This document discusses siphonic roof drainage systems and their advantages over conventional systems for commercial buildings. Siphonic systems use a single downpipe to drain large volumes of rainwater quickly. This allows harvested stormwater to be easily redirected to rainwater tanks without pumping. The increased driving head also enables various non-potable reuse options like landscape irrigation. The document outlines the benefits of siphonic drainage and opportunities for building surveyors. It describes a new large-scale testing facility built at the University of South Australia to further research siphonic drainage flow patterns and air entrainment effects.

1. Rainwater harvesting options for commercial buildings using
siphonic roof drainage systems----Lessons for Building Surveyors
Terry Lucke1, Simon Beecham 1 and George Zillante 1
1
School of Natural and Built Environments, University of South Australia
(terry.lucke@.unisa.edu.au)
ABSTRACT
Water conservation is an integral part of sustainable building practice and Water
Sensitive Urban Design (WSUD). New building design priorities have been
established in Australia that focus on reducing the consumption of both energy and
water. The stormwater runoff from commercial buildings is one area in which much
potential for improvement has been identified. Gone are the days where the only
concern with roof runoff was to ensure its rapid removal from the site. The need to
harvest this precious resource has been recognised and new technologies are emerging
to resolve this issue. Siphonic roof drainage is a relatively new building services
technology which has many benefits over conventional drainage systems. Building
designers and architects are specifying siphonic roof drainage systems on an
increasing number of commercial and industrial buildings. For example, Sydney
Olympic Stadium, the Norman Foster designed Chek Lap Kok airport in Hong Kong
and the new International Terminal Buildings at Adelaide and Sydney airports all
have siphonic roof systems. The benefits of these systems include, ability to quickly
drain high intensity rainfall events, substantial cost reductions, virtual elimination of
underground pipework and the opportunities for significant stormwater reuse options.
This paper aims to provide an overview of the many benefits that are being realised by
planners, building designers, engineers, architects, surveyors, contractors and owners
by specifying a siphonic roof drainage system. Furthermore, it will examine the water
conservation and reuse options that are possible with siphonic drainage and compare
these to conventional roof and property drainage systems.
KEYWORDS
Siphonic roof drainage, stormwater harvesting, water reuse, building services design
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using siphonic roof drainage systems- lessons for Building Surveyors’

2. INTRODUCTION
Commercial siphonic rainwater drainage systems were first developed by Ebeling and
Sommerhein in the early 1970s in Scandinavia (May, 1995). Since then, many
thousands of new buildings worldwide have been designed incorporating siphonic
roof drainage systems. The advantages of these systems over conventional roof
drainage systems are numerous and have much appeal for architects and designers.
Siphonic Drainage is growing from a once “obscure curiosity” in Europe to an
emerging market in the United States (Rattenbury, 2005). Because of the height
requirements needed for siphonic roof drainage systems, the technology is only viable
for larger commercial buildings and structures over about four metres in height (Refer
to Plates 1 and 2)
Plates 1 and 2 – Qantas Domestic Terminal in Sydney
Siphonic systems are designed to exclude air from the pipework and, once primed,
cause the pipes to flow under pressure. Syphonic roof drainage systems have strategic
advantages over conventional systems, and particularly so in respect of their cost-
effectiveness to quickly remove large volumes of rainwater safely and effectively
(Brahmall and Saul, 1999). A major advantage of siphonic systems is the greatly
increased driving head of water and consequent reduction in pipe diameter sizes. The
driving head in this case is effectively the difference in level between the water in the
gutter and the ultimate discharge point, which is usually near ground level.
The increased driving head in siphonic systems offers much potential for stormwater
harvesting and reuse. Because the building’s total roof runoff is normally discharged
from only one or two downpipes with high velocity, the water can easily be directed
to most places on a development site without the need for pumping. This means that
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 45
using siphonic roof drainage systems- lessons for Building Surveyors’

