VENTILATION
Ventilation of a soil and waste system is necessary to prevent water seals in traps being broken due to negative pressure or pressure fluctuations within the system. Broken seals permit foul air and smells to escape from the system, contaminating the air in and around the building. There are two ways of ventilating a soil stack: either externally to the atmosphere, or internally to a non-inhabited space within a building.

The termination of an externally vented system must comply with the dimensional requirements illustrated in the following diagram.



Where an internal method of ventilation is approved, the stack is terminated, e.g. in the roofspace, using an air admittance valve.



Individual branch pipes require separate ventilation if their length and slope exceed those illustrated in the diagram below.


In circumstances where maximum lengths of branch pipes exceed those permitted by Building Regulations or local Bye Laws, then the branch pipe should be ventilated using a branch ventilation pipe or an anti-vacuum trap. The diagrams below illustrates typical arrangements.



In the case of large numbers of ventilating pipes being required, or if their length is considerable, then a separate ventilating stack, at least 32mm diameter, should be considered.

In addition to the length and slope limitations placed on unvented branch pipes, there is also a maximum number of appliances which can be connected to one branch.The table below details the maximum number of appliances that can be connected to unvented branch pipes.

UNVENTED BRANCH PIPES - APPLIANCE CAPACITY