Mould growth around drains is typically a sign of persistent moisture, organic residue and limited ventilation rather than a problem within the drain itself. Pink staining around shower drains usually involves Serratia marcescens bacteria feeding on soap and biofilm, while black mould around external gullies and overflows points to repeated wetting from blocked or surcharging drains. In London basements and ground floor flats, poor airflow and shaded yards can intensify the issue. Building Regulations Part F covers ventilation requirements that help control internal humidity. Engineers usually address common causes by clearing the affected drain, improving ventilation, and treating the visible mould with appropriate proprietary cleaning products. Recurrence after cleaning often indicates a recurring drainage fault upstream that benefits from a CCTV survey to identify the underlying source.
Mould around drains
Black or pink mould growth around drain outlets, gullies and overflow areas.