3. rainwater tanks can be placed in a convenient location away from the building
footprint if so desired. The strategic location of stormwater collection tanks can then
facilitate energy-free landscape irrigation or other reuse options.
CONVENTIONAL ROOF DRAINAGE
Conventional roof drainage systems consist of a number of large diameter downpipes
which connect the roof drainage to the underground stormwater drainage system.
Conventional systems are designed to operate at atmospheric pressure (May, 1995).
The amount of water that can enter the open-ended downpipes is dependent on the
depth of water in the gutter (H) and the outlet size (D). The type of flow is categorised
as either “weir” type or “orifice” type flow (May, 1995). Refer to Figure 1.
Air
Water
H = head of water
in gutter D = diameter of
downpipe
Figure 1 – Conventional Gutter Outlet and Downpipe
Research has shown that the water flowing in the downpipe in a conventional roof
drainage system is annular in nature (Wright, Jack and Swaffield, 2006). This means
that the water spirals down the inner edges or walls of the pipe and there is a hollow,
air-filled core down the centre of the water flow (Arthur, Wright and Swaffield,
2005). The air that is drawn down by the water actually restricts the water discharge
in a pipe to between one quarter and one third of the pipe cross section area. This
means that large diameter pipes are required to enable the gutters to drain quickly
without risk of overflowing. These types of conventional roof drainage systems are
very inefficient and require extensive underground pipework systems (Figure 2).
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 46
using siphonic roof drainage systems- lessons for Building Surveyors’

4. Figure 2 – Conventional Roof Drainage System
SIPHONIC DRAINAGE
Siphonic roof drainage systems are currently designed to operate with full bore flow
without the need for any mechanical pumping either to prime or operate the system.
The gutter outlets in siphonic roof drainage systems are specifically designed to
restrict the inflow of air into the pipework and this allows a much greater volume of
water to flow in the pipe. Because there is limited air in the pipework, the falling
water generates a vacuum behind it which “sucks” the water into the gutter outlet.
This vacuum effect around the outlet allows more water to be drawn into the pipe
resulting in much greater flow rates (Arthur and Swaffield, 2001).
Unlike conventional roof drainage, in siphonic systems gutter outlet pipes are directed
into a horizontal collection pipe which often then flows into a single downpipe
(Figure 3). This collection pipe usually runs at roof level close to the gutters collecting
all the water flowing out of the gutter (Arthur and Swaffield, 2001). The total volume
of water collected then flows into the downpipe and is often discharged at a single
outlet at ground level. This outlet can be located directly in the underground
stormwater system or redirected to a rainwater tank for harvesting and reuse (Figure
3).
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 47
using siphonic roof drainage systems- lessons for Building Surveyors’

5. Figure 3 – Siphonic Roof Drainage System
Because the roof runoff from siphonic systems is usually directed into a single
downpipe, the normally extensive, underground drainage pipe system is virtually
eliminated. Besides the obvious benefit of lower excavation and pipe costs, siphonic
systems also minimise potential building damage associated with footing movements
in reactive soils. As there are generally no drainage pipes under the slab or parallel to
footings, soil heave problems caused by leaking pipes are also eliminated.
Siphonic drainage systems are designed using pipe full flow conditions which assume
no air in the system. As such, most siphonic outlets are specially designed to reduce
the amount of air entering the system. This is often achieved by means of a horizontal
baffle plate configuration at the siphonic outlet installed in the gutter floor. These
baffle plates restrict the formation of a vortex above the outlet which would suck air
into the system and break the siphon action. Other outlet configurations include a type
of “upturned dish” arrangement which forms an airlock to restrict the air entrainment
into the flow (Figure 4).
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using siphonic roof drainage systems- lessons for Building Surveyors’

