saying should be amended to “just be happy you’ve got a structurally sound roof over your head!”. After all, a roof over one’s head doesn’t sound like a great idea if it can no longer hold up all that snow!
Luckily, engineers and architects take the effect of snow on roofs seriously and design accordingly. Roofs in colder regions are designed to handle heavier snow loads than those where it never snows. Factors of safety and structural redundancy are also considered during design and required by code to ensure that the roof can withstand more than the expected amount of snow, just to be on the safe side.
Nature can be unpredictable, however.
In 2018, Pie’s Minnesota office responded to an atypically large number of claims dealing with structural collapse due to snow. The record-breaking winter was characterized by consistently cold temperatures and repeated heavy snowfalls of up to 30 inches. These conditions did not give the snow on rooftops a chance to melt in between snowfalls. Subsequent snow events led to a compounding effect with layer upon layer of un-melted snow quickly adding a lot of weight to roofs.
The roof trusses in this building failed due to the weight of snowdrift accumulation.
The snow accumulation on the roofs was not static, either. Winter winds typically blew the snow in the southeast direction creating “snow drifts”, or mounds of snow caused by wind, on the leeward side of gable-style roofs. These snowdrifts put additional stress on the framing, creating forces that exceeded a roof’s design capacity and lead to collapse.
An interesting trend in the types of buildings that collapsed emphasized the effect that codes have on various structures. Most of the collapsed structures that Pie’s engineers investigated were agricultural buildings (barns, workshops, sheds, and garages). Agricultural buildings fall under a different occupancy category. The occupancy category affects a design parameter called the “importance factor”, which considers the likelihood and severity of human casualties should a structural failure occur. Since agricultural buildings are expected to be less occupied than a house, hospital, school, etc., they do not have the same structural redundancies built in. As a result, these buildings can be designed to withstand up to 20% less load than other buildings.
Buckling and catastrophic damage to the framing members of this building caused a collapse of the roof.
The structural problems associated with too much snow load can be exacerbated by poor construction practices. Improper installation of pre-engineered wood roof trusses and lack of lateral bracing are common culprits of a poorly built roof. Lateral bracing on wood roof trusses is critical for the overall structural stability of the roof system. Improperly installed braces on critical components of the truss (top chords, bottom chords, or web members) can lead to lateral distortion or even buckling of the member, which may cause a catastrophic collapse.
About the Author
Shannon Pierce has over 12 years of experience designing and investigating structures for projects of all sizes, including residential, commercial, industrial, education, and historic preservation. A large part of his current role is to provide consulting services for forensic engineering, structural assessments, condition assessments, fire damage assessments, water damage assessments, structural and non-structural repair assessments, building rehabilitation, and structural feasibility studies. A Senior Project Manager within the Forensic Department at Pie’s Minnesota office, Shannon enjoys the challenging aspects of determining why a failure has occurred, what can be done to prevent it from happening again, and what repairs are needed to correct the problem.