Hydromax Siphonic Rainwater System


Siphonic rainwater systems have been utilised extensively across the Middle East and are ideal for large, complex roof structures such as shopping malls, airports, warehouses and buildings where space for drainage pipework is a limiting factor.

The HydroMax siphonic rainwater drainage system has been developed to rapidly remove large volumes of water during extreme rainfall events, capable of handling 10-15 times that of a traditional gravity rainwater system. The siphonic action also provides self-cleansing of the pipework, a significant feature in the Middle East region where sand deposits can prove more challenging compared to conventional gravity rainwater systems.

Properties & applications

Throughout the design process, the specially designed siphonic roof outlets allow a significant reduction in pipework, utilise smaller diameters and take up much less installed space when compared to traditional gravity systems. This has a direct result in reducing the necessary below ground connections and therefore underground pipework and associated costs.

The Hydromax system utilises the Wavin HDPE drainage pipework.

In order to appreciate the function and benefits of a Siphonic Rainwater System it is important to understand how a traditional gravity rainwater system operates.

As rain falls on the roof, the rainwater spreads out to the lowest level it can find. As the depth of water on the roof or gutter increases, it generates a force to move the water flow towards holes or openings called roof outlets that convey the water to a lower level. The roof outlets are connected to pipes that drop to ground level and into the underground drainage system.

A large amount of air is required to enable the water to move into the rainwater downpipe from the outlet. A vortex forms naturally and the water spirals down the outer wall of the pipe therefore transporting the water flow extremely inefficiently. Current European regulations limit the water/air mix within the pipe to 1/3 water and 2/3 air. This form of water movement results in a large bore pipe moving a lot of air and a little water.

The flow rate is very slow as the water is rotating round and round as it drops instead of flowing directly in the intended course.

The preferred requirements of the architect or building user for free use of the space within buildings is sacrificed due to having limited choice as to the positioning of the numerous rainwater downpipes which are generally determined by the outlets in the roof.

When using the HydroMax™ siphonic roof drainage system, the technology, custom and practice in managing the discharge of rainwater from roofs is changed. A siphonic system maximises the flow of water through the pipes by eliminating air within the pipework and uses the full available building height to drive the water directly through the pipework. Running with full-bore flow by means of simple siphonic action ensures efficient drainage of the rainwater to a connection into the underground drainage system in your preferred location. By using this method, designers can position the downpipe(s) in location(s) to suit the design team and or client’s requirements.

The number of downpipes is normally significantly reduced effecting another benefit (and a large cost saving) through a reduction in underground drainage requirements.

A constituent part of the HydroMax™ siphonic system is the special HydroMax™ siphonic roof drain. The HydroMax™ drain (outlet) is designed so that as soon as the rainwater reaches the hallow depth to the underside of the inducer plate, the unique inducer prevents air-ingress into the pipework as a first step towards making the system run under siphonic principles. Instead of multiple downpipes, small diameter tailpipes join a series of outlets to a collector pipe positioned at high level below the roof area. This horizontal collector pipe connects from the required number of roof outlets and runs to a chosen position before dropping for connection into the underground drainage system.

A HydroMax™ Siphonic Roof Drainage system utilises the full height of the building to provide the energy to create a powerful siphonic action.

The piping system is empty prior to a storm event occurrence. As the rainfall event starts, our unique HydroMax™ Siphonic Roof Drain prevents further air-entry into the pipework system whilst its Anti-Vortex fins also stop Vortex formation. Our technically advanced HydroTechnic™ analytical design software program has unsurpassed accuracy to determine the pipework diameters necessary to rapidly fill and prime the pipework system to provide optimised performance.

HydroMax™ utilises significantly smaller pipe diameters than required in a traditional gravity system. As the small diameter pipes prime and the water reaches the vertical downpipe, the water accelerates due to the forces of gravity. This acceleration creates the negative pressures that give the system its ‘Siphonic’ title and the forces literally suck the water off the roof until the rainfall event abates.

Principle of Syphonic

  • The Siphonic Roof Drain eliminates air entering the pipe promoting water filled pipework
  • The full bore of the pipe work carries the rainwater
  • Water, without air, accelerates down the vertical downpipe, creating negative pressure and naturally generating a high performance siphonic flow
  • The full height of the building provides the energy for this highly efficient Rainwater Drainage Solution

Features & benefits

  • Smaller diameter pipe work
  • No slope to pipes 
  • Fewer pipes run full-bore with self-cleaning properties
  • A significant reduction in below grade drainage (common range is from 20% to 60%)
  • Rainwater downpipes routed to the engineer’s preferred locations, freeing up valuable building space
  • Lower ponding depths on roof than gravity
  • Eliminate below grade drainage under the building floor
  • Easy co-ordination due to pipes running flat level
  • Fewer pipes = reduced construction time and cost

Standards & approvals

    • 2000 – European Standard BS EN 12056-3 introduced in 2000 (1st Standard to include a section for Siphonic drainage)
    • 2005 – ASME A112.6.9 Siphonic Drain Test issued in USA
    • 2006 – Singapore Standard SS 525 includes siphonic drainage
    • 2007 – Comprehensive Siphonic Drainage Standards published in UK – BS 8490
    • 2007 – Comprehensive Siphonic Drainage Standards published in USA – APSE 45
    • 2012 – International Plumbing Code references to use ASPE 45 and ASME 112.6.9 

Required design details

In order to carry out a siphonic system design, we will require the following information:

  1. Project Details / Client Name
  2. Design Rainfall Rate
  3. Roof Plan Drawings (AutoCAD)
  4. Roof catchment area drawing
  5. Floor Plan Drawings (AutoCAD)
  6. Sections/Elevation Drawings (AutoCAD)
  7. Underground drainage Connections
  8. Attenuation details
  9. Gutter Profile (to allow compatibility checks)
  10. Specification (we are able to provide the CSI or R10 type specification)