Wind exposure categories B / C / D
Exposure describes the surface roughness of the terrain upwind of a building. Rougher ground slows wind near grade; smooth water exposes it fully. Smoother terrain means higher design pressures.
The terrain sets the load
Exposure adjusts wind pressure through the velocity-pressure exposure coefficient Kh (Kz). As roughness drops from B to C to D, wind speed near grade climbs — and so does design pressure.
Exposure B
Urban & suburban, wooded areas — numerous closely spaced obstructions. Lowest pressures.
Exposure C · default
Open terrain with scattered obstructions under 30 ft — farmland, grassland, light development. Baseline.
Exposure D
Flat, unobstructed areas & water surfaces — open water, mud flats, salt flats. Highest pressures.
B / C / D side by side
Definition, upwind fetch, and the boundary-layer parameters that drive each profile.
| Characteristic | Exposure B | Exposure C | Exposure D |
|---|---|---|---|
| Definition | Urban/suburban, wooded — numerous closely spaced obstructions | Open terrain, scattered obstructions < 30 ft | Flat, unobstructed areas & water surfaces |
| Typical terrain | Dense suburbs, forests, downtown | Farmland, grassland, light industrial | Open water, mud/salt flats, beaches |
| Min. upwind fetch | 2,600 ft or 20h (1,500 ft if h ≤ 30 ft) | N/A — default | 5,000 ft / 20h over water, within 600 ft / 60h of shore |
| Kh at 30 ft | 0.70 | 0.98 | 1.16 |
| Relative pressure | Lowest (−29% vs C) | Baseline | Highest (+18% vs C) |
| α (power-law) | 7.5 | 9.8 | 11.5 |
| zg gradient height | 3,280 ft (1,000 m) | 2,460 ft (750 m) | 1,935 ft (590 m) |
| Selection difficulty | Moderate — verify fetch | Easy — default | Moderate — coastal proximity |
Kh climbs with height — and converges
The exposure gap is widest near grade where surface friction dominates. As height rises, the profiles climb toward the wind aloft and the three categories converge.
| Height (ft) | B | C | D | D vs B |
|---|---|---|---|---|
| 0–15 | 0.57 | 0.85 | 1.03 | +81% |
| 25 | 0.66 | 0.94 | 1.12 | +70% |
| 30 | 0.70 | 0.98 | 1.16 | +66% |
| 50 | 0.79 | 1.09 | 1.27 | +61% |
| 60 | 0.83 | 1.13 | 1.31 | +58% |
| 80 | 0.90 | 1.21 | 1.38 | +53% |
| 100 | 0.95 | 1.26 | 1.43 | +51% |
Same building, three exposures
25 ft residential building · Wilmington, NC · V = 140 mph · Risk Category II · qh = 0.00256 × Kh × V²
Kh = 0.66 · baseline
Kh = 0.94 · +42% vs B
Kh = 1.12 · +70% vs B
A 70% swing in velocity pressure drives member sizing, component ratings, and build cost — from a single terrain call.
How to pick the exposure
Evaluate upwind terrain for each wind direction. Test for D first, then B; if neither qualifies, default to C.
Test Exposure D
Within 600 ft or 60h of shore (greater) AND open water ≥ 5,000 ft or 20h upwind. Both true → D.
Test Exposure B
Numerous closely spaced obstructions extending ≥ 2,600 ft or 20h upwind (1,500 ft if h ≤ 30 ft). True → B.
Default to C
Neither B nor D satisfied → use C. Covers farmland, grasslands, light development, and short-fetch transition zones.
Evaluate every wind direction
Section 26.7.3 requires checking each of the 8 compass directions. One site can carry different exposures from different directions — design software computes each and selects the controlling values per surface.
Four exposure mistakes to avoid
Exposure errors are among the most frequent in wind-load work — and they almost always under-design.
All homes are B
B needs ≥ 2,600 ft of suburban fetch. A house on a subdivision edge bordering farmland is likely C.
One exposure, all directions
Mixed sites are common — a coastal lot can be D from the ocean but C or B inland.
Ignoring fetch
A few nearby trees and houses do not make B. The 2,600 ft / 20h upwind fetch must be met.
Misreading D distance
For tall buildings, 60h extends D far past 600 ft — an 80 ft building reaches D out to 4,800 ft.
When unsure, go conservative
Exposure resources
Per-category detail, the selection workflow, and where terrain changes.
Skip the Kh tables. Run the numbers.
WindLoadCalc.com automates directional exposure analysis — per-direction B/C/D, full velocity-pressure coefficients, and signed-and-sealed reports with built-in ASCE 7-22 compliance.