CanadaNBCC 2020Cl. 4.1.6
Determine the specified roof snow and rain load to NBCC 2020 (Cl. 4.1.6) from the 1-in-50-year ground snow basis. Builds the balanced load plus the unbalanced, drift, sliding, and valley cases, each with its governing NBCC clause.
This calculator determines the specified snow and rain load on roofs to NBCC 2020 (Division B, Subsection 4.1.6). The roof load S = Is[Ss(Cb·Cw·Cs·Ca) + Sr] is built up from the 1-in-50-year ground snow load (Ss) and associated rain load (Sr), adjusted by the importance factor (Is) and the basic roof (Cb), wind exposure (Cw), slope (Cs), and accumulation/shape (Ca) factors. It covers flat, sloped, and gable roofs including the unbalanced load case, and computes snow drift accumulation loads at steps down to lower roofs and adjacent to projections and parapets.
Method & scope
What it calculates
Canadian structural engineers calculating roof snow loads to NBCC 2020, the edition adopted across most provinces. Covers the 1-in-50-year ground snow basis, balanced and unbalanced cases, and drift, sliding, and valley loads at steps, projections, and parapets. Use the NBCC 2025 version once your jurisdiction adopts it. Replaces roughly 2-4 hours of manual calculation per roof to derive Canada’s NBCC 2020 snow load cases (balanced, unbalanced, drift, sliding, and valley loads).Calculation method
This calculator determines the specified snow and rain load on roofs to the National Building Code of Canada 2020 (Division B, Subsection 4.1.6). The roof load is built up from site climatic data and roof adjustment factors, then evaluated for every accumulation arrangement the code requires.The specified roof snow load
The balanced roof snow load is calculated as S = Is[Ss(Cb·Cw·Cs·Ca) + Sr]. Here Ss is the ground snow load and Sr the associated rain load, Is is the importance factor, and Cb, Cw, Cs, and Ca are the basic roof, wind exposure, slope, and accumulation factors. The result is reported as a specified (unfactored) load for use in NBCC load combinations.Ground snow, rain, and importance
The 1-in-50-year ground snow load Ss and associated rain load Sr are entered for the site from the NBCC 2020 climatic data. The building importance category sets the importance factor Is from Table 4.1.2.1, distinguishing low, normal, high, and post-disaster importance.Roof snow load adjustment factors
Each factor follows from geometry and site conditions: the basic roof snow load factor Cb, the wind exposure factor Cw derived from the site exposure condition (Cl. 4.1.6.2.(3),(4)), the slope factor Cs from the roof angle and whether the roof is unobstructed and slippery, and the accumulation factor Ca for the load arrangement being checked. The factors are shown individually so the build-up of the load is transparent.Unbalanced load on gable roofs
For gable roofs the calculator evaluates the unbalanced load case, redistributing snow from windward to leeward slopes. The eave-to-ridge distance and roof angle drive the shape factor for this arrangement, and the unbalanced result is reported next to the balanced case so the governing arrangement is identified.Snow drifts at steps, projections, and parapets
Where a roof steps down to a lower level, the drift accumulation load on the lower roof is built from the upper and lower roof geometry and the lower roof angle. Drifts adjacent to projections and parapets are evaluated separately. Each drift is expressed through an accumulation factor Ca applied to the base load, with the peak drift pressure and its extent tabulated.Sliding and valley snow
Sliding snow from an upper unobstructed slippery roof onto a lower roof is added as a separate accumulation case. For roofs with a valley, the valley snow load is computed from the valley slope and the two contributing surface widths b1 and b2 (Cl. 4.1.6.12), capturing the additional accumulation that collects in the valley. Every load case is reported as a specified load with its governing NBCC 2020 clause, so the balanced, unbalanced, drift, sliding, and valley arrangements can be compared and the controlling case carried into design.How to use it
1
Open the calculator
Open it from Run calc in the About this calculator panel above.
2
Enter your inputs
Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See Checks, References, Conditions and Assumptions.
3
Review the results and export
Check the utilization of each governing check in the summary, then export a PDF report. See Views and Export.
Common questions
What code edition does this snow load calculator use?
What code edition does this snow load calculator use?
It determines the specified snow and rain load on roofs to the National Building Code of Canada 2020, Division B, Subsection 4.1.6, using the NBCC 2020 basis of a 1-in-50-year ground snow load and associated rain load.
What are the key inputs?
What are the key inputs?
The 1-in-50-year ground snow load Ss and rain load Sr for the site, the building importance category, the site exposure condition, and roof geometry: eave-to-ridge distance, building height, plan dimensions, and roof angle. Multi-level roofs and valleys add the relevant lower-roof and valley dimensions.
What does it output?
What does it output?
It returns the specified roof snow load S = Is[Ss(Cb·Cw·Cs·Ca) + Sr] for the balanced case, the unbalanced load on gable roofs, drift accumulation loads at steps down to lower roofs and adjacent to projections and parapets, sliding snow loads, and valley snow loads, each with its governing NBCC 2020 clause.
Does it handle unbalanced and drift load cases?
Does it handle unbalanced and drift load cases?
Yes. It computes the unbalanced (gable) load case, snow drifts where a roof steps down to a lower level, drifts against projections and parapets, sliding snow onto lower roofs, and valley accumulation, evaluating each arrangement separately so the governing case is clear.
Should I use NBCC 2020 or NBCC 2025?
Should I use NBCC 2020 or NBCC 2025?
Use the edition adopted by the authority having jurisdiction for your project. NBCC 2020 uses a 1-in-50-year ground snow basis and the Cb, Cw, Cs, and Ca factors. NBCC 2025 moves to a 1-in-1000 basis and adds the thermal factor CT and winter wind factor Fws; a separate NBCC 2025 version of this calculator covers that edition.
Next steps
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