A combined drain carries both foul sewage from sanitary fittings and surface water from roofs and paved areas in the same pipe. This arrangement was standard practice during the Victorian and Edwardian period and remains common in older London boroughs such as Camden, Islington and Westminster. While simpler to install, combined systems can become overloaded during heavy rainfall, leading to combined sewer overflows that discharge diluted sewage into rivers like the Thames. The Thames Tideway Tunnel project was specifically designed to address this issue. Modern UK Building Regulations Part H now generally requires foul and surface water flows to be separated where practical. Owners of properties on combined drains should be aware that surface water blockages can cause internal flooding more readily than in separate systems.
Combined drain
Single drain carrying both foul wastewater and surface water rainwater to one sewer.
