Technical Booklet N Drainage·Page 17·2.15
Ventilation of discharge stacks
Discharge stacks must be ventilated except for stub stacks complying with paragraph 2.16. The upper part of a stack above the highest branch may be reduced to 75 mm. Stack termination must be in external air at least 900 mm above any building opening within 3 m with an unobstructing cage or cover, or fitted with an air admittance valve complying with BS EN 12056: Part 2 and BS EN 12380.
A discharge stack (other than a stub stack complying with paragraph 2.16) should be ventilated. The diameter of the part of a stack above the highest branch pipe may be reduced to 75 mm where the stack is of a greater diameter.
The upper end of a stack should terminate either –
(a) in the external air at least 900 mm above any opening into a building within 3 m, with a cage or cover which does not restrict the air flow (see Diagram 2.3); or
(b) with an air admittance valve which complies with BS EN 12056: Part 2 and BS EN 12380.
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