siphonic roof drainage - geberit australia · geberit’s pluvia siphonic roof drainage system...

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Why use Geberit Pluvia siphonic roof drainage?Geberit Pluvia outlets have been specially constructed to ensure rapid removal of rainwater from the roof using siphonic principles with a vacuum effect. Compared with conventional drainage, the discharge velocity and capacity is considerably increased, despite the smaller pipe dimensions. Rainwater within the pipes travels at high velocity maximising the discharge capacity and minimising blockages.

What can go wrong / What to look out for? 1. Not following the plan exactly when installing the pipework. 2. Changes to the isometric plan required during construction. 3. Horizontal pipes section must be installed flat (without slope). 4. For horizontal pipe sections, eccentric reducers must be installed with the flat section on top. 5. Check any changes with the designer prior to installation (particularly any bends / offsets). 6. 6. 90° changes in direction should be achieved by 2 x 45° bends.

How does it work?A siphonic drainage system utilises the height of the column in the downpipe to create a negative pressure and suction effect. To create the negative pressure and suction effect, air is prevented from entering the system.For this reason, the pipework dimensions must be designed properly to ensure it is completely filled with water.

Why use siphonic?Siphonic is a more efficient roof drainage system, provides less complex and extensive pipework layout.This reduces installation costs and construction time.

What are the benefits?A siphonic drainage system is many times more powerful / efficient than a conventional roof drainage system. Large areas can be drained quickly with fewer roof outlets and downpipes.This saves space which increases the saleable area.

What is siphonic roof drainage?A siphonic roof drainage systems allows large roof areas to be drained quicker and with fewer roof outlets and downpipes than a conventional system.

Conventional roof drainage system Geberit Pluvia

Siphonic Roof DrainageFAQ


What are the benefits of Geberit Pluvia Siphonic roof drainage ?Geberit’s Pluvia siphonic drainage offers many advantages and savings for builders and installers, including: • Fewer downpipes / stacks • It is possible to layout the pipework to desired locations which frees up valuable building space; • Horizontal pipework is installed without any gradient/slope, which frees up valuable building space; • Pipework installed without gradient/slope allows easy coordination of other services; • • Downpipes can be routed to the perimeter of buildings to eliminate the need for below ground drainage within the building footprint; • A significant reduction in underground drainage is achieved; • Smaller diameter pipes running full-bore that provide self-cleansing properties; • Reduced construction costs due to fewer pipes; • Environmental benefits through the ability to route rainwater pipes to water storage/attenuation/recycling tanks; • • Reduced risk of flood damage because of fast drainage of rainwater from the building roof

What types of building / projects are suitable for siphonic roof drainage?This generally depends on the location and design rainfall rate (ARI). A building with a roof area of 500 m2 or greater will normally achieve commercial benefits. However, smaller roof areas have been drained using a Geberit Pluvia siphonic roof drainage system.Geberit Pluvia has been used to drain roofs on multilevel residential buildings, commercial developments, industrial facilities, sports stadiums and entertainment complexes globally and in Australia.

How does the cost of a Geberit Pluvia siphonic roof drainage compare to a conventional drainage system?A siphonic roof drainage system is in fact normally less expensive. Major cost savings can be attained due to the significant reduction in underground drainage requirements together with the associated pipe installation and trench work (excavation, bedding, pipe laying, testing, backfill, compaction, further backfill, and re-test costs.)SimilarlSimilarly, the number of downpipes required is substantially reduced. This means ducts to house pipe-work are substantially reduced in size or eliminated, thereby increasing the useable space in a building.

How does Geberit Pluvia siphonic drainage work? Geberit’s Pluvia Siphonic Roof Drainage system utilises the full height of the building to provide the “energy” to create a powerful siphonic action. A collection of Geberit Pluvia roof outlets, pipes and fittings combine to create the Geberit Pluvia Siphonic Roof Drainage System. The piping system is initially empty prior to a storm event. As the rainfall event starts, the Pluvia Siphonic roof outlet prevents further air-entry into the pipework system. The pipework becomes filled with water, thereby creating a powerful suction ecreating a powerful suction effect. This allows the system to quickly drain large volumes of rainwater from the roof.Geberit’s proprietary design software program calculates the pipework diameters necessary to rapidly fill and prime the pipework system to provide optimised performance. Pluvia siphonic drainage will utilise significantly smaller pipe diameters than that required in a conventional roof drainage system.


Does a siphonic roof drainage system block more readily than a conventional drainage system?The Pluvia siphonic drainage system is designed with high flow velocities, thereby ensuring the pipe system is self-cleansing. Any small debris such as leaves, twigs, etc. will be pulled into the pipe system and flushed away.


Can I use conventional spreaders in conjunction with a Pluvia system? Yes, conventional rainwater spreaders are commonly used to drain small upper catchment areas to Pluvia outlets on a lower deck.

How does a siphonic drainage system terminate?Pluvia siphonic systems terminate to a vented civil stormwater pit or rainwater tank.

Is the pipework always full of water?No, at the end of a rainfall event, the rainwater will drain from the roof and the pipework will empty. During light rain-fall events, the system will not fill with water and will operate similar to a conventional drainage system.

How much height is required for Geberit Pluvia siphonic drainage to operate?Each project is unique and therefore the height necessary to generate the siphonic action can vary depending on a number of factors, including: • the total horizontal run length (the frictional energy losses through the pipework system) • the complexity of the pipework configuration (frictional energy losses through fittings) • the design rainfall intensity (the volume of rainwater to be transported). Geberit has designed siphonic drainage systems for building heights as low as 1.2m.Geberit has designed siphonic drainage systems for building heights as low as 1.2m.

