A fi tting or part of an appliance or pipe arranged to retain water acting as a seal to prevent the passage of foul air. The water in a trap acting as a barrier to the passage of air through the trap is known as the water seal and is the depth of water which would have to be removed from a fully charged trap before air could pass through the trap. General notes from BS5572:1978 Code of Practice Sanitary Pipework ‘The entry of foul air from the drainage system into the building is prevented by the installation of suitable traps which should be self-cleansing. A trap which is not an integral part of an appliance should be attached to and immediately beneath its outlet and the bore of the trap should be smooth and uniform throughout. All traps should be accessible and be provided with adequate means of cleaning. There is advantage in providing traps which are capable of being readily removed or dismantled.’ Depths of water seals as recommended by BS5572
Water closets-minimum 50 mm Other appliances-minimum 75 mm for traps up to and including 50 mm diameter. Minimum 50 mm for traps over 50 mm diameter.