Soil and vent pipe (SVP)

A soil and vent pipe combines two functions in a single vertical pipe: it carries foul wastewater downwards to the underground drain while also allowing air to enter and leave the system through an open top above the roof line. Adequate ventilation prevents pressure fluctuations from siphoning the water seals in nearby traps, which would otherwise allow drain odours into the property. UK Building Regulations Part H1 requires the open vent to terminate at least 900mm above any opening within 3 metres. In older London terraces, cast iron SVPs run up the rear elevation, while modern installations typically use PVC-U with a wire balloon at the top to keep birds and debris out. Where extending above roof level is impractical, an air admittance valve can sometimes be used as an alternative.