Essential structural considerations in roof design

Figure 9 Areas where rain loads can exceed snow loads. Illustration courtesy Dr. F.M. Bartlett

In his presentation of the new NBC: “Companion Loads, Wind/Snow Loading,” Dr. F.M. Bartlett discussed areas in Canada where the rain load can exceed roof snow loads. These areas are susceptible to “ponding” (Figure 9).

The NBC does not have many provisions for rain loading; they are extremely broad and general and require a lot of work to satisfy them. Section 4.1.6 states:

“The Rain load, S, due to the accumulation of rainwater on a surface whose position, shape, and deflection under load make such an accumulation possible, is that resulting from the one-day rainfall whether the surface is provided with a means of drainage. Where scuppers are provided as secondary drainage systems and where the position, shape, and deflection of the loaded surface make an accumulation of rainwater possible, the loads due to rain shall be the lesser of either the one-day rainfall or a depth of rainwater equal to 30 mm [1.18 in.] above the bottom of the scuppers.”

The NBC structural commentaries go on to advise designers:

“It is considered good practice when locating roof drains to take into account not only the roof slope but also deflection of the roof due snow and rain. Drains should be provided with suitable devices to prevent clogging by leaves or, where appropriate, suitable overflows should be provided through parapet walls. There is potential for the primary drainage system for a roof to become blocked due to freeze-thaw conditions. Roofs should be designed accordingly.”

In the U.S., the rain loads are covered in ASCE-7-22, Chapter 8.

Rain loads are calculated based on obtaining an equilibrium of the rainstorm rainfall and how fast the water can drain off the roof. There is a requirement for the roofs to have two drainage systems so if the primary drains are blocked, the secondary system will drain the water. The equation for rain loads used is:

R = 5.2 (ds + dh + dp)

  • ds, static head—depth of water due to the elevation difference between the primary and secondary drains.
  • dh, hydraulic head—depth of water, assuming a flow rate corresponding to a rainfall intensity for a 15-minute duration storm of a specified return period and the drain outflow.
  • dp—depth of water due to deflections of the roof subjected to rain and self-weight.

The 2022 edition of the ASCE-7 differs from previous editions by requiring that deflections due to ponding be calculated. In past editions, the standard rules were provided of what minimum stiffness and geometry should be avoided (similar to what is in the 2015 edition of the NBC).


Snow is similar to water in that it moves and follows the geometry of the roof structure. High parapets, roof valleys, snow guards, changes in roof elevations, privacy screens, mechanical units, and ducting can all cause snow accumulations and increase loading. In northern Canada (north of the tree line in exposed areas especially with constant winds), designers have experienced snow drifts up to 9 m (29 ft) in height. Care must be paid to the shape of a building to reduce snow accumulations. Problems occur when snow accumulates in an area not anticipated by the structural engineer.

Calculation of the weight of snow is also a challenge. Snow properties change over time, including the snow’s density. When snow first falls, its density is exceptionally light 1 kN/m3, within 24 hours the density doubles and over time it can increase to 4 kN/m.3 If only the depth of the snow has been measured, the change in density with time makes estimating the snow load difficult.

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