Rooftop units, screen walls, solar racks, antennas, and freestanding signs all have to be tied down for wind — and they are governed by Chapter 29 of ASCE 7-22, “Other Structures and Building Appurtenances,” not the whole-building MWFRS procedure. This article shows the velocity-pressure equation, where every factor comes from, and a full worked example for a rooftop unit.
The velocity pressure equation
Everything in the wind calculation flows from the velocity pressure qz, evaluated at the height z of the appurtenance:
with V the basic wind speed in mph. The 0.00256 constant already carries the air-density assumption, so qz comes out directly in pounds per square foot.
| Factor | What it is | Where it comes from |
|---|---|---|
| Kz | Velocity-pressure exposure coefficient | Table 26.10-1, by exposure & height |
| Kzt | Topographic factor (hills, escarpments) | §26.8; 1.0 on flat sites |
| Kd | Wind directionality factor | Table 26.6-1; 0.85 for signs/appurtenances |
| Ke | Ground elevation factor | Table 26.9-1; 1.0 is conservative |
| V | Basic wind speed (mph) | ASCE 7 Hazard Tool by Risk Category |
From pressure to force
For an appurtenance, the design wind force is:
where G is the gust-effect factor (0.85 for a rigid object per §26.11), Cf is the force coefficient from the Chapter 29 figures (it depends on the shape and aspect ratio), and Af is the projected area normal to the wind.
Worked example: rooftop unit at 30 ft
A packaged rooftop HVAC unit sits on a building in Exposure C. The unit’s top is at z = 30 ft. Basic wind speed V = 115 mph. Flat site, near sea level. The face exposed to wind is 6 ft tall × 8 ft wide.
Step 1 — pull the factors. From Table 26.10-1, Exposure C at 30 ft gives Kz = 0.98. Flat site: Kzt = 1.0. Appurtenance: Kd = 0.85. Sea level: Ke = 1.0.
Step 2 — velocity pressure.
(115² = 13,225; multiply through and you get 28.2 psf.)
Step 3 — force coefficient and gust factor. Treating the boxy unit like a solid appurtenance, take Cf ≈ 1.8 and G = 0.85. The design pressure on the face is:
Step 4 — total force and overturning. Projected area Af = 6 × 8 = 48 ft².
That 2.07 kip lateral force, applied at the unit’s centroid roughly 3 ft up, is what your curb attachment, anchors, and the roof structure below must resist — plus the overturning moment F × height. It is a surprisingly large number for a piece of equipment people assume “just sits there.”
Run it in seconds
Our free Wind Load Calculator implements this ASCE 7-22 Chapter 29 procedure — enter wind speed, exposure, height, and area, and it returns qz and the design force. It sits alongside the rest of our Structural Engineering tools, so you can carry the reaction straight into the Beam Calculator for the supporting members.
Exposure categories in one minute
- Exposure B — urban, suburban, or wooded terrain with closely spaced obstructions. Lowest velocity pressures.
- Exposure C — open terrain with scattered obstructions under 30 ft: most flat, open sites and the common default.
- Exposure D — flat, unobstructed areas and water surfaces. Highest pressures; used for coastal and shoreline sites.
Exposure sets Kz, and Kz also grows with height — an appurtenance high on a tall building sees more wind than one at grade. Always evaluate Kz at the height of the object, not the roof.
Three mistakes to avoid
- Using MWFRS pressures for a small appurtenance. Chapter 29 exists precisely because rooftop units, signs, and racks are not the main building system. Do not borrow the whole-building number.
- Skipping overturning. The lateral force acts at the centroid; multiply by the lever arm to the anchor plane and check the connection for both shear and uplift.
- Wrong directionality factor. Kd = 0.85 applies to signs and appurtenances; other structure types have different values in Table 26.6-1.
Sizing anchors for rooftop equipment or a screen wall? Get your ASCE 7-22 velocity pressure and design force instantly.
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