ASCE 7-22 Wind: MWFRS Design Pressure for Residential Gable Roof (Chapter 28 Envelope)

Technical NotesOpen StructSuite

Example

A Risk Category II single-family home is 40 ft × 28 ft in plan with 20 ft mean roof height, 6:12 roof slope. Basic wind speed 115 mph, Exposure B. The building is enclosed and qualifies as low-rise for Chapter 28. Determine MWFRS design pressures for roof and wall zones per ASCE 7-22 Chapter 28 Section 28.3.1 and evaluate minimum design wind loads per Section 27.1.5 and Section 28.3.6.

How StructSuite solves this

Select Chapter 28 — Envelope Procedure at the top of the wind module. Enter Risk Category II, V = 115 mph, Exposure B, K_d = 0.85, K_zt = 1.0, K_e = 1.0, enclosed, GC_pi = 0.18. Step 4: h = 20 ft, Kh from Table 26.10-1; after Step 6, Step 4 shows Roof angle = 6:12 = 26.57° (or θ = 26.6° in degrees mode). Step 5: qh. Step 6: L = 40 ft, B = 28 ft, gable roof, θ = 26.6° (6:12), Figure 28.3-1 GC_pf zones. Step 7: p = qh×K_d×[(GC_pf)−(GC_pi)] for each zone and load case; review Minimum Design Wind Loads and governing base shear comparison.

Steps

  1. Step 1: Determine risk category of building

    Design consideration: Residential Risk II. 115 mph inland; 130–150 mph coastal. q ∝ V²: 30% speed increase = 70% pressure increase. Hurricane clips, straps sized from zone pressures. Address or zip for ASCE 7 Figure 26.5-1.

    In StructSuite: In Step 1, use the Risk Category dropdown to select I, II, III, or IV (e.g., II for offices, residential). In the Basic Wind Speed (mph) input box enter 115. Per ASCE 7-22 Section 26.2, Figure 26.5-1. If using address lookup, use the address search to auto-fill wind speed.

  2. Step 4: Velocity pressure exposure coefficient

    Design consideration: Step 4: h = 20 ft drives Kh; Step 6: 40×28 ft plan, gable θ = 6:12. After θ is entered, Step 4 accordion shows Roof angle = rise:run = degrees. Zone size a = 0.1×least plan dimension per Table 28.2-1.

    In StructSuite: In Step 4, enter Mean Roof Height h (ft) and confirm Kh from Table 26.10-1 (this example uses h = 20 ft). Chapter 27 Directional lists Kz at each elevation z on the Step 4 blue summary. In Step 6, enter Building Length = 40 ft, Building Width = 28 ft, roof type, and roof angle θ (26.6° for this example). After θ is entered, the Step 4 accordion line includes Roof angle = …° or Roof angle = rise:run = …°.

  3. Step 3: Wind load parameters

    Design consideration: B typical suburban. C (open lot, new development): 25–35% higher. Enclosed: GC_pi=±0.18. Partial enclosure (garage door open): higher internal pressure. Kzt for hillside—ridge sites can double wind.

    In StructSuite: In Step 3, use the Exposure dropdown to select B, C, or D. If ground elevation > 1000 ft, use Table 26.9-1 for Ke. For topographic features (hills, ridges), use the Kzt section and enter parameters from ASCE 7-22 Figure 26.8-1. Kd = 0.85 for MWFRS is typical.

  4. Step 5: Velocity pressure at mean roof height

    Design consideration: qh is used in Equation 28.3-1. Step 7 zone uplift drives roof-to-wall connections; Minimum Design Wind Loads (16 psf / 8 psf wall and roof vertical projections) are separate cases—compare governing base shear.

    In StructSuite: In Step 5, velocity pressure qh is computed from ASCE 7-22 Equation 26.10-1. For Chapter 28 Envelope, continue to Step 6 for zone dimension a and GCpf from Figure 28.3-1; Step 7 shows zone pressures, load cases, and Minimum Design Wind Loads per Section 27.1.5 and 28.3.6 when applicable. For Chapter 27 Directional, Step 5 lists qz at each z; then Step 6–8 follow Figure 27.3-1 and Section 27.3.1.

  5. Step 6: External pressure coefficients

    Design consideration: Figure 28.3-1: gable roof GC_pf by zone and wind direction; zone geometry sets where pressure is normal to the roof and walls in Step 7, then horizontal base shear comes from summing along-wind components of those normals. Load Cases 1–2 are perpendicular and parallel to ridge; torsional Cases 3–4 if Section 28.3.2 applies unless an exception is attested. Hip vs gable: select roof type to match framing.

    In StructSuite: With Chapter 28 Envelope selected, in Step 6 enter Building Length = 40 ft, Building Width = 28 ft, Mean Roof Height = 20 ft, roof type Gable, roof angle = 26.6°. Confirm zone dimension a and external pressure coefficients (GC_pf) from Figure 28.3-1 (and 28.3-2 for torsional cases if required); these zones define where Equation 28.3-1 pressure acts normal to each surface in Step 7. Attest torsional exceptions per Section 28.3.2 when applicable.

  6. Step 7: Calculate wind pressure, p, on each building surface

    Design consideration: Equation 28.3-1 pressures are normal to each surface per Figure 28.3-1; sloped roof zones contribute horizontal base shear through resolution of those normal forces (not by duplicating a Chapter 27–style separate roof strip on the same check). Section 27.1.5 / 28.3.6: minimum MWFRS wind load uses 16 psf for wall vertical projection and 8 psf for roof vertical projection (enclosed/partially enclosed), applied simultaneously. Wind parallel to ridge on a gable uses gable end wall area (rectangle to eave plus gable triangle), not B×h alone. Open buildings use 16 psf×Af. StructSuite lists minimum cases and max(computed, minimum) where applicable. Load-case base shear is the sum of along-wind horizontal components per zone (windward F, leeward −F, parallel sidewalls 0, leeward roof −F×sin θ).

    In StructSuite: In Step 7, review design pressure p from Equation 28.3-1 for each zone and load case. Pressures are normal to the actual wall and roof surfaces per Figure 28.3-1 zones; normal forces on sloped roof areas are resolved into horizontal and vertical components so roof shear is included in the zone summation without double-counting. Base shear for each load case is the sum of along-wind horizontal components (windward wall F, leeward wall −F, sidewalls parallel to wind 0, leeward roof half −F×sin θ). The tables include comparison to Minimum Design Wind Loads (Section 27.1.5 / 28.3.6): 16 psf × wall vertical projection plus 8 psf × roof vertical projection for enclosed and partially enclosed buildings (applied simultaneously); open buildings use 16 psf × Af. Governing base shear uses the larger of computed and minimum where shown.

Live design (pre-filled)

The form below is the real StructSuite module with example data loaded. Display only—values cannot be changed.

Steps to Determine Wind Loads on MWFRS

ASCE 7-22 Tables 27.2-1, 28.2-1

User Note: Use Chapter 27 to determine wind pressures on the MWFRS of buildings with any general plan shape, building height, or roof geometry that matches the figures provided. These provisions use the traditional "all heights" method (Directional Procedure) by calculating wind pressures using specific wind pressure equations applicable to each building surface.

User Note: Use Chapter 28 to determine the wind pressure on the MWFRS of low-rise buildings that have a flat, gable, or hip roof. These provisions use the Envelope Procedure by calculating wind pressures from the specific equation applicable to each building surface. For building shapes and heights for which these provisions are applicable, this method generally yields the lowest wind pressure of all the analytical methods specified in this standard.