A water seal is the small volume of water retained in the U-bend of a trap that physically blocks foul air and insects from passing from the drainage system into the room. UK Building Regulations Part H1 require a minimum seal depth of 75mm on self-cleansing fittings and 50mm on fittings discharging to a gully outside the building. Seals can be lost through evaporation in unused fittings, siphonage during fast flows, back-pressure from blockages or capillary action across debris draped over the trap weir. Loss of seal is a common cause of drain odour complaints in flats and HMOs across London. Restoration is usually as simple as running water into the fitting, although recurring problems may indicate inadequate venting or a partial blockage further down the drain.
Water seal
Layer of water retained in a trap that prevents drain odours entering a building.
