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Siphonic drainage explained (we do the rest)

Siphonic Drainage Systems are analytically calculated pipe work systems, designed to receive and discharge a given volume of rainwater from roofs in line with the Clients R10 specification and BSEN 12056.

Siphonic rainwater outlets are designed and tested to allow rainwater to flow freely into the pipework below, whilst also building the level of rainwater around the outlet to rise and restrict the flow of air into the system.  At this point the horizontal pipe work below becomes full (primed), and the falling column of water in the down-pipe below causes suction in the system, drawing the rainwater through at a designed and controlled flow velocity.

As the system empties the water around the outlet drops, allowing air to enter the system breaking the siphonic action, as the rain continues the process starts again and the cycle continues until such time as the rainfall intensity decreases.

As with traditional gravity systems there are no moving parts or pumps. The terminology "traditional" or "gravity" and "siphonic systems" can be misunderstood as it’s the head of water and gravity drop within the vertical stack which creates the suction in a siphonic system, (siphonic systems are created by gravity and the controlled flow of air and water).


A  balanced siphonic system is designed, activated and controlled by the siphonic outlet, analytical design software ( Link to the outlet performance) pipe work system and material.

The pipe work material and jointing system again needs to meet a required level of performance due to the velocity and negative pressures produced and be able to withstand the forces and contain the rainwater at all times.

As siphonic systems flow at high velocity under suction they offer a great benefit over traditional gravity systems...

  • less outlets,
  • less roof penetrations,
  • smaller pipe diameters,
  • horizontal pipe runs (no drop required)

..all leading to a more efficient installation and removing the need for awkwardly positioned internal gravity down pipes, vast reduction in networks of underground drainage systems required  to collect  gravity drops

Siphonic systems summary : efficient and green, reducing carbon footprint, less work, less materials, easy co-ordination, all leading to less costs   

For a further understanding, to discuss a concept design, tender or live contract, contact our technical support 

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