Why do ASCE wind zones differ across roof areas?

• A. The geometry and exposure of different roof areas alter wind flow and pressure distribution.
• B. Wind pressures are random and not defined.
• C. The material type is the only factor affecting wind pressure.
• D. Wind speeds are uniform across the roof.

Answer: (A)

Wind loads on a roof vary because wind flow is shaped by local geometry and exposure. The roof’s shapes—ridges, valleys, edges, overhangs, and corners—create different air flow patterns, producing zones of higher suction or different pressure on different areas. The surrounding environment, or exposure, also changes how fast the wind reaches the roof, so areas upwind or near obstructions can experience different wind speeds and pressures than more sheltered parts. All of this means pressure distribution isn’t uniform across the roof; ASCE wind design accounts for these variations to reflect how wind interacts with the specific roof geometry and its surroundings. Wind pressures aren’t random, materials don’t set the pressures themselves, and wind speeds aren’t uniform across the roof—the geometry and exposure drive the differing zones.