what to specifyFor further information download the SRDA Guide to siphonic drainage

You can also download our standard docSRDA Siphonic Drainage Specification in writeable format



  • what-to specify 2Rainfall intensity - rainfall levels should be determined from BSEN 12056-3:2000, using the projected building life plus 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 source of third party certification.
  • Supermarket-collectorMaterials - the majority of siphonic drainage systems in the UK use high density polyethylene (HDPE) 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.
  • Don't bother to specify a 'self-priming outlet' - they all self prime!