(1) Except as permitted in subsection (4), there shall be provided immediately beneath every waste-water fitting an approved trap having a removable cleaning-plug or other approved means of cleaning situated below the water-seal.

(2) The nominal bore of such a trap shall be not less than—

a) the size of the waste-water fitting outlet to which it is attached; or

b) 32 millimetres in the case of a wash-hand basin and 40 millimetres in the case of a sink or a bath;

whichever is the greater.

(3) The depth of the water-seal in a trap serving a waste-water fitting shall in no case exceed 100 millimetres, and shall be not less than 40 millimetres in a two-pie system and not less than 75 millimetres in a one-pipe system or single-stack system.

(4) Notwithstanding the provisions of subsection (1) –

a) a bath, a shower, a wash-hand basin or a sink used only for laundry purposes may discharge without the interposition of a trap into an open channel which—

i) is semi-circular in cross-section and has a diameter of at least 100 millimetres and is made of glazed ceramic material or other approved material; and

ii) is accessible for cleaning throughout its length and its placed immediately beneath the point or points of discharge; and

iii) discharges into a trap capable of receiving the discharge and having a corrosion-resistant metal grating;

b) a bath, a shower or a wash-hand basin installed in a compartment containing a urinal or squat pan may, with the prior written approval of the local authority, discharge into the urinal channel, or squat pan, if the outlet from any such bath, wash-hand basin or shower is above the highest part of the urinal channel or squat pan;

c) a range of wash-hand basins or sinks may discharge into a common waste-pipe without the interposition of traps:

Provided that such common waste-pipe does not exceed two metres in length, has a diameter of at least 50 millimetres and is provided with a trap as described in subsection (3) beneath the final fitting.