Per ASCE 7-10, buildings are composed of 5 different zones, depending on the wind loading they are subjected to. These zones are defined as follows:
Zone 1: Has the lowest load; this zone accounts for approximately 80% of the roof surface, represented in the interior zones of the roof.
Zone 2: Higher loading than Zone 1; this zone accounts for approximately 15% of the roof surface, represented in the perimeter of the roof.
Zone 2H: Used for roof overhang area along the perimeter of Zone 2.
Zone 3: Has the highest load; this zone accounts for about 5% of the roof surface, represented the corners of the roof.
Zone 3H: Used for roof overhang area along the perimeter of Zone 3.
Zone 4: Any areas between the wall corners that are not included within Zone 5.
Zone 5: 10 percent of least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4 percent of least horizontal dimension or 3 ft (0.9 m).
Key Terms and Definitions:
Mean Roof Height (h)
The interpretation of zones should be left to a licensed professional engineer when in doubt. Otherwise, the most critical zone is suggested.
An illustration of where zones are applied comes from ASCE 7 table 30.7-2 for enclosed buildings less than or equal to h = 160′; whereas, each zone is equal to ‘a’ unless noted otherwise:
Below is a video that helps illustrate how the zone 5 vortexes form and how to better understand the theory of the zone 5 effect. Keep in mind when watching that the wind has to come from an opposing direction (all structures are analyzed with wind approaching from all angles), and create the turbulent effects in the video. Areas protected from the turbulent effect are generally not wind zone 5.
Additionally per AAMA TIR A-15-14, zones 4 & 5 can be further explained as:
170 Degrees: Unobstructed exterior corner is considered Zone 5 if angle < 170 degrees open as shown