Roof drainage and guttering

Siphonic drainage systems

Siphonic drainage systems were first designed in the late 1960s in Sweden and introduced to the UK in the 1980s. The system is now in use throughout the world, although still treated with caution by some owing to a legacy of poor design decisions in the 1990s.

When the systems were first introduced, rainwater drainage design was based on conventional gravity systems which were covered by BS 6367:1983. The code contained advice on rainfall intensity (based on mm/hour) but was open to interpretation. The incorrect selection of design rainfall intensity could lead to serious under capacity and according to the Siphonic Roof Drainage Association it was the selection of an inadequate intensity rather than a fundamental flaw in siphonic drainage that gave the system a bad name.

Siphonic action. The siphon will only work if the outlet is at a lower level than the inlet. The higher the head, h, the more efficient the system.

Early design code (now superseded)

BS 6367:1983 Code of practice for drainage of roofs and paved areas provided advice in respect of rainfall intensity.

The code advised that it was not possible to ensure complete safety from flooding or overflow. A design rate of rainfall of 75mm/h was considered generally satisfactory for roof gutters where overflow was unlikely to occur inside a building and for other gutters where some risk to the contents of the building might have been acceptable.

Current standards and experience tell us that a design rate of 75mm/h is inadequate for valley and parapet gutters. Who was to decide whether or not risk to contents might be acceptable, particularly if a speculative building is being designed and the end user is unknown?

The standard went on to set out 5 categories of design rate:

  • Category 1: A design rate of rainfall of 50mm/h for paved areas on which ponding can be tolerated during a heavy storm and for a few minutes after the storm has ceased.
  • Category 2: A design rate of rainfall of 75mm/h for sloping surfaces where ponding cannot occur and for flat surfaces where ponding cannot normally be tolerated. (Note: 'In some cases (i.e. some valley gutters) the risks associated with categories 1 and 2 will be considered too great and higher design rates of rainfall should be used (categories 3 to 5).')
  • Category 3: Provided a method of computation 'in cases where the building or its contents require an additional measure of protection'.
  • Category 4: Provided a method 'that should be used if a higher degree of security than that provided by category 3 is desirable'.
  • Category 5: For cases where 'the highest possible degree of security is required'.

All siphonic roof drainage systems function in the same way. They comprise a special baffled roof outlet that is connected to a vertical tail pipe. The baffle is designed to prevent air from being drawn into the system so that during rainfall the outlet drain fills above the baffle, cutting off air flow into the pipe. This lack of air, coupled with the downward pull of the water under gravity, creates a vacuum in the tail pipe.

The tail pipe or pipes are then connected to a horizontal collector pipe and then to a downpipe. The collector pipes and downpipe are intended to run at full bore to maximise the effectiveness of the system. For the system to work properly, careful balancing of pipe diameters, fall and configuration is needed to ensure that the pipes fill with water and do not continue to work under simple gravity flow. The system will take time to 'prime', i.e. develop proper siphonic action. If the system primes in 2 minutes there is every likelihood that the gutters will be overcome before the system works properly. Many systems work on a priming time of around 50 seconds, but in the past, longer priming times have led to problems.

Essentials of a siphonic system. By comparison with a gravity system, the head of water, h, is equivalent to the height of the building.

Cavitation is a potential difficulty as this can cause serious problems and if severe can lead to the implosion of the pipework. The system must be designed so that the peak negative pressure in the system is within the working capability of the pipework. The overall height of the building contributes to the effectiveness of the system, but if the fall is too great and/or the pipework is not sized correctly, the negative pressures induced by the falling water in the downpipe could create an excessive pressure drop and resultant cavitation.

The siphonic system of drainage reduces the number and size of rainwater downpipes and therefore the extent of underground drainage. So care must be taken to ensure that the underground drainage is able to deal with the volume of water from the downpipe without surcharging, as this could lead to problems with the effectiveness of the system. A vented manhole is usually provided so that if surcharge does occur, water can escape clear of the building. Design codes now dictate that possible surcharging must be factored into the design.

Although the basic theory is simple, when more than one roof drain is connected to a common collector main, the hydraulic calculations become very complex. Therefore drainage design is usually completed with the assistance of appropriate software packages. The final design is the product of specialist engineering; it is usually the remit of a specialist subcontractor, so clear lines of communication and liability are essential. For example, difficulties could arise if the specialist retains responsibility for specifying the design rainfall intensity whereas design parameters should be the responsibility of the architect.

Note that the siphonic system is merely a means of removing a large volume of water from a roof effectively; its ability to do so relies on outlet and pipe design. The system still relies upon adequate gutter design for which the traditional sizing calculations under BS EN 12056-3 apply. Furthermore, the provision of emergency overflows should still be made to safeguard against blockage events or a failure of the system to prime sufficiently rapidly.