Storm drainage design gone wrong
ASPE Research Foundation paper reveals roof drain inconsistencies.
At the recent 2012 ASPE Biennial Convention in Charlotte, N.C., the ASPE Research Foundation presented its paper on storm drainage testing. The study included the performance testing of 60 roof drains. The paper was an eye-opener for any plumbing engineer when it comes to the design of a roof drainage system.
For the last 80 years, roof drainage systems have been designed based on the capacity of the storm drainage piping. No consideration has been given to the amount of rainwater that flows through a roof drain. Furthermore, no consideration has been given to the amount of ponding required to achieve the required flow through a roof drain.
The 60 roof drains tested were provided by 10 manufacturers. The manufacturers are not identified in the report. The testing of the roof drains was done through three different piping arrangements. The first test had the roof drain open and not connected to any piping. The second arrangement featured a straight section of pipe connected to the roof drain and directed into the reservoir. The third was an offset piping arrangement where a short section of vertical pipe was connected to a horizontal offset pitched ¼-in. to the foot, then turned downward with another vertical section of pipe discharging into the reservoir.
The Research Foundation thought the roof drain without any piping would provide the control values to use for comparison to the other two piping arrangements. The assumption was the system without piping would flow the most water. This turned out not to be true. The flow rate through the roof drain without any piping was the lowest value. There were a few exceptions to this. The flow through the roof drain connected to the straight vertical pipe almost always produced the highest flow rates. With a few drains, the values for the straight pipe were 3 1/2 times greater than the flow rate through the roof drain without any piping.
For most of the tests, the flow rate through the roof drain connected to the horizontal offset was lower than the flow rate through the roof drain with a straight vertical pipe. However, in almost every test the flow rate through the roof drain with the offset piping arrangement exceeded the flow rate through the roof drain without any piping. The Research Foundation considered the test results through a roof drain without piping to be somewhat meaningless since it did not provide any control values, and the typical installation is a roof drain with connected piping.
One of the noticeable differences between the manufacturers was the flow rate through their roof drains. There was no consistency in flow rates. Sometimes the flow rate through one manufacturer’s roof drain was three or more times the flow rate through another manufacturer’s roof drain of the same size with the same amount of water ponding.
Examining flow rates
For certain sizes, some manufacturers provided multiple models to be tested. There was no consistency in the flow rates through these roof drains. The Research Foundation identified three roof drains from one manufacturer that were all 3 in. diameter. With 6 in. of water ponding around the roof drain, the first model flowed at 195 gpm with the offset piping arrangement. The second model flowed 408 gpm and the third model flowed 358 gpm. The difference between model one and two was more than double the flow rate.
One of the most startling factors identified in the report is how quickly the roof drain can reach a point where the flow rate exceeds the capacity of allowable flow for the storm drainage piping. Roof drains that were 2 in. and 3 in. diameter exceeded the pipe capacity of the same size with 1 in. of water ponding around the roof drain. For a 4 in. roof drain, it could occur with 2 in. of ponding, and for 6 in. roof drains it could occur at 3 in. of ponding.
Roof drains 2 in. through 6 in. diameter allow more flow through the drain than the design of the piping system. This means rather than having open channel flow in the storm drainage system, there could be pressurized flow during a heavy rainfall occurrence. This also accounts for the number of failures in storm drainage systems that were located remotely from the roof.
As pointed out in the Research Foundation report, the design of a roof drainage system does not include one important factor, the flow rate through the roof drain. A roof drainage system is based on the rainfall rate, the area of the roof, the area of the side walls projecting onto the roof and the maximum flow rate through the pipe. If you analyze this sizing method, the assumption is the rainwater gradually flows to the roof drain at a uniform rate. There is no analysis of the slope of the roof and the amount of water that may be ponding at the roof drain.
At the end of the report, there is a recommended sizing method for a roof drainage system. That recommended method includes analyzing the roof configuration to determine ponding at the roof drain during a design storm. The pipe sizing is based on the flow rate through the roof drain with the maximum anticipated ponding.
Based on the reported test results, it would appear future roof drainage systems may have smaller roof drains and large storm drainage piping. The recommended sizing method will require more engineering than is currently applied to a storm drainage system. The sizing method also requires manufacturers to report the flow rates through their roof drains. This will make it difficult if not impossible to interchange roof drains once the specifications and calculations are submitted.
The Research Foundation report will be made available in electronic format to ASPE members online at www.aspe.org or through the ASPE store. The report is available in both electronic and print format. Members and nonmembers can download the report at no charge at http://aspe.org/sites/default/files/webfm/pdfs/rf_report_roof_drain.pdf.
This is one of the most important reports issued on storm drainage design. Every engineer, architect and roofing contractor should obtain this 121-page report, read it and heed the recommendations.