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# Calculate Snow Loads (NBCC 2025)
> Get balanced, unbalanced, drift, and sliding roof snow loads to NBCC 2025 Cl. 4.1.6, including the new thermal factor CT and winter wind factor Fws.
Determine the specified roof snow and rain load to NBCC 2025 (Cl. 4.1.6) from the 1-in-1000 ground snow basis. Builds the balanced load and the unbalanced, drift, sliding, and valley cases, each reported with its governing NBCC clause, including the new thermal factor CT and winter wind factor Fws.
This calculator determines the specified snow and rain load on roofs to NBCC 2025 (Division B, Subsection 4.1.6). The roof load S = Is\[Ss(Cb·Cw·Cs·Ca·CT) + Sr] is built up from the 1-in-1000 annual probability 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), accumulation/shape (Ca), and thermal (CT) 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
| Property | Detail |
| ------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Design standards | NBCC 2025, Cl. 4.1.6 |
| Regions | Canada |
| Who it's for | Canadian structural engineers adopting NBCC 2025 roof snow load requirements (new thermal factor CT, winter wind factor Fws, 1-in-1000 ground snow load basis) without rebuilding their NBCC 2020 workflow. Available now to Core Calcs users (Beta). |
| What it helps with | Replaces roughly 2-4 hours of hand calculation per roof to derive the NBCC 2025 adjustment factors (including the new CT and Fws), build the balanced, unbalanced, drift, sliding, and valley load cases, and tabulate drift pressures at every roof step, parapet, and projection. |
| What it checks | The specified roof snow load, Ground snow, rain, and importance, Roof snow load adjustment factors, Unbalanced load on gable roofs, Snow drifts at steps, projections, and parapets, Sliding and valley snow |
#### What it calculates
Canadian structural engineers moving to NBCC 2025 for roof snow loads, when the new thermal factor CT, winter wind factor Fws, and 1-in-1000 ground snow basis change the numbers carried over from NBCC 2020. Balanced, unbalanced, drift, sliding, and valley cases run together. Use the NBCC 2020 version for projects still on that edition.
Replaces roughly 2-4 hours of hand calculation per roof to derive the NBCC 2025 adjustment factors (including the new CT and Fws), build the balanced, unbalanced, drift, sliding, and valley load cases, and tabulate drift pressures at every roof step, parapet, and projection.
#### Calculation method
This calculator determines the specified snow and rain load on roofs to the National Building Code of Canada 2025 (Division B, Subsection 4.1.6). The roof load is built up from site climatic data and a set of 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·CT) + Sr]. Here Ss is the ground snow load and Sr the associated rain load, Is is the importance factor, and Cb, Cw, Cs, Ca, and CT are the basic roof, wind exposure, slope, accumulation, and thermal factors. The result is reported as a specified (unfactored) load for use in NBCC load combinations.
##### Ground snow, rain, and importance
The 1-in-1000 annual probability ground snow load Ss and associated rain load Sr are entered for the site from the NBCC 2025 climatic data (Appendix C). NBCC 2025 also introduces the winter average temperature and winter average wind speed inputs (Fws), which feed the wind exposure treatment. The building importance category sets the importance factor Is from Table 4.1.2.1.
##### Roof snow load adjustment factors
Each factor is derived from geometry and site conditions: the basic roof snow load factor Cb, the wind exposure factor Cw (adjusted for sheltered or significantly obstructed roofs), the slope factor Cs from the roof angle and whether the roof is unobstructed and slippery, the accumulation factor Ca for the load arrangement being checked, and the thermal factor CT. CT is the NBCC 2025 addition: where the average indoor operating temperature and overall roof thermal transmittance U are known, CT is computed from them per Cl. 4.1.6.2.(10),(11); otherwise a conservative default applies.
##### Unbalanced load on gable roofs
For gable roofs the calculator evaluates the unbalanced load case, in which snow is redistributed from windward to leeward slopes. The eave-to-ridge distance and roof angle drive the shape factor used for this arrangement, and the unbalanced case is reported alongside 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 calculator builds the drift accumulation load on the lower roof from the upper and lower roof geometry and the lower roof angle (Cl. 4.1.6.11). Drifts adjacent to projections and parapets are evaluated separately. Each drift is expressed through an accumulation factor Ca applied to the base load, and the peak drift pressure and its extent are 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 extra accumulation that collects in the valley.
Every load case is reported as a specified load with its governing NBCC 2025 clause, so the balanced, unbalanced, drift, sliding, and valley arrangements can be compared and the controlling case carried into design.
## How to use it
Open it from **Run calc** in the About this calculator panel above.
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](/docs/running-calculations/checking-results/checks-references-conditions-assumptions).
Check the utilization of each governing check in the summary, then export a PDF report. See [Views and Export](/docs/running-calculations/exporting-reports/views-and-export).
## Common questions
It determines the specified snow and rain load on roofs to the National Building Code of Canada 2025, Division B, Subsection 4.1.6. It applies the NBCC 2025 basis of a 1-in-1000 annual probability ground snow load, together with the new thermal factor CT and winter wind factor Fws introduced in that edition.
The 1-in-1000 annual probability ground snow load Ss and associated rain load Sr for the site, the building importance category, roof geometry (eave-to-ridge distance, height, plan dimensions, and slope), the wind exposure condition, and, where known, the indoor operating temperature and roof thermal transmittance used to establish CT.
It returns the specified roof snow load S = Is\[Ss(Cb·Cw·Cs·Ca·CT) + 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 2025 clause.
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. Each accumulation case is evaluated separately so the governing arrangement is clear.
NBCC 2025 moves the ground snow load to a 1-in-1000 annual probability basis and adds the thermal factor CT and winter wind factor Fws to the roof snow load build-up. If your jurisdiction has not yet adopted NBCC 2025, use the NBCC 2020 version of this calculator instead.
## Next steps
Complete list of Calcs.com design calculators available for Canadian projects (CSA O86, CSA S16, NBCC, and related standards).
Get balanced, unbalanced, drift, and sliding roof snow loads to NBCC 2020 Cl. 4.1.6 from the 1-in-50-year ground snow basis, with each code clause shown.
Pull the reference wind pressure and climatic data for any Canadian site from the NBCC 2020 tables (Appendix C) as the basis for design wind loads.
Determine design base shear to NBCC 2020 (Art. 4.1.8) with the Equivalent Static Force Procedure, and distribute forces story by story.
Design simple and continuous wood beams, joists and girders to CSA O86:19 with unlimited supports and loads. Checks moment, shear and deflection.
