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Last edited 25 Jul 2022

Trap

In its broadest sense, the word ‘trap’ refers to a something that catches and holds things.

The 2018 International Residential Code (IRC), published by the International Code Council (ICC) defines a trap as: ‘A fitting, either separate or built into a fixture, that provides a liquid seal to prevent the emission of sewer gases without materially affecting the flow of sewage or wastewater through it… The trap seal is the maximum vertical depth of liquid that a trap will retain, measured between the crown weir and the top of the dip of the trap.’

The Illustrated Guide to Hot and Cold Water Services (BG 33/2014), written by Reginald Brown and published by BSRIA in 2014, it states: ‘Traps containing a water seal are essential to prevent toxic gases from the sewer system entering the building through the appliance drain. The minimum trap size and seal depth is detailed in Table 1 of Section 1 of Approved Document H. Traps are fitted to baths, basins, sinks and shower trays as a version of the so-called “U-bend”. In most cases the trap will connect on to the waste fitting plughole with a standard thread (1¼ inch BSP for basins and 1½ inch BSP for baths and showers). Traps for WCs are incorporated in the design of the WC pan. The water seal is continuously maintained in use but can disappear through evaporation if the appliance is not used for several months, for example when a property is vacant.’