Syphonic Explained

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In principle, syphonic drainage is a very simple process. Unlike traditional roof drainage, which is designed to flow part full, a syphonic system operates at full capacity, when water is sucked or syphoned from the roof down into the drain at high velocity. The Fullflow system allows architects, consultants and contractors alike to specify a lower number of roof outlet drains and have them flow into a single downpipe. Whereas a conventional outlet is simply a hole set into the lowest point on the roof, into which the water pours, the syphonic drain incorporates an anti-vortex plate that acts as a baffle, allowing only water to be drawn of the roof. During heavy rainfall the outlet drain fills to above the anti-vortex plate, cutting off air flow into the pipe. This lack of air, coupled with the downward pull of the water creates a vacuum. The drainage pipes then flow at 100% full over the entire system. The priming of a Fullflow syphonic system takes place in four typical stages: Gravity Flow, Plug Flow, Bubble Flow and Full Bore. Further reading regarding flow patterns.

Several outlet drains can be connected to a single collector pipe (see diagrams below). This means that the pipe can be laid without fall directly under the roof covering, with numerous outlet drains discharging into it. The collector pipe is routed to a single downpipe. When the pipes fill, the water in the downpipe wants to fall. This action causes the water in the horizontal collector pipe to be pulled through the downpipe to replace the water flowing out. The water pressure in the system falls below atmospheric pressure and water on the roof is sucked into the connected drains. On reaching the ground, the water travels into a vented manhole or inspection chamber where it is discharged at atmospheric pressure into the storm sewer.


Benefits of a Fullflow Syphonic Rainwater Drainage System:

  • Syphonic systems require fewer outlets and downpipes than a Gravity equivalent
  • Collection mains can be routed horizontally throughout the building, no need for pipes to be fitted on a gradient
  • Multiple rainwater outlets can be connected to a single connector
  • Up to 80% fewer downpipes are required, resulting in cost savings in materials and reducing associated groundwork. Also gives architects more freedom with design
  • Pipe diameters are smaller due to full volume discharge
  • Complete control over downpipe discharge location gives increased design and programme flexibility
  • Rainwater can be easily routed to collection tanks for future recycling, e.g. irrigation, fire ponds, sanitation etc.
  • The rainwater pipes are designed to run 100% full of water at high velocity from roof level to ground level, vastly increasing the capacity of the system when compared to traditional methods of roof drainage
  • Acceleration of construction programme due to reduced installation period
  • Designs can be varied to cater for a range of specifier requirements (for example protection levels & pipe specification)
  • The system is technically sophisticated with each individual building requiring its own specially-designed system based on well-established hydraulic engineering principles
  • Syphonic systems are self-cleansing because of the high flow rates, thereby minimising maintenance costs
  • Downpipes are generally located inside buildings which provides visual enhancement in the majority of situations

What to specify?

Please see our Specification information page.

Further Reading:

The document ‘Syphonic Drainage Explained’ provides further detailed information, covering the following questions:

  • What are roof drainage systems?
  • What are self-priming syphonic roof drainage systems?
    • Syphonic action: Flow patterns
    • Syphonic outlets: The key element
  • How are self-priming syphonic systems designed?
    • Syphonic systems design criteria: Basic fluid dynamics
    • Syphonic system specification: Roof, building and drainage interfaces.
  • What is required to ensure that self-priming syphonic roof drainage systems are effectively interfaced with buildings?
    • Design and construction implications of Fullflow syphonic systems.

PDF Download the PDF brochure ‘Syphonic Drainage Explained’ or alternatively request a hard copy.

PDF Download a bibliography of further reading and additional information

Request a CPD seminar on ‘Understanding Syphonic Drainage’


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 Galvanised Mild Steel Outlet


 PVC Coated Aluminium Outlet


Download Videos & Animations:

Syphonic Introductory Video

Download a video explaining Syphonic drainage (57.1mb, Quicktime MOV)
(Get Quicktime for Windows or MacOS)


Clamp Ring Outlet Gutter Assembly

Clamp Ring Outlet Gutter Assembly (13.8mb, AVI)


Clamp Ring Outlet Assembly

Clamp Ring Outlet Assembly (6.5mb, AVI)


Pipe System

Pipe System (2.7mb, AVI)


Engineered Railing System

Engineered Railing System (2.7mb, AVI)























Fullflow - Syphonic Drainage Explained

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