Key Differences Between Siphon Drainage Systems and Gravity Drainage Systems

For the same drainage capacity, a siphon drainage system requires a shallower water depth at the drain outlet than a gravity flow system. This is highly advantageous for both the architectural and structural design of the roof. Not only are the pipe diameters used in siphon systems smaller than those in gravity flow systems, but the number of pipes required is also fewer.

For the same drainage capacity, a siphon drainage system requires a shallower water depth at the drain outlet than a gravity flow system. This is highly advantageous for both the architectural and structural design of the roof. Not only are the pipe diameters used in siphon systems smaller than those in gravity flow systems, but the number of pipes required is also fewer. The system shown in the figure on the left—consisting of one horizontal pipe and one vertical pipe—can connect to more than ten rainwater drains, whereas a gravity flow system would require multiple vertical pipes.
The horizontal pipes in a siphon system can be installed horizontally, whereas in a gravity flow system, horizontal pipes must have a slope of at least 0.5%, which lowers the pipe’s outlet end and thus affects usable space or requires special structural considerations. Because siphon systems require fewer vertical pipes, they can be routed through stairwells or alongside columns without occupying additional usable space. Additionally, horizontal pipes can be routed through non-sensitive areas such as public corridors, providing favorable conditions for architectural design.