Technical Guidance

Question:

Is it acceptable to have a ‘Y’ junction on a drain under a building?

Considerations:

• NHBC Standards clause 5.3.4 refers to BS EN 752 for guidance on drainage design.

• BS EN 752 : 3 ‘Drains and sewer systems outside buildings - planning’, says ‘Every drain and sewer length should be accessible for maintenance and cleaning without the need to enter the building’.

• Access to ‘Y’ branches from upstream may be difficult or impossible to achieve and it is generally unacceptable to require the removal of sanitary fittings within the building to clear a blockage.

• Experience has shown that the risk of a blockage occurring in a drain run serving solely a wash hand basin, shower or bath is low
  

Answer:

Apart from the exceptions described below, a drain from a WC, kitchen sink, washing machine, or similar drain carrying heavily

soiled waste, should not connect into another drain under a building. Such drains should run direct to an access point outside the

building which can enable each drain to be rodded without the need to enter the building. An external access chamber may serve

up to three separate branch drains provided the angles of the connections and bends and distance to the rst bend on a branch

comply with diagram 5 below. (Where practicable the advice in Technical Guidance 5.3/01 on the arrangement of the connections

into a pre-formed inspection chamber should be followed).

Exceptions:

1. A drain serving relatively clean waste discharges e.g. from a wash hand basin (WHB), shower or bath, may connect into another drain via a ‘Y’ junction under the building (see diagrams 1 & 6 below).

2. Where the main drain extends the full depth of the building and has external rodding access at both ends*, as shown in diagram 6 below, branch connections may be made via ‘Y’ junctions under the building. Branch drains carrying heavily soiled wastes should be provided with rodding access which does not involve the removal of any sanitary fittings. Therefore, wastes from WC’s, kitchen sinks and washing machines, or similar, should connect into a roddable stub stack or SVP fitted with a rodding access located above the flood level of any adjoining fitting within the ground floor accommodation.

(See Technical Guidance 8.1/25 - regarding provision of rodding access points on SVP’s).

* Where it is not possible to extend the main drain to a rodding access outside the building the head of the main drain may terminate within the building and accessed via a roddable stud stack or SVP fitted with a rodding access located above the flood level of any adjoining fitting within the ground floor accommodation.