Plastic pipe pressures in siphonic roof drainage systems

Siphonic roof drainage is a highly efficient type of drainage system that is particularly suitable for large buildings and other structures over approximately 4 m in height. Although siphonic roof drainage systems are enjoying ever-increasing popularity with architects, there is still some uncertainty regarding the minimum pipe pressure class required for siphonic pipework, especially in tall buildings. This is particularly the case in warmer countries since higher temperatures can drastically decrease the strength of the pipework material used in siphonic systems - typically unplasticized polyvinylchloride (PVC-U) and high-density polyethylene (HDPE). However, there is very limited information available on how plastic pipes behave under the sub-atmospheric pressures that occur under operating conditions in siphonic systems. This paper describes experiments conducted to investigate sub-atmospheric pressures in siphonic systems and how they may be controlled by injecting air into vertical downpipes. Recommendations for minimum pipework pressure classes are provided together with methods for limiting the minimum pressures without significantly decreasing the system capacity. This paper will help engineers design siphonic systems with more confidence so that such systems will continue to perform adequately over their intended design life.