NBC 2025 Snow Loads and Roof Design Factors: Professional Guide for London, Ontario
Sarah's Expert Insight
Senior Consultant at empcontracting.ca Inc.
"In London, Ontario, our unique lake-effect snow patterns mean we often deal with heavy, wet accumulation that tests the limits of standard roof designs. At empcontracting.ca Inc. your construction contractor in London, Ontario, we focus heavily on the 2025 NBC updates regarding snow drifting and rain-on-snow loads to ensure our clients' roofs are resilient against the specific localized weather challenges of the Forest City."
As climate patterns shift across Canada, the National Building Code of Canada (NBC) 2025 has introduced refined standards for snow load calculations. These updates are designed to ensure that modern structures can withstand the increasing variability of winter precipitation. For residential and commercial projects, partnering with empcontracting.ca Inc. your construction contractor in London, Ontario ensures that your building is designed to meet these rigorous safety requirements.
The Specified Snow Load Calculation
The NBC 2025 calculates the specified snow load (S) based on a formula that integrates local climatic data with the physical characteristics of the roof. This ensures that the structural framing is sufficient to prevent collapse during peak winter months.
Ground Snow Load (Ss) and Rain Load (Sr)
The foundation of the calculation is the ground snow load ($S_s$), representing the 1-in-50-year snow accumulation, and the associated rain load ($S_r$). In southwestern Ontario, the interaction between heavy snow and subsequent rainfall is a critical factor, as rain trapped in the snowpack significantly increases the weight exerted on the roof structure.
The Importance of the Exposure Factor (Ce)
The exposure factor ($C_e$) accounts for whether a roof is sheltered from or exposed to the wind. A roof in a densely built urban area may accumulate more snow than one in an open field where wind can clear the surface. empcontracting.ca Inc. meticulously evaluates the surrounding terrain to apply the correct exposure coefficients.
Roof Geometry and Snow Accumulation Factors
Roof shape plays a major role in how snow is distributed. The NBC 2025 provides specific guidelines for various roof types to prevent structural imbalance.
Slope Factor (Cs) and Accumulation (Ca)
Steeper roofs (high $C_s$) allow for snow shedding, which can reduce the required structural capacity. Conversely, multi-level roofs or roofs with parapets must account for the accumulation factor ($C_a$). This factor addresses 'snow drifting,' where wind moves snow from one part of the roof to another, creating localized heavy loads that can compromise structural integrity if not properly addressed during the design phase.
Sliding Snow and Unbalanced Loads
The 2025 code emphasizes the danger of sliding snow from higher roof surfaces onto lower ones. Professional design must account for the impact force and the total weight of this accumulated surcharge to protect the safety of the occupants and the longevity of the building.