Urban and suburban residential areas with closely spaced obstructions
Exposure B is one of four terrain roughness classifications defined in ASCE 7-22 (and ASCE 7-16) Section 26.7. It represents urban and suburban areas with numerous closely-spaced obstructions, producing the lowest wind pressures of all exposure categories.
The exposure category determination is critical because it directly affects the velocity pressure exposure coefficient (Kh or Kz), which can change design wind pressures by 30% or more compared to other exposures.
Numerous closely-spaced obstructions the size of single-family dwellings or larger characterize Exposure B terrain.
Exposure B terrain must extend ≥2,600 ft (or 20× building height) upwind, or ≥1,500 ft for buildings ≤30 ft tall.
Produces 30-33% lower wind pressures than Exposure C due to terrain roughness reducing wind speeds.
Defined in Section 26.7 of both ASCE 7-22 (latest) and ASCE 7-16 with same requirements.
Exposure B represents urban and suburban terrain with numerous closely-spaced obstructions having the size of single-family dwellings or larger. This is the most common exposure category for residential construction and urban commercial buildings.
Exposure B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.
Key Requirements:
Exposure B produces the lowest design wind pressures of all exposure categories because terrain roughness (buildings, trees, structures) significantly reduces wind speeds near the ground.
Single-family neighborhoods, townhomes, subdivisions with mature trees, residential streets with houses on 1/4 - 1 acre lots.
Downtown areas with multi-story buildings, strip malls surrounded by development, industrial parks with large structures.
Dense forests, mature tree groves, orchards with full canopy. Trees must be closely spaced (not isolated clumps).
Manufacturing facilities with large buildings, warehouses clustered together, commercial/industrial mixed use.
University campuses with multi-story buildings, hospital complexes, corporate office parks with mature landscaping.
Main street commercial districts, village centers with 2-3 story buildings, mixed-use urban neighborhoods.
| Location Type | Exposure | Example Locations |
|---|---|---|
| Suburban subdivision | B | Cary NC, Naperville IL, Plano TX typical neighborhoods |
| Urban downtown | B | Charlotte uptown, Raleigh downtown, Atlanta midtown |
| Mature residential | B | Established neighborhoods (30+ years old) with large trees |
| Industrial park (developed) | B | Warehouses, manufacturing with surrounding development |
| Rural farmhouse | C | Isolated home surrounded by crop fields - NOT Exposure B |
| New subdivision (no trees) | C | Recently cleared land - use C until mature landscaping |
To qualify for Exposure B, the terrain must meet specific upwind distance requirements. This is one of the most commonly misapplied provisions in wind load analysis.
Exposure B terrain must exist for a distance of:
Greater of: 2,600 feet OR 20× building height
Measured upwind from the building in the wind direction being considered.
| Building Height | 20× Height | 2,600 ft Minimum | Required Distance |
|---|---|---|---|
| 15 ft (residential) | 300 ft | 2,600 ft | 2,600 ft (governs) |
| 25 ft (2-story commercial) | 500 ft | 2,600 ft | 2,600 ft (governs) |
| 40 ft (3-story office) | 800 ft | 2,600 ft | 2,600 ft (governs) |
| 100 ft (8-story building) | 2,000 ft | 2,600 ft | 2,600 ft (governs) |
| 150 ft (12-story building) | 3,000 ft | 2,600 ft | 3,000 ft (20× governs) |
| 200 ft (high-rise) | 4,000 ft | 2,600 ft | 4,000 ft (20× governs) |
Key Insight: For typical low-rise buildings (≤130 ft tall), the 2,600 ft minimum governs. That's nearly half a mile of suburban terrain required upwind!
Buildings ≤ 30 ft tall with specific characteristics may use Exposure B if:
This exception is commonly applied to residential construction (houses ≤ 30 ft tall) in suburban neighborhoods.
Exposure B uses the lowest velocity pressure exposure coefficient (Kh) values, resulting in reduced design wind pressures compared to Exposures C and D.
| Height (ft) | Exposure B Kz | Exposure C Kz | Exposure D Kz | B vs C |
|---|---|---|---|---|
| 0-15 | 0.57 | 0.85 | 1.03 | -33% |
| 20 | 0.62 | 0.90 | 1.08 | -31% |
| 25 | 0.66 | 0.94 | 1.12 | -30% |
| 30 | 0.70 | 0.98 | 1.16 | -29% |
| 40 | 0.76 | 1.04 | 1.22 | -27% |
| 60 | 0.86 | 1.13 | 1.31 | -24% |
Critical Impact: At 15 ft height, Exposure B produces 33% lower wind pressures than Exposure C. This translates directly to component DP rating selection and structural member sizing.
Single-family house
Suburban neighborhood
Mean roof height: 20 ft
Risk Category II
V = 115 mph (ASCE 7-22)
Exposure B (suburban)
Kzt = 1.0, Kd = 0.85
Window - Zone 5
Effective area = 10 ft²
GCp = -1.00
Enclosed: GCpi = ±0.18
p = -20.5 psf
Required: DP-25
(vs DP-30 for Exposure C)
1. Kh for Exposure B, h=20 ft:
Kh = 0.62 (from ASCE 7 Table 26.10-1)
2. Velocity pressure qh:
qh = 0.00256 × 0.62 × 1.0 × 0.85 × 1.0 × 115²
qh = 17.4 psf
3. Wind pressure (governing case):
p = qh[(GCp) - (GCpi)]
p = 17.4[(-1.00) - (+0.18)] = -20.5 psf
4. Component selection (ASD with 0.6W):
ASD pressure = 0.6 × 20.5 = 12.3 psf
Select DP-25 (25/0.6 = 41.7 psf > 20.5 psf ✓)
Comparison - Same building in Exposure C:
Kh = 0.90 → qh = 25.3 psf → p = -29.8 psf
Would require DP-30 (vs DP-25 for Exposure B)
WRONG: "This house is in a subdivision, so it's Exposure B" (only 800 ft of suburban terrain upwind)
RIGHT: Must verify ≥2,600 ft (or 1,500 ft for h≤30 ft) of suburban terrain in ALL wind directions being considered.
Impact: Under-designed components, potential failures in windstorms
WRONG: Farmhouse surrounded by crop fields or pastures classified as Exposure B
RIGHT: Farmland is Exposure C. Isolated trees or farm structures do NOT create Exposure B.
Impact: Significant under-design (30%+ reduction in wind loads vs correct Exposure C)
WRONG: Site has 2,600 ft of suburban terrain to the west, so use Exposure B for all wind directions
RIGHT: Must check EACH wind direction. If terrain is Exposure C to the east, use C for east winds (or use C for all directions conservatively).
Impact: Non-compliance with ASCE 7, potential under-design
WRONG: New development with houses but no mature trees/landscaping → Exposure B
RIGHT: Newly cleared land is Exposure C until sufficient mature vegetation and structures exist. Must have "numerous closely-spaced obstructions."
Impact: Under-designed structures in early phases of subdivision development
Verify terrain has numerous closely-spaced obstructions (house-sized or larger)
Confirm upwind distance: ≥2,600 ft OR 20× building height (whichever is greater)
For buildings ≤30 ft tall: verify ≥1,500 ft OR 10× height of Exposure B terrain
Check ALL wind directions - each must meet Exposure B criteria (or use most critical exposure)
Document exposure determination with aerial photos or site plans showing upwind terrain
If ANY doubt exists, use Exposure C (conservative and most common default)
Re-evaluate exposure if site conditions change (new development, tree removal, etc.)
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