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Blocked outlets - is this due to faulty workmanship of the gutter or the drainage system? A correctly designed siphonic system will run in siphonic mode sufficiently often to minimise the risk of blockage. The system will run full-bore and at a speed of several metres per second and be self-cleansing. If blockages still do occur, the system may not prime correctly and so will run at greatly reduced capacity. This, in turn, could cause over-topping of gutters and/or overloading so regular checks to clean gutters are essential.

Connection to storm drainage: how do you reduce the pipe size? It is normal to connect the down pipes of a siphonic system via
a manhole. A major headache for siphonic systems can be inadequate underground service pipes. For this reason, the inlet pipe should not be reduced in size as it enters the manhole. This follows normal gravity-flow practice of not reducing diameters in the direction of flow.

Connection to combined drainage: where to trap it? Connection of a siphonic rainwater system to a combined sewer is slightly unusual because the underground system may well be rather old and bordering on inadequate. Another point is that a sealed industrial building would not be affected if the normally inaccessible roof area contained 'vent pipes' – the siphonic system working in reverse direction. Alternatively, a manhole connecting to the foul drainage can have rod access running trap built into the outlet. Another method could be to fit a non-return flap valve into the inlet pipe, one that at rest will always fall shut, preventing smell from passing back up the siphonic pipework.

Damaged gutter joints – whose fault is it? There is a lot of energy around in a siphonic system when it is operating at high capacity. A correctly installed system should be suitably clamped such that vibration in pipework is not able to create noise or transfer stress to the pipework itself (and its joints) or other components. The tailpipe should be isolated from the gutter/outlet by a flexible joint to isolate (a) any thermal movement in the gutter from the pipework and (b) any vibration in the pipework from the outlet.

A correctly installed gutter should not be weakened by vibration. Proprietary polymeric gaskets should be used to form seals at clamped joints in steel gutters. IMAG4808 2These are more effective than application of sealant. Alternatively, single ply membrane-lined gutters offer an effective way to seal joints in gutters because the waterproofing is heat-welded at seams. A 'Component Quality Standard' is available by free download from www.siphonic-roof-drainage.co.uk

What can go wrong? In the 1990's there were a number of high profile failures of siphonic roof drainage systems, which undermined confidence. In fact 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, such as is used by all members of the SRDA.
  • 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 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 BSEN12056-3:2000 closed this loophole, and so it is no longer an issue with siphonic drainage, but continues in some gravity systems.

Gravity and siphonic: can you mix them in a single system? Definitely not in the siphonically active areas! There should be no problem with mixing the systems beyond the siphon break provided the pipe is sized sufficiently to take the volume of water from both systems. The siphon break, usually at or near ground level and is where the bore of the siphonic pipework is enlarged to break the siphon and reduce the velocity of the discharge.

Pipe-work: should it be installed horizontally or sloping? There are three elements to the pipe-work system:

IMAG4760 1

  • The vertical tailpipe from the outlet. 
  • The collector pipe linking the tailpipes.
  • The downpipe taking the collector pipe to ground level

One of the key advantages of a siphonic system is the ability to run the collector pipe horizontally and so minimise the depth of services voids. Slope affects the rate at which the system primes (i.e. begins to switch from low capacity gravity mode to high capacity siphonic mode). The design software assumes this level condition, and it would be pointless and highly complex to adjust the calculation for the chosen slope. You would also have to construct to precisely that slope! In any case, all software and products should have a margin of safety to allow for tolerances but a traditional 1:60 fall in the collector pipe is not what is needed.

Pipework sleeves - do you need them? SRDA strongly recommends the use of manufactured sleeves to protect siphonic downpipes where they pass through masonry. 

Retrofit – what are the main criteria that determine whether retrofit siphonic is feasible?

  • The main point to consider is the roof void space. With all the pipework normally being placed between deck and ceiling (if any) the effect of existing – and planned - other services should be assessed. Routing siphonic pipes may become overly complicated and expensive if it needs to avoid the air conditioning and the electrics trays for example.
  • The structural capacity of the roof needs to be considered as the roof may need to be strengthened to cope with the extra weight of the new pipework and support bracketry (which is not normally supported by the roof in a gravity system) plus the water that will be carried at full bore. Manufacturers will be able to supply the data on the weight of their systems.

Retrofit – if roof falls have been designed for gravity drainage, what modifications are likely to be required at roof level? The top side of the roof needs to have troughs or low points less than 20 metres apart if it is to become a flat roof installation. Extra penetrations through the roofing membrane are likely to be20140328 First Direct Leeds-crop2 needed, so roof finishing may need hot works if using plastic membranes. Gutter installations are less problematic, but routing is still very important, as is the width of the guttering which will need to be wide enough to fit a siphonic outlet into.

Retrofit – if the existing outlets are perished or no longer available, can they be replaced with a different product? Generally yes. SRDA members should be able to incorporate new outlet products into the system, but some redesign of tail and collector pipework may be necessary.

Secondary systems – how do they work? A secondary system uses siphonic outlets placed slightly higher than the primary system. These operate only at rainfall intensities close to the design maximum, when the head of water reaches a high level. Secondary systems involve separate pipework, which may be designed to discharge above ground and clear of the building. Thus the primary system can be designed at a lower capacity (and cost) whilst ensuring that it will prime frequently and hence self-clean. On buildings for which its effects are acceptable, the alternative to a secondary system may be a simple overflow.

Siphonic break – what is it and where should it be? Siphonic systems run at higher capacity and higher velocity that a gravity system because the water collected from individual roof outlets is literally sucked along the collector pipe suspended in the space about 1 metre below the roof membrane or gutter sole level. Supermarket-collectorThe power creating this suction is the energy generated in the downpipe by descending water. This solid column of water, under negative pressure and completely filling the down pipe needs the energy in it released before it enters the underground system, which is only designed to run under more normal conditions.

In practical terms when running as designed, the water can be a concentrated jet and if allowed to enter a manhole like that will, in the long term cause damage to the underground drainage components.

Therefore the siphonic effect is broken in the last metre or so of the downpipe. This is done just by increasing the diameter of the pipe – giving the water more space will bring the negative pressure back to atmospheric pressure. The optimum point will depend a little on the site circumstances – ease of access to manholes, for example, but the exact point will be defined by the agreed design of the whole system.