Alternative low-cost overflows for siphonic roof drainage systems: Proof of concept

• SRDS can drain entire roof areas using a single downpipe.
• Expensive, secondary overflow systems generally required with SRDS.
• Alternative low-cost secondary overflow techniques were explored in this study.
• Increasing downpipe length increased flowrates through the outlets.
• Lower-cost, alternative overflow systems found to be feasible.

This paper describes an experimental investigation of alternative overflow systems for roof structures that are drained siphonically. Many buildings that have siphonic roof drainage systems currently incorporate a separate secondary overflow system, which is quite often also siphonic. An alternative low-cost technique that is explored in this study is to connect each overflow outlet to a single, vertical downpipe. Seven different overflow configurations, each with five different downpipe lengths, are investigated in terms of maximum flowrate and corresponding water depth in the gutter. The results of this study are significant in that they do not support the common theoretical assumption that there is a limiting length of a siphonic downpipe over which pipe-full flow may occur before gravity causes the water to accelerate enough for it to no longer occupy the full cross-sectional area. Instead, this study has found that once a downpipe is flowing full, it is possible for pipe-full flow conditions to be maintained over the entire pipe length.