What it calculates
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.
Who uses this calculator
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.
How it calculates
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 each accumulation arrangement the calculator covers. Domed and curved roofs, intersecting drifts, and sawtooth or other non-standard geometries are outside its scope.
The specified roof snow load
The balanced roof snow load is calculated as S = Is[Ss(Cb·Cw·Cs·Ca·CT) + Sr], with Sr not exceeding Ss(Cb·Cw·Cs·Ca·CT). 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 ULS importance factor Is from Table 4.1.6.2.-A. The SLS importance factor is 0.6 for every category.
Roof snow load adjustment factors
Each factor is derived from geometry and site conditions. The basic roof snow load factor Cb follows the roof size. The wind exposure factor Cw is calculated from the winter average temperature and wind speed, with a minimum of 0.5. It is taken as not less than 1.0 for sheltered roofs, High and Post-disaster buildings, and the drift and unbalanced cases. The slope factor Cs comes from the roof angle and whether the roof is unobstructed and slippery. The accumulation factor Ca depends on the load arrangement being checked, and CT is the thermal factor. 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 case applies to roof angles above 15 degrees. The roof angle and the winter wind factor Fws set the downwind accumulation factor, 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 step height, the gap, and the source roof length, width and parapet height (Cl. 4.1.6.5, 4.1.6.6). Case I and Cases II and III are evaluated, with Fws entering the fetch factor, and the larger accumulation governs. No drift is required where the gap exceeds 5 m or the step is below the balanced snow depth. 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
Where the upper roof is sloped and either unobstructed and slippery or at 20 degrees or more, sliding snow is added to the drift load at each lower roof step (Cl. 4.1.6.11). You can mark sliding as blocked where a parapet or other measure prevents it. For roofs with a valley, the Case I valley load is a uniform Is(Cb·Ss + Sr). Where the valley slope exceeds 10 degrees, the further valley load cases are tabulated by zone, with zone widths set from the two surface widths b1 and b2 (Cl. 4.1.6.12).
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.
