What is wind load, and why does it matter?
Wind load is the lateral force exerted on a structure by moving air. Unlike gravity loads (dead load, snow load), wind load acts horizontally — pushing on windward walls, pulling with suction on leeward walls and roofs, and creating uplift on flat and low-slope roofs. These forces must be resisted by the building's structural system: shear walls, diaphragms, connections, and foundations.
In Canada, wind loads vary significantly by location. St. John's, Newfoundland has a 1-in-50 reference wind pressure of 0.95 kPa — nearly twice the 0.48 kPa in Toronto. A building designed to inland Ontario wind loads would be structurally deficient on the Newfoundland coast. The National Building Code of Canada (NBCC 2020) provides city-specific reference wind pressures to account for this variation.
NBCC 2020 wind load formula (Clause 4.1.7)
The specified wind pressure on a surface is calculated as:
- p — specified wind pressure (kPa)
- Iw — wind importance factor (0.8 to 1.25, depends on building importance category)
- q — reference velocity pressure (kPa), city-specific from NBCC 2020 Table C-3
- Ce — exposure factor (varies with height and terrain roughness category)
- Cg — gust factor (2.0 for cladding and components; dynamic factor for flexible structures)
- Cp — external pressure coefficient (+0.8 windward wall, −0.5 leeward wall, suction on roofs)
The net lateral pressure is the sum of the windward pressure and leeward suction. For a building wall, this is typically (+0.8) + (−(−0.5)) = 1.3 times the base wind pressure, meaning the windward and leeward loads are additive.
Exposure categories
The exposure category describes the terrain roughness at the building site. It affects the exposure factor Ce — how quickly wind speed builds up with height.
| Category | Description | Typical sites |
|---|---|---|
| A | Open, unobstructed | Coastlines, lakes, airports, open prairie |
| B | Suburban / open flat | Suburbs, farmland, open terrain with scattered trees |
| C | Dense urban | City centres surrounded by tall buildings |
Most residential and light commercial buildings in Canada fall under Exposure B. Coastal buildings in BC, Nova Scotia, PEI, and Newfoundland may require Exposure A — check NBCC commentary or consult a structural engineer for sites near water.
Reference wind pressures by Canadian city
The following values are from NBCC 2020 Appendix C, Table C-3. The 1-in-50 year value (q50) is used for strength design; the 1-in-10 year value (q10) for serviceability (deflection) checks.
| City | q10 (kPa) | q50 (kPa) |
|---|---|---|
| Toronto, ON | 0.39 | 0.48 |
| Ottawa, ON | 0.35 | 0.44 |
| Vancouver, BC | 0.45 | 0.55 |
| Victoria, BC | 0.5 | 0.6 |
| Calgary, AB | 0.45 | 0.55 |
| Lethbridge, AB | 0.55 | 0.68 |
| Edmonton, AB | 0.38 | 0.47 |
| Winnipeg, MB | 0.42 | 0.53 |
| Montreal, QC | 0.38 | 0.47 |
| Halifax, NS | 0.55 | 0.7 |
| Moncton, NB | 0.48 | 0.6 |
| Charlottetown, PE | 0.53 | 0.67 |
| Regina, SK | 0.47 | 0.58 |
| St. John's, NL | 0.75 | 0.95 |
| Iqaluit, NU | 0.55 | 0.7 |
Source: NBCC 2020 Appendix C, Table C-3. Values are reference velocity pressures (kPa) at 10 m height, open terrain.
Importance factor (Iw)
The importance factor scales the wind load based on the consequences of failure. Per NBCC 2020 Table 4.1.6.2:
| Category | Iw | Examples |
|---|---|---|
| Low | 0.8 | Storage buildings, farm structures |
| Normal | 1.0 | Houses, offices, retail, industrial |
| High | 1.15 | Schools, arenas, large assembly buildings |
| Post-disaster | 1.25 | Hospitals, fire/police stations, communication centres |