You are correct in your assumption that it seems wrong to have negative windward roof pressures that subtract from the net base shear (but is real because the internal pressure is larger than the exterior wind pressure on the windward roof, usually in the case of a "ballooning" partially...
A canvas roof membrane(and probably with very light framing like a tent framework) would fit into what ASCE 7-02 refers to as "buildings and other structures, flexible " in Section 6.2; that means the gust factor,Gf, will be different from the normal defaut value of 0.85 given in Section...
