ASCE 7-22 · SECTION 26.7 · EXPOSURE CATEGORIES
Exposure B — suburban, urban & wooded terrain
Numerous closely-spaced obstructions slow the wind near grade. Surface Roughness B yields the lowest velocity-pressure coefficients of any category — and the lowest design pressures.
SURFACE ROUGHNESS B
How the terrain drags wind down
Houses, trees and structures pack the boundary layer with friction. Wind speed near the ground is suppressed and only recovers high above the rooftops.
TABLE 26.10-1 · VELOCITY PRESSURE COEFFICIENT
Kₘ for Exposure B, height by height
Below the 30 ft minimum height (zₘᵢₙ), Exposure B holds a constant Kₘ = 0.70. Values rise with height as the building reaches faster air. C and D are shown at the same heights for scale.
| Height z (ft) | Exposure B Kₘ | Exposure C Kₘ | Exposure D Kₘ |
|---|---|---|---|
| ≤ 30 (zₘᵢₙ floor) | 0.70 | 0.98 | 1.16 |
| 40 | 0.74 | 1.04 | 1.22 |
| 50 | 0.79 | 1.09 | 1.27 |
| 60 | 0.83 | 1.13 | 1.31 |
| 70 | 0.86 | 1.17 | 1.34 |
| 80 | 0.90 | 1.21 | 1.38 |
| 100 | 0.95 | 1.26 | 1.43 |
| 120 | 1.00 | 1.31 | 1.48 |
| 160 | 1.08 | 1.39 | 1.55 |
| 200 | 1.14 | 1.44 | 1.61 |
Book Table 26.10-1 lists Kₘ = 0.57 for B at 15 ft and below; the zₘᵢₙ = 30 ft rule overrides it, so 0.70 is applied for all heights below 30 ft.
TERRAIN CONSTANTS · TABLE 26.11-1
The three numbers that define Exposure B
zₘᵢₙ = 30 ft
Minimum height. Kₘ is frozen at its 30 ft value below this elevation.
FLOOR HEIGHTα = 7.5
Power-law roughness exponent. The lowest of B/C/D, giving the steepest near-ground velocity gradient.
ROUGHNESSzₒ = 3,280 ft
Gradient height — where terrain friction fades and wind reaches free-stream speed.
GRADIENTSECTION 26.7 · QUALIFYING TERRAIN
When Exposure B applies
Surface Roughness B must prevail upwind for the greater of 2,600 ft or 20× building height — or 1,500 ft / 10× height for buildings ≤ 30 ft (Section 26.7.3). Check every wind direction.
Suburban residential
Single-family neighborhoods and subdivisions with mature trees on quarter- to one-acre lots.
EXPOSURE BUrban commercial
Downtown blocks of multi-story buildings and dense strip-mall development.
EXPOSURE BWooded areas
Dense forest and full-canopy groves — closely spaced, not isolated clumps of trees.
EXPOSURE BIndustrial zones
Clustered warehouses and manufacturing plants surrounded by built development.
EXPOSURE BInstitutional campuses
Universities, hospital complexes and office parks with multi-story buildings and mature landscaping.
EXPOSURE BTown centers
Main-street commercial districts and mixed-use neighborhoods of two- to three-story buildings.
EXPOSURE BDoes not apply to isolated rural farmhouses in open fields or new subdivisions on freshly cleared land — both are Exposure C until numerous obstructions exist.
SAME HEIGHT · DIFFERENT TERRAIN
At 30 ft, B carries the lightest load
Holding height fixed at 30 ft, the velocity-pressure coefficient climbs sharply as terrain opens up — driving design pressures with it.
Exposure B — 0.70
Suburban / urban / wooded. The baseline.
Kₘ 0.70Exposure C — 0.98
Open terrain. +40% over B at 30 ft.
Kₘ 0.98Exposure D — 1.16
Coastal / open water. +66% over B at 30 ft.
Kₘ 1.16Worked example — suburban window, h = 20 ft, V = 115 mph (ASCE 7-22): with Kₘ = 0.70 (zₘᵢₙ floor), Kₖₜ = 1.0 and Kₔ = 0.85, qₕ works out near 17 psf. For an enclosed building GCₚᵢ = ±0.18; with C&C GCₚ = −1.00 the governing pressure is roughly −20.5 psf — about a DP-25 window. The identical building in Exposure C (Kₘ = 0.85 below 15 ft / 0.90 at 20 ft) jumps to a DP-30.
QUALITY CONTROL
Four ways Exposure B gets misapplied
Too little upwind terrain
A subdivision with only ~800 ft of roughness upwind does not qualify. You need ≥ 2,600 ft (or 1,500 ft for h ≤ 30 ft) in the direction analyzed.
UNDER-DESIGNRural buildings as B
A farmhouse in open fields is Exposure C. Isolated trees or a lone barn do not create Roughness B.
WRONG CATEGORYAssuming all directions qualify
B to the west doesn't mean B to the east. Evaluate each wind direction, or take the most critical exposure for all.
NON-COMPLIANTCleared-land subdivisions
New houses with no mature trees are Exposure C until numerous closely-spaced obstructions develop.
UNDER-DESIGNRule of thumb: if any doubt remains, use Exposure C. It is conservative, the most common default, and re-evaluation is required whenever site conditions change.
KEEP READING · EXPOSURE CATEGORIES
Related exposure topics
Exposure Selection Guide
How to determine the correct exposure category for a site.
GUIDEExposure C — Open Terrain
Grassland and the most common default exposure.
CATEGORY CExposure D — Coastal
Open water and the highest wind pressures of the three.
CATEGORY DTransition Zones
Where exposure categories change across a site.
TERRAINExposure Categories Overview
Compare all ASCE 7 exposure categories side by side.
OVERVIEWASD Wind Loads
Allowable Stress Design methodology for wind pressures.
METHODRELATED · WIND SPEED RESOURCES
Look up the wind speed too
ASCE 7-22 Wind Speed Maps
Updated MRI-based maps and changes from ASCE 7-16.
MAPSWind Speed by Location
Find design wind speeds for a specific project site.
LOOKUPWind Speed by Zip Code
Quick wind-speed lookup by zip code.
TOOL3-Second Gust Wind Speed
The 3-second gust parameter in ASCE 7 explained.
PARAMETERBasic Wind Speed Explained
A full guide to the basic wind speed parameter (V).
CONCEPTASCE 7 Wind Load Calculator
Professional wind-pressure calculation software.
CALCULATORASCE 7-22 COMPLIANT · ALL EXPOSURES
Exposure B wind loads, solved automatically
WindLoadCalc.com applies the correct Kₘ for suburban terrain, all exposure categories and component DP ratings — with instant results and latest ASCE 7 (7-22) wind-speed lookup.