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Wind loads on roof-parallel (“flush-mounted“) solar PV systems

It is tempting to calculate wind loads on flush-mounted solar PV systems simply by applying Figures 30.4-2A through 30.4-2C of ASCE 7-10 which give external pressure coefficients for gable and hip roofs. However, simple approaches are often conservative. As wind tunnel studies conducted in the large I.F.I. boundary layer wind tunnel have shown, reductions in the range of 30% to 50% of ASCE 7-10 code values may be easily achieved.

The reason is that in the wind tunnel the favorable effect of pressure equalization between upper and lower module sides is modeled, whereas code values simply represent external pressure coefficients.