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For generating out-of-plane wind loads on cladding and secondary structural members supporting the cladding, using the low-rise method, a combined external pressure co-efficient and gust coefficent CpCg is given in NBC Figure 4.1.7.6.-B. There are higher pressures on end zones defined in the figure. Using NBC 2010, this is Figure I-8 or the Structural Commentaries.
The figure is based on the tributary area of the load on the component. For the purpose of designing sheathing and for nail withdrawal calculations, a tributary area A is chosen to be less than 1 m, a range over which CpCg is constant. This is because the area tributary to each fastener for nail withdrawal design does not exceed 2 square feet (.2 m), and the area tributary to each line of fasteners on a panel for sheathing bending design does not exceed 8 square feet (1 sq m.).