The first siphonic system was installed in Scandanavia in the late 1960’s, and since the 1980’s siphonic systems have been installed throughout the UK, Europe and Worldwide. They are without doubt the best way of draining large roof areas, but unfortunately much of the information available to specifiers in the past has been fairly partisan, and in some cases misleading. This section will explore the workings of a siphonic system, explode some of the myths.
How it all works
Siphonic drainage is actually very simple in principle and all systems work in exactly the same way. Baffle plates inserted in the outlets restrict air entering the top of the system which, when combined with carefully sized pipework, causes the system, horizontal and vertical, to run full. In a very similar way to a simple tube siphon (such as you would use to empty a fish tank), the action of water dropping down the downpipe will cause a negative pressure to form at the top. This negative pressure can be harnessed to suck water along a collector pipe installed horizontally connecting the outlets at high level.
The benefits this gives are:
Each gutter will have only one or two downpipes, and these can be located at the end of the building, allowing free use of floor space by eliminating downpipes and therefore reducing columns.
The horizontal collector pipe can be very close to gutter, allowing full use of internal space.
Underground drainage can be eliminated internally in building, and can be significantly reduced externally, which can provide considerable cost savings and enhance construction programme on all sites, and particularly contaminated ones.
For sites with a requirement for SuDS, siphonic drainage will allow water to be delivered at a designated point at shallow depth, which can significantly reduce the construction costs, especially for pond based solutions.
What to specify
When specifying siphonic drainage there are a number of key factors which must be covered.
Rainfall intensity - The rainfall levels should be determined from BSEN 12056-3:2000, using the projected building life, and a suitable factor of safety. The contents of the building should be considered as well as building type. The more years specified the lower the risk to the building, but the more expensive the system, so it is always a balance to suit the acceptable level of risk.
Filling time and gutter calculations - It is vitally important that the siphonic contractor provides calculations to show that the system will fill within 1 minute, and that the gutter will function correctly i.e. will not over-top. In the UK the design rainfall event (the most intense period of a storm) is 2 minutes, and so a siphonic system must begin to function within half this time, or the roof may flood. In the past some companies have claimed that their systems do not need to fill to operate, but this is simply not correct. Gutter calculations should be to BSEN12056-3:2000, using outlet data from a BBA certificate or other third party source.
The majority of siphonic drainage systems in Britain use high density polyethylene pipework. HDPE can be connected using either electrofusion couplings, which are heated by internal elements, or by butt-jointing, where the cut ends of the pipe are melted and then forced together under pressure to make a joint. Butt-joints should only be made using a machine incorporating a jig and control system to monitor the temperature, time and pressure required. Site butt jointing of HDPE should only be allowed in a specification if ’factory’ conditions are set up on site so that consistent quality can be guaranteed.
Metal pipe systems (cast-iron, galvanized or stainless steel) can also be used for siphonic drainage. The specification should detail that installation should be according to their manufacturers recommendations for negative pressure.
Where should siphonic systems be specified?
The answer is that almost any building can be drained siphonically but with the following provisions:
Large industrial, storage or retail buildings will show much greater benefit gains over gravity drainage - in fact it would be almost impossible to drain some of these buildings by gravity.
The gutters or flat roof areas must be large enough to accept a siphonic outlet, and must have adequate access for maintenance. Gutters in inaccessible locations may not be so suitable.
All drainage can produce unwanted noise. In areas sensitive to sound, siphonic systems like gravity pipework may need acoustic insulation.
What can go wrong?
In the 1990’s there were a number of high profile failures of siphonic roof drainage systems, which led to the technology becoming suspect in some people’s eyes. This is however a great shame, as in all cases poor design was the cause, not a failure in the system.
The key reasons for failure were:
One or two companies set up in the industry without an adequate level of technical knowledge, and designed systems where the negative pressure was so great that pipework actually collapsed under the pressure, causing serious flooding of the building. This would not have happened if the pipework had been designed using suitable software.