Sudden Roof Collapse of an Assembly Building
July 29, 2021
Nabi Goudarzi, PhD, P.Eng.
Technical Experts Involved:
Nabi Goudarzi, PhD., P.Eng.
Yasser Korany, PhD, P.Eng, P.E., LEED AP
Darren Keuhl, Drone Operator
A wood-framed assembly use building located near Ottawa had a large extension that was constructed in the 1960s. The gable roof of the extension collapsed without warning during winter, resulting in an estimated loss of over one million dollars. Thankfully, the building was not occupied at the time of collapse. The roof over the original portion of the building did not collapse. Origin and Cause Inc. was retained to investigate the probable cause of the sudden roof collapse.
Because the building was unsafe to enter or approach, drone-assisted inspection was conducted to examine the framing of the intact roof of the original building and the failure mechanism of the collapsed roof of the extension. An advanced compact drone was flown by our experienced pilot very close to the areas of interest. The depth of ground snow reported by Environment and Climate Change Canada was reviewed to estimate the roof snow load at the time of collapse. The 1950 edition of the National Building Code of Canada was reviewed to determine the structural design requirements at the time the addition was constructed.
Cause of Loss
We determined that the most likely cause of the roof collapse was pre-existing construction deficiencies in the roof framing, mainly the lack of proper connection between the roof rafters and the ceiling joists. The rafter-to-ceiling joist connection was significantly under designed, which caused the rafters and rim board to withdraw from the ceiling joists under the weight of snow. Available weather records revealed that the amount of snow at the time of collapse was within the design limits. Had this building been constructed to meet the requirements of the building code of the time, it would not have collapsed.