Essential structural considerations in roof design

Table 1 shows the ASCE-7 edition with the basic wind speed and a conversion equation between ASCE-7 to NBC.

It is accepted by the construction and design communities that areas where the basic wind speed is greater than 125 mph (201 km/hr) 1/700-year wind, ASCE-7-22, 100 mph (160 km/hr) ASCE-7-05, the area was considered a high-wind zone requiring special detailing. Converting design wind speed for use in Canada, the same criteria is used for buildings in areas where the 50-year pressure is greater than 0.55 kPa (11.5 psf); therefore, special framing needs to be considered.

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 percent (return period of 50 years) at a height of 10 m (33 ft) above ground in open terrain.

Exposure factor (Ce): the exposure factor accounts for the effect of the surrounding terrain. Rough and open terrain are the only two exposure categories considered.

In Canada, the NBC covers wind load in Article 4.1.7. The wind load equation is:

P = Iw q Ce Ct Cg Cp

A quick explanation of three of the variables:

Design pressure (q): this is calculated from a design wind (0.00256V2 [psf]). The design wind in Canada is the average wind pressure taken over 60 minutes, with a probability of exceedance of two percent (return period of 50 years), at a height of 33 ft (10 m) above ground in open terrain. When wind speeds over a 60-minute period are compared, there are likely wind gusts in that time which are
60 percent higher than the average wind speed.

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—equivalent to the use of risk categories.

Gust factor (Cg): is similar to G, Cg converts the design pressure into a wind pulse of three seconds on the structure and is a function of the sample period and the size of the area considered.

Figure 2  shows a roof section of a project in southern Alberta. The detail shows attention being paid to building science consideration. The project was constructed in an area where the design wind pressure is 0.9 kPa (18.8 psf); this pressure is the equivalent to winds from a Category 3 hurricane or an EF-2 tornado.


Tornadoes are rotating columns of air from the ground to the base of a thunderstorm and hurricanes. They cut a narrow path of destruction about 300 ft (90 m) wide, extending out as wide as 1,700 ft (520 m). The greatest wind intensity is in the central path. Wind speeds decrease rapidly away from the vortex of the tornado, but they can still be damaging.

With large tornadoes, more damage can occur in the periphery than from the central path. Field surveys performed after tornadoes have discovered common structural failures. Some of these include:

  • Bottom chord buckling of roof trusses, column base plates.
  • Loss of roof decks and wall siding due to inadequate connection to the structural supports.
  • Inadequate connection of walls to the foundation.

Tornadoes are classified based on the level of damage they cause. In the 1970s, the Fujita scale was developed and enhanced in the 2000s (Enhanced Fujita scale). Figure 3 provides a description of each scale, estimated wind speeds, and equivalent gust pressures.

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