A single droplet of rain means no true threat, and similarly, rainfall, at ordinary intensities, is rarely a cause for immobilizing concern. However, by weighing over 8 pounds per gallon and having a propensity to conglomerate due to its dipolar nature, a substantial accumulation of water can be detrimental to building roofs. For this reason, it is essential to incorporate appropriate provisions for rain loads into building and structure design.
Calculating Rain Loads in Building Construction
Section 1611 of the 2021 Edition of the International Building Code is focused on rain loads, and it stipulates that each portion of a roof be designed to sustain the load of rainwater that will accumulate on it under certain conditions. The amount of rainfall and water flow influencing these design considerations is to be based on the 100-year hourly rainfall rate of the region, as indicated in Figure 1611.1 of the code. However, the use of other rainfall rates from approved local weather data is permissible as well.
The accumulation of water caused by the deflection of the roof structure is called ponding, and, as it results in added load, it can lead to roof instability. The key to managing rain loads in buildings is roof drainage, including a primary drainage system and a secondary drainage system, which is set at an elevation higher than the primary system to drain any water that builds up after the lower system is blocked or not working.
Guidance for roof drainage systems can be found in Chapter 8 of ASCE/SEI 7-2022: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. This chapter has a general focus on rain loads, and uses the same 100-year return period for determining the design flow rate, with different frequencies for the secondary and primary drainage systems.
About ASCE/SEI 7-2022
ASCE/SEI 7-2022 is the standard for design considerations for building loads, and it provides minimum loads, hazard levels, associated criteria, and intended performance goals for buildings, other structures, and their nonstructural components. With this wide-reaching scope, the standard covers many different weather- and climate-related loads. This includes not only snow but also the extreme forces of flood and tsunami loads.
Among the many changes made to the 2022 edition of this standard, it updated design rain loads to explicitly consider a ponding head and added new commentary for low slope roofs and drainage to existing roofs. You can learn more about this standard in this post: ASCE/SEI 7-2022: Minimum Design Loads for Buildings
ASCE/SEI 7-2022: Minimum Design Loads and Associated Criteria for Buildings and Other Structures is available on the ANSI Webstore.