Essential structural considerations in roof design

The snow load provisions in ASCE-7 are similar to Canada’s provisions, but with some significant differences. Snow loads are dealt with in Chapter 7 and the snow loads calculated from the following equation:

pf = Cb Ce Ct pg

ps = Cs pf

= Cs Cb Ce Ct Is pg


Pg = Ground snow load based on a reliability analysis

Ce = Exposure factor

Cb = Ground to roof factor set at 0.70

Ct = Thermal factor

Cs = Sliding factor 1.0 ≥ Cs ≥ 0.0

The differences are the ground snow load pg and the basic roof factor Cb. In Canada, it is a combination of the basic roof factor and the thermal factor. In the 2022 edition of ASCE-7, the thermal factor was increased by 20 per cent in value to reflect the effects of increased roof insulation or vented roof and increased 30 per cent where a complete building was used as a refrigerator.

In 2020, a major project was launched to generate ground snow loads across the country to reflect a more consistent level of safety matching what is required by ASCE-7. Achieving a more consistent level of safety was done following a “reliability approach.” This method could only be done at this time because of the amount of computing capacity required to do this. The work was done in the following manner:

The snow load was compared to the resistance of a steel roof.

  • The steel resistance probability distribution was used based on research by the American Institute of Steel Construction (AISC).
  • The snow load probability distribution was calculated based on a ground to roof conversion probability distribution.

○ The ground snow load probability distribution based on annual maximum snow load records across the country.

○ Snow depth to load conversion equations for different parts of the country at locations where only snow depth was measured.

Using all these distributions, Monte Carlo simulations were done. Snow loads that would cause failures were found at each site for low-occupancy buildings, normal occupancy buildings, and high importance only for an acceptable number of times.

The new snow maps have improved the prediction of ground snow loads across the U.S. The project results improved the prediction of snow loads across the country. The new ground snow load maps have less areas requiring special studies to establish ground snow loads and the new maps have dealt with areas where local codes used different loads than in ASCE-7.

It must be noted, for the ground-to-roof conversion, the data from the snow study performed by the NRCC between 1957 and 1968 was used.

Occupancy loads

Roofs not only have to resist environmental loads but also loads from human use of the roof area. Most of the occupancies are ones which architects are used to dealing with. The one loading that is different, is an allowance for doing maintenance work on the roof. Both Canadian and U.S. codes have requirements for a minimum roof live load for maintenance work on the roofing system to prevent collapse.

The minimum live load requirements for both countries are similar. Prior to 1985, there was confusion about the purpose of the minimum roof live load. The load allowance was considered by many as a minimum snow load. Since 1985, the NBC  clarified the minimum roof live requirements in Article 4.1.5, which directs the designer to Table for an area load of 1.0 kPa (20.9 psf) and Table for a point 1.3 kN (290 lb).

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