Underground Drainage

Underground drainage is the general term to sub-surface drainage. The system of pipes and fitting that is installed below ground level is known as underground drainage. This system is mainly used to transport foul drainage and rainwater flow to the sewage treatment facility.

Most underground drainage systems today are manufactured in PVC-u, which is hardwearing and virtually maintenance-free. The smooth inner wall of the pipe also promotes a much better flow when compared to traditional materials. Underground drainage pipes and fittings are almost always terracotta orange in colour. This makes identification easy, as soil pipes installed above ground level are usually black, grey or white.


Underground drainage pipes are usually laid in a trench that has been lined with a bed of pea shingle. The installation must be laid to the correct fall and should always follow the basic drainage design principle that all points in the drainage system should be accessible for testing, inspecting and cleaning.

It is always a good idea to plan all drainage installations – most building projects in the UK will include a drainage layout drawing that has been designed by the architect. The key components in terms of design location are generally gullies, inspection chambers, rodding eyes and rainwater adaptors.