Snow Loads (NBCC 2020)

How it calculates

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.