Essential structural considerations in roof design

Codes and standards

In Canada, building codes are a municipal responsibility. Most municipalities do not want to have their own code, so they rely on the provincial government product and maintain a provincial building code. Most of the provinces and territories use the NBC as a basis for their building codes. Having different provincial building codes has led to different editions of the NBC being used. Presently, the following building codes are in force:

  • Federal jurisdictions: NBC 2020
  • Territories: NBC 2020
  • British Columbia: The BC Building Code 2018 using provisions from NBC 2015
  • Alberta: NBC (Alberta Edition) 2019
  • Ontario: Ontario Building Code (OBC) using provisions from NBC 2015
  • Quebec: Quebec Construction Code: Chapter I: Buildings using provisions from NBC 2015
  • Manitoba: NBC 2010 with modifications
  • Maritime Provinces: NBC 2015 with modifications

Fortunately, the structural provisions of the NBC 2020 are very similar to NBC 2010. In the U.S., building codes are also a municipal responsibility. Most municipalities do not want to have their own code, so they rely on the state government to maintain building codes. Most of the states use the International Building Code (IBC) as a basis for their state building codes. The IBC has adopted the American Society of Civil Engineering’s Standard, ASCE-7, Minimum Design Loads for Buildings and Other Structures.

However, each state having its own building code led to situations similar to Canada’s, where different states used different editions of the IBC.

Figure 1 Gust Front. All illustrations of this page courtesy


  • Four states have adopted IBC 2021, ASCE-7 -16
  • 26 states have adopted IBC 2018, ASCE-7 -16
  • 16 states have adopted IBC 2015, ASCE-7 -10
  • Four states have adopted IBC 2012 or older, ASCE-7 -10
    or older

The use of various editions of IBC presents a challenge. The provisions of the most recent edition of ASCE-7 -22 have been adopted for the next edition of the IBC.

ASCE-7 -22 has made significant changes to how rain and snow loads are handled. ASCE-7 -10 changed the specified wind speed from a 1/50-year wind speed to an ultimate wind speed for each category of building importance.

Figure 1 Tornado.

While these editions are not accepted in all locations, this article will discuss the provisions from the NBC 2020 and ASCE-7, Minimum Design Loads for Buildings and Other Structures.


Several types of winds are behind the wind loads used in Canada. Synoptic winds (winds associated with large-scale events, such as warm and cold fronts) make up everyday weather. In four provinces (Alberta, Saskatchewan, Manitoba, and Southern Ontario), short-term storm events cause the major wind events. Short-term storms start as thunderstorms, but they can cause microbursts which can turn into tornadoes or hailstorms. In Alberta, on the east side of the Rocky Mountain Range, there are areas where downslope winds (known as Chinooks) contribute to extreme wind events. A comparable situation occurs on the maritime coasts. The U.S. (and rarely the Atlantic maritime provinces) have tropical storms, hurricanes, and/or typhoons.

Figure 1 Microbust.

The wind speeds of all these weather events have been recorded by Environment Canada. As mentioned earlier, this department of the government has supplied design values in NBC.

Code provisions

The wind load provisions in Canada and the U.S. are different, but they also have some provisions that are similar. In Canada, NBC covers wind load in Article 4.1.7. The wind load equation is:

P = Iw q Ce Ct Cg Cp

This article will focus on four of the variables:

Figure 1 Hail Storms

Importance factor (Iw): accounts for whether the structure is low occupancy, regular use, or high importance. For example, the structure could be used as a post-disaster shelter or a critical structure such as a hospital.

Design pressure (q): this is calculated from a design wind. The design wind in Canada is the average wind pressure taken over 60 minutes with a probability of exceedance of two per cent (return period of 50 years) at a height of 10 m (33 ft) above ground in open terrain.

Gust factor (Cg): design pressure is a function of the sample period and the size of the area considered. The pressure increases with a shorter sampling period and the smaller the sampling area. However, the gust factor converts the design wind pressure to a pressure taken over a sample for a time of three seconds. The simplified gust factor for a whole building is 2.0 and it is increased to 2.5 for smaller areas (cladding).

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