Winter 2018–2019 was particularly difficult in Quebec.
Several damages have been observed by building owners due to the snow load on the roofs.
Damages extended from simple cracks in the gypsum finishing of the ceilings to the partial collapse of the roof’s structures all the way to the complete collapse of the building.
This winter was characterized by numerous rainfalls followed by sudden freezing temperatures within only a few hours. This pattern happened mostly between mid-February to the beginning of April. During this time period, the rainwater would infiltrate the existing snow on the roof’s of the buildings. However, not having enough time to evacuate, the water would freeze at different levels in the snow built-up. The snow density on the roofs would then increase becoming an ice and snow mix.
During a “normal” winter, the snow that falls onto a roof is partially swept away by the wind. A reduction factor is applied during the design process in order to consider the wind gusts effect on the roofs (default 0.8). The roofs shape, the type of sheathing, and the slope of the roof, are also considered when calculating the roof’s snow load with the National Building Code of Canada. However, this winter, snow was denser than usual (and therefore was less swept by the wind); which increased the roofs’ snow load.
Were the design loads of the National Building Code exceeded?
During several forensic investigations, especially when we had access to the building a few hours after the damage occurred, we measured the snow load on the ground (or on a part of the roof that was not affected by the collapse) to validate if the measured load was more than the design load. We were then able to conclude that, that the measured snow loads were rarely greater than the design loads required by the National Building Code.
We noticed construction defects and/or gradual structural damages on several buildings when the design load was not exceeded.
Several farms and commercial buildings’ roofs collapsed during winter. These buildings usually have big surface areas and their roofs are composed of prefabricated wood trusses.
During the installation of the wood roof trusses, the installation of the following two elements are required between the roof trusses to ensure an accurate structural resistance of the roof:
- Continuous support perpendicular to the members in compression according to the manufacturer plans (red arrow in figure 1);
- A diagonal support to brace (lock) the continuous support mentioned in point 1) (blue arrow on figure 1).
Figure 1—Example of a continuous and diagonal support
One element does not go without the other. In the absence of diagonal bracing, the continuous supports are useless. While the diagonal support is absent or partially installed, the dimensioning of the roof trusses are no longer valid. The installation of the continuous support and their bracing is fundamental for the structure’s integrity of a roof structure.
During the previous winter (2018-2019), we repeatedly observed the absence of one or the other of these elements.
During a rupture, the compression members buckled (cambered) and broke because of a gravitational load caused by the snow that was probably more then in the preceding years due to the particular winter conditions (see figure 2).
Figure 2—Breaking of the wooden roof trusses chords without continuous connections
In conclusion, the snow load accumulated on the roofs were not exceptional as per the National Building Code in most investigation cases. The buildings would not have undergone damages in the absence of construction deficiencies. In a buildings structural collapsing case, a forensic investigation could identify the construction deficiencies, and eventually, allow you to assert your rights.
Do you have questions? Contact the author!