6. Figure 4 – Typical Siphonic Outlet Showing Baffle Plate
SYSTEM PRIMING
Siphonic rainwater drainage systems are designed to operate under both part-full and
pipe-full conditions. The transition between these two states involves priming or
unpriming of the system, both of which involve considerable air entrainment.
Priming is the term used to describe the process where resistance to flow is sufficient
to cause the pipe system to become full of water. It is the friction and form losses
which are present in every pipe flow which resists the movement of the water and
assists in the development of pipe-full flow conditions.
The design of siphonic systems involves dynamic balancing of the pipe systems when
they are flowing under pressure. Friction and form losses in pipes are proportional to
the square of the velocity of the fluid and are cumulative. This means that the further
the water travels, the more energy it loses and consequently the less volume of water
can flow. In order for the siphonic outlet flows to be balanced, smaller tail pipe
diameters are often used closer to the vertical downpipe to reduce the flow volumes to
similar flows upstream.
The priming sequence usually occurs in three phases. As the water level in the gutter
increases, so does the flow into the vertical tailpipe. This flow enters the lateral
collector pipe in a supercritical state and thereafter forms a hydraulic jump. As long
as the subcritical depth downstream of the jump is less than the pipe diameter, the
flow in the vertical stack remains annular, as for conventional gravity flow systems.
As the flowrates increase, the subcritical depth downstream of the jump also increases
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 49
using siphonic roof drainage systems- lessons for Building Surveyors’

7. until it eventually reaches the soffit of the pipe. The plug of air trapped upstream of
the jump then moves downstream until it is expelled down through the vertical stack.
At this point the system is said to be primed. The weight of the water in the full
flowing vertical downpipe leads to the generation of negative pressures in the
upstream horizontal collector pipe.
UniSA SIPHONIC DRAINAGE TESTING FACILITY
As part of an ongoing PhD research project in siphonic roof drainage systems titled,
“The role of air entrainment in the performance of siphonic roof drainage systems”
, a full scale siphonic drainage rig has been constructed in the hydraulics laboratory at
the University of South Australia (UniSA) Mawson Lakes Campus (Refer Plate 3).
This research is being undertaken in collaboration with Syfon Systems of Melbourne,
a leading Australian siphonic drainage company since 1992.The dimensions of the
UniSA rig are 32m long by 6m high by 3m wide. This is, according to the literature
reviewed so far, the largest laboratory-based siphonic testing facility in the world.
In order to allow visual observation of the flow patterns, perspex was chosen for the
pipework material. Hydraulic calculations undertaken for this system using
commercial siphonic software predicted that the expected maximum flow rate in the
laboratory will be 69 litres per second. This is equivalent to the rainfall of a 1 in 300
year storm event falling on the entire roof of the hydraulics building in which the
model is housed. The maximum flowrate measured through the rig has been 70 litres
per second which thoroughly agrees with the calculations. This demonstrates the
accuracy of the commercial software used in designing this system.
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using siphonic roof drainage systems- lessons for Building Surveyors’

8. Plate 3 – UniSA Siphonic Drainage Testing Rig
(Mezzanine floor shown is 6m above ground floor)
OBSERVED FLOW PATTERNS
Research into siphonic roof drainage systems has been undertaken at Heriot Watt
University in Edinburgh, Scotland. The principal researchers have been Swaffield,
Wright and Arthur of the Heriot-Watt Drainage Research Group. They described the
formation of hydraulic jumps within the pipework and the role they play in priming.
Arthur and Swaffield identified three main effects that air entrainment has on system
performance. Air affects system operating pressure, propagation velocity and friction
losses. Their research has identified various interesting phenomena of siphonic
drainage which will be further investigated on the UniSA rig.
The design of the testing facility at UniSA has evolved from studying the limitations
of existing experimental models identified in the research literature. The UniSA rig
has much larger pipe diameters than previously built models and has four gutter
outlets. At six metres high, the apparatus is considered to be representative of real
siphonic roof drainage systems in use.
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using siphonic roof drainage systems- lessons for Building Surveyors’

