ASCE 7-22 · SECTION 26.7 · EXPOSURE SELECTION
Pick the right exposure category for every wind direction
Read the upwind Surface Roughness, classify it B, C, or D, then apply it per wind direction. When in doubt, the more open exposure governs.
SECTION 26.7.2 → 26.7.3
The selection workflow
Exposure is set for each wind direction from the upwind Surface Roughness over a sector. Walk the four steps.
The more open of the applicable exposures may govern — never assume the sheltered case by default.
SECTION 26.7.2 · SURFACE ROUGHNESS
Three roughness categories
Each terrain type carries its own minimum height (zmin) and velocity-pressure coefficient. Values shown are Kz at 30 ft from ASCE 7-22 Table 26.10-1.
Roughness B
Wooded, suburban, and urban terrain with numerous closely-spaced obstructions.
zmin = 30 ft Kz @ 30 ft = 0.70Roughness C
Open terrain with scattered obstructions generally less than 30 ft tall — the default.
zmin = 15 ft Kz @ 30 ft = 0.98Roughness D
Water surfaces, mud flats, salt flats, and unobstructed open areas exposed to wind flow.
zmin = 7 ft Kz @ 30 ft = 1.16WHY THE CHOICE MATTERS
One step that swings the whole load
At 30 ft, the velocity-pressure coefficient climbs steeply from B to C to D. Misclassify the terrain and every downstream pressure is wrong.
Call B where C truly governs and you under-design the structure by 40% in Kz alone — before the gust, directionality, and pressure factors compound it.
FIELD-TESTED PITFALLS
Common selection mistakes
The errors that most often turn an exposure call non-conservative.
Assuming suburban = B
Roughness B needs numerous closely-spaced obstructions upwind. Edge-of-town and new developments often classify as C.
26.7.2Ignoring direction
Exposure is set per wind direction from the upwind sector. A coastal site can be D from the water and C from land.
26.7.3Skipping the more-open case
When more than one exposure applies across the sector, the more open exposure may govern — do not default to the sheltered one.
26.7.3Forgetting zmin
Below zmin (B 30 ft, C 15 ft, D 7 ft) the coefficient is held at its floor value — not extrapolated lower.
Table 26.10-1GO DEEPER
Per-category guides
Detailed pages for each roughness category and the transition zones between them.
Exposure B
Suburban and urban terrain — the lowest pressures.
SUBURBAN / URBANExposure C
Open terrain — the default exposure for most sites.
OPEN TERRAINExposure D
Coastal and open-water terrain — the highest pressures.
COASTAL / OCEANTransition Zones
Where exposure changes along the upwind fetch.
WHERE IT CHANGESPUT IT TO WORK
Exposure built into every wind load calculation
WindLoadCalc.com applies the correct Kz and Kh coefficients for your selected exposure category, ASCE 7-22 compliant.