Dye test (drain tracing)

A dye test is a simple but effective drainage tracing technique in which a brightly coloured non-toxic dye is introduced at a specific fitting and observed downstream to confirm which drain it discharges into. The method is widely used to determine whether a property's wastewater connects to the foul or surface water sewer, to identify misconnections in older London streets with separate systems, and to confirm responsibility under Building Regulations Part H. Engineers typically run the test after a visual inspection, watching adjacent inspection chambers and outfalls for the dye to appear. Results inform the remedial approach where misconnection is suspected and are sometimes reported to Thames Water or the local authority. Dye testing complements CCTV inspection and is carried out by qualified engineers using approved tracing products.