FAQ

Q: What are the differences between conventional and siphonic drainage systems?

A: To answer this question let’s look at conventional drainage first: Most downpipe systems have a very inefficient method of discharging water from roofs, as water spirals down around the inner walls of the pipe with an air column in the centre. During drainage, the water in a conventional downpipe never exceeds a third of the pipe capacity, due to this air presence and its effects.
Siphonic drainage works very differently. Special siphonic roof outlets prevent air entering the pipes with only a small amount of water covering the outlet. The downpipe backs up (primes) and the gravity force pulls the water through the downpipes at high velocity. This high velocity creates friction loss within the pipe, which results in a negative pressure situation or vacuum, hence the rainwater gets sucked into the downpipe.

Q: How does the cost of installation of a siphonic system compare to an equivalent “traditional” storm water system?

A: This often asked question is one of the more difficult questions to answer, as there are many variables involved. However, historical data show that savings of up to 30% can be achieved in most cases. Much of these savings are in the reduction or elimination of excavation and backfill cost and the reduction in trenching depths on site. The reduction in number of downpipes and their size also contributes to the lower material costs. As part of our service, Siphonic Solutions provides obligation free concept designs and budget estimates tailor-made for your project.

Q: What are the main benefits? Why use it?

A: Siphonic drainage offers many benefits, both technical and economic. Smaller pipe diameters and the horizontal ability of siphonic systems create several benefits that many architects, hydraulic engineers and builders have already realised. Among these benefits are:

• Smaller pipe diameter (less materials, lower cost).
• No pitch in pipework required.
• Invert coordination is eliminated.
• Further overhead horizontal runs are possible.
• Under-slab piping is reduced significantly or eliminated.
• Cost of excavation, backfill, and compaction is reduced significantly.
• The depth of rain retention ponds or tanks is reduced.
• Cost of rainwater harvesting cisterns is reduced. Above ground tanks
  are possibilities. Underground tank depth is reduced. Tank location is
  highly flexible.
• Higher operating velocities contribute to effective interior cleansing.
• Renovation and historic preservation is aided with smaller piping and there is no
  need to saw existing slabs to install new pipe.
• Flexible gutter design without limitation of downpipe locations.

Q:What special considerations are needed in the layout of roof drainage when using a siphonic roof drainage system?

A: None, the roof can be designed exactly as it would be if a traditional roof drainage system was to be used. The siphonic roof drainage pipework is designed to cater to the needs of the roof structure and architecture, not the other way around.

Q: Where do siphonic systems terminate? How is the connection to the soakwell system made?

A: Siphonic systems exit a building in the same manner as traditional systems, but at typically shallower invert levels (usually just deep enough for protection from weight loading). Common practice is to run the piping in its siphonic size and then increase the pipe diameter to the “equivalent” gravity pipe size to reduce the discharge velocity to 1.5m/s which is then connected to the soakwell or rainwater disposal system with a vented cover. This type of connection “breaks“ or terminates the siphonic condition. The receiving stormwater system sees no difference between siphonic and conventional downpipe systems in the amount of water received.

Q: Can the horizontal piping run uphill or can it offset up and over an obstruction?

A: This is not recommended. If the pipe was run uphill, there would always be water trapped in the piping with no means of getting out. In addition, such presence of water and the air pocket formed would delay or prevent priming during the next rain event.

 

Q: Is there any special maintenance required for siphonic drainage systems?

A: No. The piping system is effective at cleansing itself due to the flow patterns developed within the system. There are no moving parts or devices requiring maintenance or replacement.
All roof drains are subject to the same environmental conditions including debris from trees, industrial activity and wildlife activity. Therefore, regular and appropriate roof and gutter maintenance is always recommended.


Q: Are secondary (overflow) systems necessary?

A: The building code of Australia requires secondary roof drainage systems for any roof with a parapet wall or some other condition that will retain water on the roof if the primary drainage system fails. This code requirement applies specifically to traditional roof drainage design, but also applies to siphonic roof drainage as well. Plumbing code requirements for the design and capacity of secondary roof drainage should be followed.

Q: What are the typical down pipe sizes?

A: Downpipe size depends on many factors! The table below is indicative only and needs to be confirmed by our design engineers, however it acts as a helpful guide. (For flowrates below 10 l/s and above 100 l/s please contact our office for further assistance).

Dropper height

10 l/s

20 l/s

40 l/s

60 l/s

80 l/s

100 l/s

4m

Ø 90mm

Ø 110mm

Ø 125mm

Ø 160mm

Ø 200mm

Ø 250mm

6m

Ø 75mm

Ø 90mm

Ø 110mm

Ø 125mm

Ø 160mm

Ø 200mm

8m

Ø 75mm

Ø 90mm

Ø 110mm

Ø 125mm

Ø 160mm

Ø 200mm

10m

Ø 63mm

Ø 90mm

Ø 110mm

Ø 125mm

Ø 160mm

Ø 200mm

12m

Ø 63mm

Ø 90mm

Ø 110mm

Ø 125mm

Ø 160mm

Ø 160mm

Q: Is the design of the box gutter incorporating siphonic drainage different to that of a conventional downpipe system?

A: In general there is greater flexibility in the design of the box gutter. The size of the gutter is still determined by the size of the roof and the design rainfall intensity, but the location of the sump can be chosen. The traditional sump can be eliminated or reduced to 50mm depth depending on the total gutter depth. Part of our professional support package service can include the sizing of the box gutters.

Q: How much room is required for the siphonic drainage system underneath the (box) gutter?

A: This depends on the number of outlets connected and their flowrates. In most cases 500 – 600mm from the bottom of the gutter to the underside of the siphonic header pipe is sufficient. If only one outlet is connected per downpipe, the shallowest connection is 150mm from gutter to the underside of the pipe. If your requirements are outside these measurements please contact our office for information.

Q: Is there a minimum roof area required for siphonic drainage?

A: Yes, the siphonic roof outlet requires a minimum design flow of 1 l/s in order to work with the siphoning effect. In the Perth metropolitan area this is equivalent to a 20m2 roof. Downpipes for this size of roof can be as small as Ø 40mm. Therefore, balconies and small canopies are not recommended to be drained by a siphonic drainage system but roof areas above 20m2 are well suited. There is no limit to the maximum roof area, which can be drained by siphonic drainage systems.

Q: How far can a siphonic drainage system run horizontally?

A: This depends on the flowrate and the height of the building but as a rule of thumb; the horizontal pipe run can be 10 times the downpipe height (e.g. with a 8m height from the gutter to ground, the siphonic drainage system can drain 80m horizontally without the need for gradient). Even at ground level, the siphonic drainage system can run flat in normal (siphonic) pipe size to a drainage point (swale, soakwell or culvert) away from the building.

Contact Us with any enquires at info@siphonic.com.au

 
Office Locations || Perth || Melbourne