According to a report by the National Climate Assessment, the frequency of "very heavy" precipitation has been on the rise for nearly 60 years. These events can be accompanied by high-speed, localized winds. The National Climate Assessment defines very heavy precipitation as a two-day precipitation total that is exceeded on average only once in a five-year period, also known as a once-in-five-year event. Since 1958, such heavy rain events have risen 71 percent in the Northeast, 37 percent in the Midwest and 27 percent in the South. In fact, since 1991, the amount of rain falling in very heavy precipitation events has been significantly above average, and increases in the frequency and intensity of such events are projected for all U.S. regions.
Such storms often result in the overloading of storm water drainage infrastructure and localized flooding around drains that cannot handle the amount of incoming water. However, the effects that microburst storms can have on roofs and roof drainage systems often are overlooked. Significant rainfall has led to significant damage of some buildings' plumbing systems and contributed to roof system failures. Roofs that incorporate roof drains should be inspected at least twice per year as well as after every major storm. However, for most companies, it seems such maintenance inspections are few and far between. Poor maintenance and blockages in drains and strainer domes have contributed to roof system failures and even roof collapses because of excessive amounts of water backing up on the roof.
It's in the code
The 2015 International Plumbing Code® includes performance-based criteria for roof drains. In prior code editions, drain selection exclusively was based on the pipe diameter of a building's plumbing system. However, a study conducted in 2012 by the American Society of Plumbing Engineers Research Foundation, with help from the International Association of Plumbing and Mechanical Officials, found roof drains of like sizes from different manufacturers (and even from the same manufacturers) did not necessarily perform at similar or even expected levels. In fact, 4-inch diameter roof drains tested were found to have flow rates, measured in GPM, or gallons per minute, anywhere from 180 to 754 GPM at a 4-inch head of water, which is quite a wide range.