Wind loads are calculated per ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), referenced by the IBC and adopted (with local amendments) by essentially every building code jurisdiction in the United States. The calculation converts a basic wind speed from the ASCE 7 wind speed maps into design pressures on walls, roofs, and components.
This guide covers the directional procedure for the main wind force resisting system (MWFRS) of enclosed and partially enclosed buildings. The procedure involves several lookup factors that account for terrain, topography, height, exposure, building geometry, and enclosure classification.
Velocity Pressure: From Wind Speed to Force
The fundamental equation that converts wind speed to pressure is:
qz = 0.00256 × Kz × Kzt × Kd × Ke × V²
This equation comes from Bernoulli's principle (dynamic pressure = ½ρV²) with air density at standard conditions, unit conversions, and the various modification factors built in.
- V = basic wind speed (mph) from ASCE 7-22 wind speed maps, which vary by Risk Category. The maps give 3-second gust speeds at 33 feet above ground in Exposure C terrain.
- Kz = velocity pressure exposure coefficient (ASCE 7-22 Table 26.10-1). Varies with height above ground and Exposure Category. At ground level in Exposure B (suburban), Kz ≈ 0.57. At 30 feet, Kz ≈ 0.70. At 100 feet, Kz ≈ 0.90.
- Kzt = topographic factor. 1.0 for flat terrain. Increased for buildings on hills, ridges, or escarpments where wind speeds up due to terrain acceleration.
- Kd = wind directionality factor. 0.85 for buildings. Accounts for the low probability that peak wind comes from the worst-case direction.
- Ke = ground elevation factor. Adjusts for air density variation with altitude. 1.0 at sea level, decreasing at higher elevations.
Wind Load Calculator (ASCE 7)
Design wind pressures for low-rise buildings per ASCE 7-22. Velocity pressure, MWFRS wall and roof pressures, exposure coefficients, and internal pressure for enclosed/partially enclosed buildings.
Exposure Categories: The Most Argued Variable
Exposure category has a massive effect on design wind pressure. The difference between Exposure B and Exposure C at the same height can be 40% or more in velocity pressure. Exposure categories per ASCE 7-22 Section 26.7:
- Exposure B: Urban and suburban areas with closely spaced obstructions (buildings, trees) having the size of single-family dwellings or larger. The surface roughness must prevail in the upwind direction for at least 2,600 feet or 20 times the building height, whichever is greater.
- Exposure C: Open terrain with scattered obstructions generally less than 30 feet in height. Includes flat, unobstructed areas and grasslands. This is the default when site conditions do not clearly qualify for B or D.
- Exposure D: Flat, unobstructed areas adjacent to large bodies of water (oceans, Great Lakes). Requires at least 5,000 feet of open water upwind.
In practice, most buildings in suburban neighborhoods qualify for Exposure B, and most buildings in open agricultural or industrial areas are Exposure C. Buildings near the coast may transition from B to D depending on the wind direction being analyzed.
When in doubt, use Exposure C. It is conservative, universally accepted, and avoids the arguments about whether there are "enough" obstructions upwind to qualify for B.
Wind Load Calculator (ASCE 7)
Design wind pressures for low-rise buildings per ASCE 7-22. Velocity pressure, MWFRS wall and roof pressures, exposure coefficients, and internal pressure for enclosed/partially enclosed buildings.
Design Pressures: External, Internal, and Net
The net design pressure on any surface is the combination of external pressure and internal pressure:
p = q × (GCp) − qi × (GCpi)
External pressure (GCp): Varies by surface location. Windward walls experience positive pressure (wind pushing in). Leeward walls, side walls, and most roof surfaces experience negative pressure (suction pulling outward). The external pressure coefficients come from ASCE 7-22 Figure 27.3-1 for MWFRS and depend on building geometry and roof slope.
Internal pressure (GCpi): ±0.18 for enclosed buildings, ±0.55 for partially enclosed buildings. The ± sign means you must check both positive and negative internal pressure and use whichever creates the worse effect on each surface. For windward walls, positive internal pressure reduces the net outward push; negative internal pressure increases it. For roof uplift, positive internal pressure adds to the suction.
The internal pressure coefficient is arguably the most important single variable after wind speed. A building classified as "partially enclosed" (due to a large opening like a garage door on the windward wall) sees internal pressures three times higher than an enclosed building. This tripling of internal pressure can double the net uplift on the roof.
Wind Load Calculator (ASCE 7)
Design wind pressures for low-rise buildings per ASCE 7-22. Velocity pressure, MWFRS wall and roof pressures, exposure coefficients, and internal pressure for enclosed/partially enclosed buildings.
Components and Cladding: Higher Pressures, Smaller Areas
The MWFRS pressures discussed above are averaged over large tributary areas for the overall structural frame. Individual components and cladding (C&C), windows, doors, siding, roof panels, fasteners, experience higher local pressures, especially at edges, corners, and ridge lines where wind flow separates and creates concentrated suction.
ASCE 7-22 divides the building envelope into zones with different C&C pressure coefficients. Corner zones and edge strips experience pressures 1.5–3 times higher than the interior field of the wall or roof. The pressure also varies with effective wind area: smaller areas (individual fasteners, small panels) see higher peak pressures than larger areas (whole wall panels).
This is why you see roof damage starting at corners and edges during windstorms. The fastening pattern for roof sheathing at corners and edges must be tighter (6" o.c. nailing vs. 12" o.c. in the field) to resist these concentrated pressures.
Wind Load Calculator (ASCE 7)
Design wind pressures for low-rise buildings per ASCE 7-22. Velocity pressure, MWFRS wall and roof pressures, exposure coefficients, and internal pressure for enclosed/partially enclosed buildings.
Wind Load Calculator (ASCE 7)
Design wind pressures for low-rise buildings per ASCE 7-22. Velocity pressure, MWFRS wall and roof pressures, exposure coefficients, and internal pressure for enclosed/partially enclosed buildings.