Internal sewer flooding

Internal sewer flooding occurs when wastewater backs up into a property through low-level fittings such as toilets, showers, baths or floor gullies. The cause is typically a downstream blockage or a surcharging public sewer in heavy rainfall, particularly affecting basement flats and lower ground floor properties in older London streets with combined drains. The incident is classified as an internal flooding event by Thames Water and is reportable, with the water company recording cases on the DG5 register where the public sewer is implicated. Engineers usually address common causes by clearing the line urgently and disinfecting affected surfaces. Building Regulations Part H1 requires anti-flood devices for fittings below the level of the upstream manhole cover. After clearance, a CCTV survey is typically recommended to confirm the cause and prevent recurrence.