As seen in illustration 1, a traditional gravity drainage system consists of a network of roof drains connected by open outlet to a vertical downpipe. The pitch in the piping allows rainwater to flow to a discharge point. This configuration necessitates relatively large diameter stacks which connect into an even larger underground drainage network.

A traditional system is sized and pitched to be at atmospheric pressure throughout. Since pressure is constant from inlet to outlet, the only thing inducing flow is the pipe pitch. In horizontal pipe segments, illustration 2,

water cascades along the invert of the pipe. About 1/2 of the pipe cross section is used to convey water and the remaining 1/2 is air at the maximum expected rainfall rate. Conceptually, if air can be removed, you need only 1/2 of the traditional pipe size to drain the same amount of water. Since the air is not removed, it works at only a fraction of its design capacity. This reduced capacity results in low flow velocities and poor internal cleaning of debris. This type of design is inherently inefficient in the use of materials since large pipe sizes are specified to handle a rainfall event that may occur only a few times during the life span of a building.

Siphonic systems induce flow by creating a full-bore continuous path of water making pitch unnecessary, as seen in illustration 3. The full-bore flow in a siphonic roof drainage system is achieved through natural hydraulic action and is not produced by any sort of moving part, special fitting or control in the piping network. There is no need for any utilities such as electricity, compressed air, vacuum, etc.

Siphonic systems do not require any special installation kit or procedure. The pipe materials and fittings used with siphonic roof drains are the same as those required for traditional drainage systems. Siphonic roof drainage is not so much a ‘system’ in terms of a pre-engineered product or package; it is instead a technique of nopitch pipe design used to achieve desired flow from roof drains to point(s) of discharge. With a flat, level design, long horizontal runs above overhead ceilings are possible, as shown in illustration 4. This reduces or even

eliminates the need for buried pipe and the associated costs with trenching, bedding, and backfilling within the building’s footprint. Siphonic systems are designed to operate under sub-atmospheric pressure when primed full. The horizontal piping in the system can have higher velocities than the terminal velocity that can be achieved in a traditional vertical stack. This means rainwater is moved off the roof faster during the heavy but infrequent storms. During light rainfall events, that are more common, the piping still drains but in the traditional open channel flow mode. Therefore, siphonic roof drainage systems are more efficient in the use of materials since smaller pipe diameters can be specified to handle a wide range of rainfall events.