2024 Ontario Building Code: Snow Load Requirements for Horizontal Roof Gaps
Sarah's Expert Insight
OBC Compliance Specialist
"In snow-heavy regions like London and Muskoka, the 5-meter gap rule is a frequent point of oversight in commercial expansions. At empcontracting.ca Inc. your construction contractor in London, Ontario, we emphasize that even if buildings don't touch, the aerodynamic 'shadow' of a taller neighbor can double the expected snow weight, requiring reinforced joists or modified drainage for the lower roof."
Understanding OBC Section 4.1.6.6: Horizontal Gaps and Snow Accumulation
In the 2024 Building Code Compendium, specifically Division B – Part 4, the regulation of snow loads on lower-level roofs is critical for structural safety. When a building sits adjacent to a higher structure, the horizontal gap between them dictates how snow accumulation factors (Ca) are calculated. As empcontracting.ca Inc. your construction contractor in London, Ontario, we prioritize these engineering standards to ensure every build is resilient against the Canadian climate.
Snow Load Cases and Parameters (Table 4.1.6.5.-B)
The code identifies three primary cases for snow load evaluation on lower roofs, determined by the source area dimensions and parapet heights. Key parameters include:
- Case I: Baseline accumulation with a factor of 1.0.
- Case II & III: Reduced factors (0.67) applied to specific source areas on the lower roof for upwind and downwind-facing steps.
- Source Area Dimensions: Calculations involve the shorter (ws) and longer (ls) dimensions of the upper roof or source area to determine the accumulation length (lcs).
The 5-Meter Rule for Adjacent Buildings
Article 4.1.6.6. defines the influence of an adjacent higher roof based on the horizontal distance 'a':
- If a > 5 m: The influence of the adjacent higher roof can be ignored, and the lower roof is treated as an independent structure regarding snow drift.
- If a ≤ 5 m: The accumulation factor (Ca) must be calculated in accordance with Article 4.1.6.5. for all areas where x ≥ a. This ensures that wind-blown snow from the higher roof is accounted for on the lower structure.
Disclaimer: This content is generated from the Ontario Building Code for educational purposes. Always consult with a licensed professional engineer or your local building department before starting any construction project in London, Ontario.