Essential structural considerations in roof design

Table 2 indicates tornado categories, wind speeds, and wind pressure.

Design for tornadoes is an evolving situation with extensive ongoing research. Tornadoes are different than other wind events as their highest wind speeds are near the ground, are very localized, and cause up to three times larger uplift forces compared to other types of wind events. With the high winds at the ground, windborne debris becomes a major concern.

In the U.S., ASCE-7 covers the design for tornadoes in Chapter 32. The provisions are the result of research done since 2011, and it reveals tornadoes mainly occur on the east side of the Rockies.  The tornado risk occurring is smaller than required for a regular building. Though rare, tornadoes are more common than people may believe. On average, more than 1,200 tornadoes are reported each year and the loss of life due to tornadoes is higher than hurricanes and earthquakes combined. Tornadoes need to be considered for buildings such as hospitals (risk level 3 and 4).  The first part of the chapter describes the types of buildings which do not need to be specially designed for tornado winds:

  • Structures outside tornado-prone areas can rely on the wind provisions for the area (Figure 4).
  • Low occupancy and regular buildings can rely
    on general wind provisions for the area.
  • If the tornado wind speed is below 96 km/hr
    (60 mph), ASCE-7 states can rely on the wind provisions for the area.
  • If the tornado wind speed is below a specified speed for each exposure, ASCE-7 states can rely on the wind provisions for the area.

If the building is required to be designed against tornadoes, Chapter 32 provides the basic provisions to calculate the design wind speeds. These wind speeds represent up to EF-2 tornadoes, which are 97 percent of all reported tornadoes. If there are requirements to design against the most intense tornadoes, designers are directed to “ICC 500 Storm Shelters,” or tornado performance-based design procedures are to be followed. The other wind parameters in Chapters 27, 29, and 30 can be used in the same manner as other wind loads to calculate wind loads on the structure.

In Canada, tornadoes are addressed in the “Structural Commentaries of NBC 2015.” The commentaries list three levels of risk for tornado-prone regions:

  1. “regions prone to significant tornadoes” are defined as regions where the estimated probability of occurrence of a significant tornado (F2–F5)
  2. “regions prone to tornadoes” are defined as regions where the estimated probability of occurrence of a tornado (F0–F2)
  3. “regions where tornadoes are possible” are defined as regions where tornadoes have been observed.

The key guidance from the commentary is:

  1. Details should be designed on the basis of a factored uplift wind suction of 2 kPa [41.8 psf] on the roof, a factored lateral wind pressure of 1 kPa [20.9 psf] on the windward wall, and suction 2 kPa on the leeward wall.


Hail is not considered a structural load. However, the occurrence of hailstorms raises one of the key specifiers’ criteria: Is the product durable?

In the U.S., Factory Mutual has identified the high-risk areas in Property Loss Prevention Data Sheets 1-34 (Figure 5).

In Canada, hail occurs in the same areas as thunderstorms. Figure 6 shows the location of hail activity, with Alberta being the most active area.


Designing for rain is vastly different compared to other roof loads, and for several reasons. The structural engineer is dependent on decisions made by the mechanical engineer and architect on water drainage from the roof. Water moves and follows the geometry of the roof structure. The code provisions give the designer a method to calculate the volume of water. For example, 100 mm (4 in.) of rain over a 1,000-m2 (10,764-sf) roof gives 1,000 m3 (26,400 gal) of water. The designer is responsible to work out how the water would sit on the roof, accounting for roof slopes and how much the roof structure deflects. The rain loading is dependent on the stiffness of the roof system and its components, if the decking and supporting members are not stiff enough, the water will flow to that area and overload the structure. This is called a ponding failure.

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