NBC 2025 Alert: The New "Uniform Risk" Snow and Wind Load Factors Are Here

By the Engineering Editorial Team

If you are still applying a 1.5 load factor to your snow calculations in 2026, your designs may no longer be code-compliant—and worse, they might be legally indefensible.

The release of the 2025 National Building Code of Canada (NBC) has introduced a fundamental shift in how structural engineers must calculate environmental loads. The transition from a "uniform hazard" to a "uniform risk" model represents one of the most significant changes to Part 4 and Part 9 structural design in decades. With provinces now moving to harmonize their local codes with these national standards, the window for transition is closing.

This update is not merely a refinement of data; it is a complete overhaul of the safety philosophy underlying Canadian structural engineering. Here is what you need to know to protect your practice and ensure your designs meet the new standard of care.

The "What": 1-in-1000 Snow and 1-in-500 Wind

According to the Canadian Board for Harmonized Construction Codes (CBHCC) and the National Research Council of Canada (NRC), the 2025 edition of the NBC has revised the specified wind and snow loads to reflect a "uniform risk" approach.

Previously, the code used a "uniform hazard" approach, where loads were based on a 1-in-50 year return period regardless of the location's specific climate volatility. The new code calibrates these loads to achieve a consistent probability of failure (target reliability index) across the country.

This update fundamentally alters the input parameters for structural calculations:

• Snow Loads: Now based on a 1-in-1000 year return period (0.001 annual probability of exceedance), a significant increase from the previous 1-in-50 year baseline.
• Wind Loads: Now based on a 1-in-500 year return period (0.002 annual probability of exceedance), up from the previous 1-in-50 year baseline.
• Load Factors: To offset these higher specified baseline values, the load factors for wind and snow in the Ultimate Limit States (ULS) load combinations have been reduced to 1.0 (down from 1.5 for snow and 1.4 for wind).

Revisions to the Snow Load Formula

Beyond the base probabilities, the calculation methodology itself has evolved. The NBC 2025 revises the snow load formula to better account for thermodynamic and aerodynamic realities:

• Wind Exposure Factor ($C_w$) and Accumulation Factor ($C_a$): These have been adjusted to account for differences in winter average temperature ($T_{ws}$) and winter average wind speed ($V_{ws}$) across Canada.
• Thermal Factor ($C_T$): A new factor introduced to explicitly account for the reduction in snow depth due to heat loss through the roof, replacing previous implicit assumptions.

The "So What": Why This Matters for Your Liability

The shift to uniform risk is a change in the philosophy of safety. By calibrating loads to a target reliability index rather than a simple return period, the code aims to provide a consistent level of safety across different geographical regions and hazard types. However, this creates a dangerous transition period for engineering firms.

For structural engineers, the implications of mixing methodologies are severe:

1. The Over-Design Trap: If you use the new 1-in-1000 year snow loads but retain the old 1.5 load factor hard-coded in your spreadsheets, you will result in massively over-designed structures. While safe, this renders your designs economically uncompetitive and wasteful.
2. The Under-Design Catastrophe: Conversely, if a designer mistakenly applies the new 1.0 load factor to old 1-in-50 year climate data, the resulting structure will be grossly under-designed, with a safety margin far below the code minimum. In the event of a failure, this would likely constitute professional negligence.
3. Climate Data Integration: The NBC 2025 Table C-2 now incorporates forward-looking climate data. For the first time, design values are adjusted for locations where climate change is projected to intensify weather conditions (Global Warming Level of 2.5°C). Ignoring these forward-looking values in favor of purely historical data is no longer an option.

Part 9 Alignment: Housing and Small Buildings

This shift is not limited to complex Part 4 structures. The changes have been harmonized into Part 9 (Housing and Small Buildings) to ensure consistent safety levels.

• Lateral Bracing & Trusses: The new wind and snow loads impact conventional construction requirements. Manufacturers of roof trusses and pre-engineered wood products must update their software to reflect the 1/1000 snow and 1/500 wind parameters.
• Radon Mitigation: Alongside structural changes, Part 9 now requires a passive vertical radon stack in dwelling units where a floor separates conditioned space from the ground, reflecting a broader push for health and safety.

The "Now What": Your Action Plan

To mitigate risk and ensure compliance, engineering teams should take the following steps immediately:

• Audit Calculation Models: Review all in-house design spreadsheets and software settings. Ensure that where the new 1-in-1000 year snow or 1-in-500 year wind loads are used, the corresponding load factor is set to 1.0.
• Verify Climate Data Sources: Stop using static historical weather data from previous code editions. Ensure your inputs are drawn from the NBC 2025 Table C-2 or the ClimateData.ca Future Building Design Value Summaries, which align with the new code provisions.
• Update General Notes: Revise your standard drawing notes to explicitly state that design loads are based on NBC 2025 uniform risk parameters (0.001 annual probability snow, 0.002 annual probability wind).
• Educate Junior Staff: Ensure that engineers-in-training understand why the load factors have changed. They must recognize that a factor of 1.0 does not imply a lack of safety margin, but rather a shift in where that margin is applied (in the specified load rather than the factor).

Bridging the Gap

Transitioning to the uniform risk model requires more than just reading the code; it requires a practical update to your engineering judgment and design processes. As provinces adopt these changes over the coming months, the "standard of care" will shift instantly. Engineers who fail to adapt their workflows risk being left behind—or held liable for ignoring the new reality of climate risk.