Essential structural considerations in roof design

Snow management case study
In the early 1990s, commercial snow guards were installed on the roofs of Alberta’s Chateau Lake Louise which failed. The firm the author worked for was asked to investigate the failure and, over time, the assignment grew into a snow management plan and replacement of the snow guards. The project provided vast experience in designing snow guards, dealing with microclimates, and managing snow.

Lake Louise is a glacier-fed lake in the Rocky Mountains. The Chateau Lake Louise is located at the east end of the lake and is 165 m (541 ft) higher and 1.8 mi (3 km) west of Lake Louise Village. In the Alberta Building Code 1990 Edition, Lake Louise Village was listed as having a design snow load of Ss = 6.3 kPa (132 psf) and Sr = 0.3 kPa (6.3 psf).

When Environment Canada was asked what the snow load for the Chateau was, the firm was told it was the same as the village. Environment Canada’s ground snow models predicted there would be a greater depth of snow, but it would be lighter due to the higher elevation. The model did not consider the effect of moisture from the lake. The firm received advice from local ski guides, later confirmed by site measurements, and used a much higher design ground snow load. In the 2019, Alberta edition of the National Building Code (NBC), the Chateau Lake Louise site design ground snow load was 0.7 kPa (14.6 psf) higher than Lake Louise Village.

The roofs at the chateau and parkade were standing seem metal roofs to encourage the snow to slide off. However, the roofs would have shed snow on pedestrian walkways or cars below; therefore, the snow needed to be held on the roofs for safety.

When replacing the existing snow guards, there were several challenges. The guards needed to be much higher and larger than anticipated. In addition, the roof structures needed additional reinforcement.

Often, people think of snow as monolithic light material, as it is when it first falls; however, snowpacks are not. Snow compacts with time; if held in place for a lengthy period, it can turn into ice. A 1 m (3.3 ft) depth of fresh snow compacts to 500 mm (19 in.) in a day and can reduce to about 250 mm (9 in.). For example, in Lake Louise, the design ground snow load represents up to a depth of 1.6 to 3.2 m (5.2 to 10.4 ft) of snow on the roof. When using guards, all the snow must be held on the roof over the winter season; one cannot rely on some of the snow sliding off. Over the winter, there will be weather conditions that create weak layers in the snowpack on the roof, similar to how snow behaves on a mountain. The guard must prevent snowpacks from sliding on the roof’s surface in multiple areas.

In the areas the author was assigned to design, his firm used 1200 mm (47 in.)-high hollow structural section (HSS) posts anchored to the roof’s perimeter steel structure, with the top of the HSS post tied back to the peak of the roof by 12 mm (0.4 in.) cables. This sounds excessive, but snow guards did not look out of place when buried in 2 m (6.5 ft) of snow with cars parked directly below the roof.

In high snow load areas, such as the Rocky Mountains, the author recommends architects design the snow sheds safely away from any pedestrian traffic below or consider having a near flat roof to carry the full ground snow load.

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