Essential structural considerations in roof design

Calculation of snow loads on roofs is relatively new. The 1953 edition of the NBC, first dealt with design snow loads. The roof loads were equal to the ground snow load, with reductions allowed for sloped roofs only. The load values were approximate and resulted in over-design for some roofs and under-design for others, particularly in areas subject to high-drift loads.

Between 1957 and 1968, The National Research Council of Canada (NRC) undertook a countrywide survey of snow loads on roofs. This survey provided evidence on the relationship between ground and roof loads and enabled the committees responsible for the 1960 edition of the NBC to adjust the code requirements.

In 1960, roof loads were set at 80 per cent of the ground load, and they were adjusted to allow for the increase in the load caused by rainwater absorbed by the snow. At the time, NRC researchers recognized, if a roof was cold and in a sheltered area, the snow load remained at 100 per cent of the ground snow.

However, this possible variation (now Cb) was not documented in the structural commentaries or the NBC. By 1965, all roof loads were directly related to the snow load on the ground. The basic design load remained at 80 per cent of the ground load, but a snow load of 60 per cent of the ground load (Cw) was allowed for roofs exposed to the wind.

Snow accumulations were accounted by means of snow load coefficients or accumulation factors, and these were shown in the form of simple formulas and diagrams, similar to those still used in 2015.

A new slope reduction formula was given for unobstructed slippery-sloped roofs in 1990. The minimum Cw was reduced to 0.5—rather than 0.75—for exposed roofs north of the tree line. Design roof snow loads were separated into snow Ss and rain Sr components. In 2005, the return period for snow loads was increased from 30 years to 50 years. This change harmonized the Canadian and American standard. The increase in loading on large-area roofs (which was previously captured in the accumulation factor Ca) was taken out of Ca and incorporated into the basic snow load factor, Cb.

Presently, in NBC 2022, snow load provisions are found in Article 4.1.7. The equation for snow load is:

S = Sr + Cw Cb Ca Cs Ss

  • Sr rain load: for the winter months, a probability of exceedance of two per cent (return period of 50 years).
  • Ss ground snow load: based on weather data and has a probability of exceedance of two per cent (return period of 50 years).
  • Cb ground to roof factor: set at 0.80
  • Cao accumulation factor
  • Cw wind factor: 1.0 ≥ Cw ≥ 0.5
  • Cs sliding factor: 1.0 ≥ Cs ≥ 0.0

Two of the variables are affected by architected details.

Many engineers believe the value of Cb remains constant at 0.8, no matter what roofing system or amount of roof insulation is used. When this value was first considered, the researchers involved debated whether Cb should be one or less. As Dr. D.A. Taylor reported in his paper, “Roof Loads in Canada,”

“The design load coefficient for a uniformly on a well-insulated or unheated roof in a perfectly sheltered location Cb = 1.0.”

An R30 insulated roof is well-insulated and a ventilated roof is unheated. This is demonstrated in the ponding case study presented in this article.

For the sliding factor Cs to be considered, the snow needs a path to slide off. If snow is prevented from sliding off due to snow guards, parapets, or another structure, the roof must take the full snow load.

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