Essential structural considerations in roof design

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.

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 percent (with a return period of 50 years)

Ss ground snow load: based on weather data and has a probability of exceedance of two percent (with a 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 0.7, 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.”

Cold roofs, as mentioned earlier, have been addressed by modifying Ct in ASCE-7-22.

For those people considering an energy retrofit or meeting the new energy requirements, an R-30 insulated roof is well-insulated and a ventilated roof is considered as unheated.

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.

Occupancy loads

Roofs not only have to resist environmental loads but also loads from human use of the roof area. Most of the occupancies are ones which architects are used to dealing with. The one loading that is different, is an allowance for doing maintenance work on the roof. Both U.S. and Canadian codes have requirements for a minimum roof live load for maintenance work on the roofing system to prevent collapse.

The minimum live load requirements for both countries are similar.

The U.S. has had minimum load provisions since 1943. The provisions, in their present form, first appeared in the Uniform Building Code (UBC) in 1949. They allow the minimum load to be reduced based on the tributary area the roof member is supporting and on the roof’s slope. The requirements are as follows:

  • 20 psf (0.96 kPa ) reducing to 12 psf (0.58 kPa), depending on a combination of tributary area and roof slopes.
  • Greenhouses, and farm accessory buildings
    10 psf (0.48 kPa).

In 1988, a lower value for the minimum roof load was allowed if repair work is done from the ground and approved by the local authority.

Prior to 1985, there was confusion in Canada about the purpose of the minimum roof live load. The load allowance was considered by many as a minimum snow load. Since 1985, the NBC clarified the minimum roof live requirements as an area load of 20.9 psf (1.0 kPa) and a point load of 290 lb (1.3 kN).

The structural commentaries, since 1985, explain these load allowances are for use and occupancy loads, not a minimum snow load intended to provide for maintenance loadings, workers, and so forth, and the load allowances are not reduced as a function of area or roof slope.

Dead loads

With roofs, the contractor needs to know what weights are always on the roof structure and what allowances are provided for items that may or may not be there. There are many terms for describing these dead loads that might be there. This author prefers the term “collateral dead load.”

The load allowances for mechanical ductwork and architectural bulkheads are examples of collateral dead loads. The reason for identifying these is when:

  • checking for uplift due to wind load on the roof structure, the structural engineer should only use the dead load


  • checking for loads which are pushing down on the roof structure, the structural engineer should use the dead load and collateral dead load.

The locations of mechanical units, ductwork, and architectural bulkheads should be reviewed with the structural engineer. Often, these items are handled by delegated design and the structural engineer is never made aware of them. This is especially true for architectural bulkheads and ceilings.

Leave a Comment


Your email address will not be published. Required fields are marked *