chapter
Interpretations and Statutory Requirements (1)
CI/SfB52 BS5572:1968 SANITARY PIPEWORK ABOVE GROUND WATER SEALS IN TRAPS
Pages 6

Section 1.12-sizes of branch pipes. Pipes serving a single appliance should have at least the same diameter as the appliance trap (see Table 1), if a pipe serves more than one appliance and is unventilated, the diameter should be at least the size shown in Table 2.