9. OPPORTUNITIES FOR WATER RECYCLING
Conventional roof drainage systems consist of a number of large diameter downpipes
which connect the roof drainage to the underground stormwater drainage system. As
the flow in these downpipes is annular, the flow volumes are relatively small
compared to the downpipe diameters. This means that most of the potential energy of
roof gutter water is expended by the time it enters the underground drainage system.
This results in low energy stormwater distributed along the entire length of the
underground drainage pipe system. In order to collect this water for re-use, some type
of collection pit and pumping system would normally be needed. This not only
increases system costs and energy consumption but also places landuse restrictions on
the building’s surrounding area.
As previously discussed, the discharge from siphonic roof drainage systems is usually
from a single, full-flowing downpipe at high velocity. This can generally enable the
stormwater to be directed to any part of a development site, even to the highest
elevated areas. Rainwater tanks or other collection devices can then be used to harvest
and store the rainwater for later reuse. This stormwater can be utilised for many
different activities ranging from simple landscape irrigation to toilet flushing, vehicle
washing and other uses.
Other areas of the research to be undertaken at UniSA include the effects of air
entrainment on the performance of siphonic systems and investigation into the
transition area between siphonic downpipe outlets and underground drainage systems.
The design of siphonic outlets to minimise air entrainment will also be investigated.
BENEFITS FOR BUILDING AND BUILDING SURVEYING
As discussed previously, the normally extensive, underground drainage pipe system is
virtually eliminated. Besides the obvious benefit of lower excavation and pipe costs,
siphonic systems also minimise potential building damage associated with footing
movements in reactive soils. As there are generally no drainage pipes under the slab
or parallel to footings, soil heave problems caused by leaking pipes are also
eliminated.
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 52
using siphonic roof drainage systems- lessons for Building Surveyors’

10. In summary the major benefits of siphonic roof drainage systems are:
• fewer downpipes are needed
• smaller pipe diameters are needed
• high flowrates for self cleansing
• pipes can be laid without fall
• pipework can be concealed in roof cavity (aesthetics)
• single outlet means minimal ground excavation
• building damage from leaking underground pipes minimised
• significant material cost savings
• single outlet enables stormwater harvesting and reuse
CONCLUSIONS
Water conservation is an integral part of sustainable building practice. New building
design priorities have been established in Australia that focus on reducing the
consumption of both energy and water. Stormwater runoff from commercial buildings
is one area that has much potential for water harvesting and reuse.
Figure 5 – Conceptual Siphonic Roof Drainage Reuse Configuration
The increased driving head of siphonic roof drainage systems compared to
conventional systems offer many stormwater harvesting and reuse options. Because
the building’s total roof runoff is normally discharged from a single downpipe with
high velocity, the water can easily be directed to most places on a development site
without the need for pumping. This means that rainwater tanks can be placed in a
convenient location away from the building footprint if so desired. The strategic
location of stormwater collection tanks can then facilitate energy-free garden watering
or other reuse options.
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 53
using siphonic roof drainage systems- lessons for Building Surveyors’

11. A full scale siphonic drainage rig has been constructed in the hydraulics laboratory at
the University of South Australia (UniSA) Mawson Lakes Campus and
comprehensive testing has been conducted. A method of measuring the harvesting
potential has been established.
References:
Arthur, S. and Swaffield, J.A. (2001). “Siphonic roof drainage: current understanding.” Urban
Water, 3, Taylor and Francis, pp. 43-52.
Arthur, S., Wright, G.B., and Swaffield, J.A. (2005). “Operational performance of siphonic
roof drainage systems.” Building and Environment 40, pp. 788-796.
Bramhall, M.A., and Saul, A.J. (1999). “Hydraulic performance of siphonic rainwater
outlets.” Proceedings of the 8th international conference on urban stormwater drainage.
Sydney. Australia
May, RWP, (1995), Design of conventional and siphonic roof drainage systems, Public
Health Services in Buildings - Water Supply, Quality and Drainage, IWEM Conference,
London.
Rattenbury, John.M, (2005), Siphonic Roof Drainage: Where Is It Headed?, PM Enginneer,
Plumbing and Architecture & Construction Groups, Troy, MI, USA
Wright, G.B., Jack, L.B., and Swaffield, J.A. (2006). “Investigation and numerical modelling
of roof drainage systems under extreme events.” Building and Environment 41, pp.126-135.
AIBS 2007, Beecham, S. et al ‘Rainwater harvesting options for commercial buildings 54
using siphonic roof drainage systems- lessons for Building Surveyors’