Methodology · NBCC 2020 Clause 4.1.7

How the Wind Load Calculator Works

Step-by-step walkthrough of the NBCC 2020 wind pressure formula used in this calculator.

Use the calculator first: Wind Load Calculator — NBCC 2020 →
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Step 1 — Select city and return period to get reference velocity pressure (q)

NBCC 2020 Appendix C, Table C-3 lists reference velocity pressures for Canadian cities at two return periods: 1-in-10 year (q10, for serviceability/deflection checks) and 1-in-50 year (q50, for strength design). For a house in Toronto: q50 = 0.48 kPa. These pressures are measured at 10 m height over open terrain and do not account for exposure or gust effects — those are handled by later factors.

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Step 2 — Determine the exposure factor (Ce) for building height and terrain roughness

Ce accounts for how wind speed varies with height and terrain roughness. NBCC 2020 Clause 4.1.7.1(5) defines three exposure categories: A (open/coastal), B (suburban/flat), and C (dense urban). For Exposure B at 7 m height, Ce ≈ 1.0. At 20 m, Ce ≈ 1.27. Coastal buildings (Exposure A) reach Ce = 1.0 at lower heights; dense urban (Exposure C) buildings see reduced Ce at low levels due to turbulence.

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Step 3 — Apply gust factor (Cg = 2.0) for cladding and components

The gust factor Cg accounts for dynamic amplification — gusts are shorter than the averaging period used to measure q. For cladding and components of most buildings, NBCC 2020 Clause 4.1.7.1(2) specifies Cg = 2.0. For dynamically sensitive structures (slender towers, long-span bridges, suspension roofs), a dynamic analysis is required per Supplementary Technical Information, but that is outside the scope of this calculator.

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Step 4 — Select external pressure coefficient (Cp) for the surface

Cp represents the fraction of dynamic wind pressure applied to a given surface. NBCC 2020 Table 4.1.7.5 provides values by surface: windward wall Cp = +0.8 (pressure, pushing in), leeward wall Cp = −0.5 (suction, pulling out), flat roof Cp = −0.7 (suction). The positive windward pressure and negative leeward suction are additive for lateral force — the net lateral pressure coefficient for walls is 0.8 + 0.5 = 1.3.

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Step 5 — Calculate specified wind pressure and total lateral force

Apply the NBCC formula: p = Iw × q × Ce × Cg × Cp. For a Toronto house (Iw=1.0, q50=0.48, Ce=1.0, Cg=2.0, Cp=0.8): p_windward = 1.0 × 0.48 × 1.0 × 2.0 × 0.8 = 0.77 kPa. Leeward: 1.0 × 0.48 × 1.0 × 2.0 × (−0.5) = −0.48 kPa. Net lateral pressure = 0.77 + 0.48 = 1.25 kPa. For a 10 m wide × 3 m tall wall face, total lateral force = 0.77 × 10 × 3 = 23.1 kN.

Code references

Note: This calculator uses the simplified analytical method from NBCC 2020 Part 4. It is suitable for preliminary design of regular, enclosed buildings. Complex building geometries, dynamically sensitive structures, or buildings in unusual terrain require a site-specific analysis by a licensed structural engineer.