More intense rains seem to be occurring and with greater frequency in many parts of the United States. In 2016, the U.S. Environmental Protection Agency (EPA) reported that total annual precipitation has increased in the United States and worldwide at an average rate of 0.08 in. (2 mm) per decade since 1901, and during this same period, precipitation in the contiguous 48 states has increased at a rate of 0.17 in. (4 mm) per decade (Fig. 1).1 The EPA went on to say, “In recent years, a higher percentage of precipitation in the United States has come in the form of intense single-day events. Nationwide, nine of the top 10 years for extreme one-day precipitation events have occurred since 1990.” Furthermore, from 1910 to 2015, “the portion of the country experiencing extreme single-day precipitation events increased at a rate of about half a percentage point per decade.”2
So, have the codes kept up with the changes in the frequency and intensity of rainfalls? We all refer to the building codes, at least as a starting point, when calculating the rainwater capacity of roof drains, scuppers, gutters, downspouts, and secondary drainage systems (overflow devices). Specifically, for most jurisdictions, we refer to Fig. 1106.1 of the International Plumbing Code (IPC),3 which provides 100-year, 1-hour (60-minute) rainfalls for various regions of the United States, or IPC Appendix B, “Rainfall Rates for Various Cities.” The latter, which may or may not be adopted as part of the local building code, also provides rainfall rates in inches per hour for rain events having a duration of 1 hour and a return period of 100 years. Knowing the code sets minimum standards, it is possible for designers to exceed code and use more conservative rainfall intensity data—such as a 100-year, 15-minute or 100-year, 5-minute rainfall intensity—for the cities in which our projects are located. Nevertheless, the question remains: Have the plumbing codes we use in our design work kept pace with what is going on around us? Or, are the 100-year, 1-hour rainfall rates we are using outdated, resulting in the design of undersized rainwater conduction systems; risking the overflowing of gutters or, worse, the overloading of low-slope roofs if such accumulated water load time periods are not accounted for by the design?
This paper reviews U.S. rainfall intensity data reports and various plumbing codes from 1935 to the present. This review suggests that plumbing codes have remained relatively static, rarely contain current rainfall intensity data, and truly represent a minimum standard with regard to the design of roof drainage systems.