(1)
Except as provided in Sentence (4), where a stack receives the discharge from fixtures located on more than 11 storeys, a yoke vent shall be installed
(a)
for each section of 5 storeys or part of them counted from the top down, and
(b)
at or immediately above each offset or double offset.
(2)
The yoke vent shall be connected to the stack by means of a drainage fitting at or immediately below the lowest sanitary drainage pipe from the lowest storey of the sections described in Sentence (1).
(3)
The yoke vent shall connect to the vent stack at least 1 m above the floor level of the lowest storey in the section described in Sentence (1).
(4)
A yoke vent need not be installed provided the stack is interconnected with the vent stack in each storey of the section in which fixtures are located by means of a vent pipe equal in nominal pipe size to the branch or fixture drain or NPS 2, whichever is smaller.