Please clarify your question as we haven't read the book, and neither have you by the sounds of it.

There are load case A and B at different roof angle in the Figure 6-18A of ASCE-7, 05, I could not figure out what is case A or B. I design a structure to protect equipments ( 480V transformer & switchger) from raining and snowing. The steel deck roof is 15' -0" width x 51'-0" long, its slope is 1/2" to 1 foot. I think it is an open structure ,since there is no walls. How to calculate the wind load on it ? I am not sure I shoud use the Figure 6-18A of ASCE-7, 05 to get the Net Pressure Corfficient. Thank you.