Investigating Roof Damage Under Snow
November 3, 2020
Often when an unusually heavy accumulation of snow occurs in a location, insurers receive many roof damage claims for sagging, cracked, or broken roof or ceiling members. The weight of snow and ice is usually blamed because the reported occurrence of the roof damage coincided with a period of heavy snowfall. However, the structural damage that occurs after a heavy snowfall is often the manifestation of other unrelated issues, such as latent defects in the design or construction of the roof, improperly designed or executed alterations to the original structure, undesirable accidental connections (load paths) in the framing, or lack of proper maintenance. While this technical note covers roof damage due to snow load only, it should be noted that when investigating any structural damage it is necessary to consider all loads acting on the building.
This technical note was developed to help the non-expert reader gain general understanding of the structural design approach for roof structures under snow load that is adopted by the National Building Code of Canada and all provincial building codes such as the Ontario Building Code. The note discusses how design values for snow load on roofs are determined and explains the difference between the anticipated maximum load under normal service conditions (service load) and the extraordinary amount of snow beyond which structural failure of the roof framing members would be expected (ultimate load). The technical note also presents examples of roof damage in the presence of snow without snow being the root cause. Building owners, property managers, risk assessors, insurance adjusters, and construction litigators will find this technical note to be a useful resource in assessing whether pre-existing conditions could have caused or contributed to roof damage even though snow was present during the incident.