Snow Loads (NBCC 2025)

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 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.