What are the disadvantages of conventional drainage systems?Conventional gravity drainage systems are less efficient in draining rainwater from buildings and roofs than siphonic drainage systems. Their performance is restricted by a number of factors, including: • The driving force is the depth or ‘head’ of water in the gutter above the roof outlet, as distinct from the driving force of a siphonic system, being the total height from the roof outlets and discharge to a stormwater system at ground level. • • The gradient or fall required to allow rainwater to flow in a horizontal pipe (this gradient also restricts the distance a pipe can travel). This typically leads to multiple downpipes and large amount of below ground drainage. • Conventional drainage relies on gravity and typically require approx. ⅔ air to transport ⅓ water.

How does the siphonic action occur?The siphonic action is caused by simple hydraulics rather than any special equipment. The height of the building provides the energy and as the small diameter pipes prime and the full bore plug of water reaches the vertical downpipe, the water accelerates due to the forces of gravity. This acceleration creates negative pressure within the pipe and creates the siphonic action. The forces generated literally suck the water off the roof until the rainfall event abates.


Can a Geberit Pluvia system be used to drain covered balcony areas?No, small wind-driven catchment areas do not provide sufficient flow-rates to allow siphonic action to occur.

Are access points/rodding points/cleaning eyes required in the siphonic drainage system?No. The Geberit Pluvia siphonic roof drainage system is designed to run full-bore at high velocities. This ensures that the pipes are self-cleaning. Access points would create air-pockets in the system which would disrupt the siphonic action.

Do any components need replacing due to wear and tear?There are no moving or mechanical parts in a Geberit Pluvia siphonic roof drainage system. There should not be any necessity to replace any parts. It is recommended during routine maintenance that all roof outlets are inspected to ensure leaf guards are not blocked and the pipework be checked to see that the pipe supports are firmly fixed.

Can Geberit’s Pluvia siphonic roof outlets be used to drain car park areas?Geberit Pluvia can be used to drain the top deck of a multi-level car park which is the main rainfall catchment area. Being the top-deck it should also provide the height necessary to operate a siphonic system.

Can a Geberit Pluvia siphonic drainage system be altered?A siphonic drainage system is a hydraulically engineered solution and therefore any alteration requires a hydraulic calculation to be carried out prior to making any changes.Geberit’s technical support team is available to discuss any design changes or proposed alterations on site during the construction phase.

Is there a minimum size roof which can be drained by a siphonic drainage system? Yes. A minimum volume of water inflow into a Geberit Pluvia roof outlet is required to “prime” the pipework and create the siphonic action. Practically speaking, it is best to aim for flow-rates of at least 3 Litres / second to each Pluvia outlet. As a rule of thumb for the Australian setting, this requires minimum catchment areas to each outlet of around 50m2.

What maintenance is required for siphonic roof drainage systems?Geberit Pluvia siphonic roof drainage systems have a proven track record of low maintenance. All roof drainage systems, including conventional and siphonic drainage should be maintained to suit local conditions and a regular maintenance regime is recommended. Local codes or standards should be followed. In locations surrounded by trees or within marine or industrial areas, intervals between maintenance should be reduced as there is a higher possibility of debris gathering. Roof cleanliness is the most important issue. Clearing away of all debris from around roof outlets and from within gutters is required however care should be Clearing away of all debris from around roof outlets and from within gutters is required however care should be taken not to clean the debris into the roof outlets.



Who is responsible for the design of Pluvia siphonic systems?The Hydraulic Consultant/Designer for the project is responsible for planning the pipe routes and ensuring proper coordination of the siphonic system. Sizing of the Pluvia system and hydraulic calculations are then completed using Geberit’s proprietary design software. It is important that any changes requested during the installation phase are checked using the design software.Geberit’s experienced Design Engineers are available to offer advice and assistance at any stage during the design process.design process.

What are the bracketing requirements for Pluvia Siphonic Systems?Forces are generated at changes of direction in the pipework due to the high velocities encountered in siphonic systems and thermally induced expansion and contraction of the HDPE.The following bracketing recommendations should be observed for all Pluvia systems- Maximum spacing between pipe clips 10 x diameter of pipe (see table)- Fixed/anchor points at all direction changes and every 5.0m on straight sectionsGeberit recommends the use of a continuous rail fastening system for siphonic systems. Please consult your Geberit recommends the use of a continuous rail fastening system for siphonic systems. Please consult your preferred bracketing supplier for additional information on HDPE bracketing.


What pipe materials can be used?HDPE has the material properties to withstand the negative pressures and high flowrates experienced in a siphonic drainage system. The engineering characteristics of HDPE make it possible to fabricate the pipe layouts to install them on site as they are. In this way, sections can be prefabricated in a safe, clean environment for quality assurance in installation.

Can you have only 1 roof outlet draining a roof area?It is always good practice to have a minimum of two roof outlets serving a single roof area or gutter section.

Are overflows required?In the case of roof areas with internal drainage such as box gutters, an emergency overflow must be present, regardless of the size of roof area. This is applicable whether a conventional or siphonic roof drainage system is installed. AS/NZS3500.3 states that the emergency overflow system must be able to drain the same capacity as the primary system.


siphonic roof drainage - geberit australia · geberit’s pluvia siphonic roof drainage system...

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