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Give a sucker an even chance

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First installed in Scandinavia in the late 1960s, siphonic systems are used worldwide as the best way of draining large roof areas. Unfortunately, information available to architects on their performance has often been misleading. Malcolm Wearing attempts

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 drainage 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 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 the gutter, allowing full use of internal space;

underground drainage can be eliminated internally in a building, and can be reduced significantly externally, which can provide considerable cost savings and enhance construction programmes on all sites - particularly on contaminated ones; and l for sites with a requirement for SuDS (sustainable drainage systems), siphonic drainage will allow water to be delivered at a designated point at shallow depth, which can reduce the construction costs significantly, especially for pond-based solutions.

What to specify When specifying siphonic drainage, there are a number of key factors that must be covered. These are:

Rainfall intensity - the rainfall levels should be determined from BS EN 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 the building type. The more years that are specified, the lower the risk to the building will be, but the more expensive the system, so there 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 one minute, and that the gutter will function correctly - meaning it will not over-top. In the UK, the design rainfall event (the most intense period of a storm) is two minutes, and so a siphonic system must begin to function within half this time or the roof could 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 BS EN 12056-3: 2000, using outlet data from a British Board of AgrÚment (BBA) certificate or other third-party source.

l Pipework - the majority of siphonic drainage systems in Britain use high-density polyethylene (HDPE) pipework. This can be connected either by using 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, galvanised or stainless steel) can also be used for siphonic drainage. The specification should detail that installation should be according to the manufacturer's recommendations for negative pressure.

l Don't bother to specify a 'self-priming outlet' - they all self-prime.

Where should siphonics be specified?

The answer is that almost any building can be drained siphonically, but with the following provisos:

l 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;

l 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 might not be so suitable;

l 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 1990s there were a number of highprofile 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:

l 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, such as is used by all members of the newly set-up Siphonic Roof Drainage Association (SRDA);

l many specifiers exploited a loophole in the previous drainage standard and designed systems to operate at 75mm/hr.

These systems worked perfectly well, but were overwhelmed by higher rainfall events, which in the south and east of England could occur every year or so. It should be stated that this was also a problem with gravity drainage, but the link was not so obviously made. The new standard BS EN 12056-3: Keeping horizontal pipework at high level is one of the benefits of a siphonic system 2000 closed this loophole, and so it is no longer an issue with siphonic drainage, but continues in some gravity systems.

What is happening today?

The current siphonic roof drainage industry is more mature than the one that existed in the 1980s, with a wider spread of major companies. A draft British Standard, dealing specifically with siphonic roof drainage, has been drawn up, and is about to go to the BSI committee procedure. This will help define good practice in an industry that for so many years, in Britain, has been forced to set its own procedures for good practice.

In another step towards raising the industry profile, the SRDA was established. The association has the goal of bringing together companies to promote good practice. Members of the association must show that they have:

l a suitable outlet, pipework and hanging system; and l a functional computer-based flow-balancing program.

All these systems should have third-party accreditation. In addition to these requirements, member companies will be audited by the association to determine, on randomly chosen projects, whether their design, installation and customer service meet the required standards.

With these stringent criteria for membership, any specifier using an SRDA member can be sure that they are placing this vital part of their project in the hands of a company that has the expertise to do the job properly.

Dr Malcolm Wearing is secretary of the Siphonic Roof Drainage Association (SRDA).

For more information on the association or siphonic roof drainage in general, email info@siphonic. org or visit www. siphonic. org

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