TECHNICAL GUIDANCE
CURTAIN WALLING
‘Y’ JUNCTIONS FORMED
IN DRAINS FROM
UNDER BUILDINGS
STACKED WINDOW ARRANGEMENTS
–
5.3/04 – December 2016 (Fourth issue - supersedes Jan 2016)
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 first 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.
NHBC
NHBC House, Davy Avenue, Knowlhill, Milton Keynes, Bucks MK5 8FP
Tel: 0344 633 1000 Fax: 01908 747 255 www.nhbc.co.uk page 1 of 2
TECHNICAL GUIDANCE
6.1/25 –- December
5.3/04 2015 2016
WC, Kitchen Sink or Utility WHB, bath or shower WHB, bath or shower ANY
WHB, bath or shower WC, Kitchen Sink or Utility
WC, Kitchen Sink or Utility
No Access Point For Rodding
Diagram 1 Diagram 2 Diagram 3 Diagram 4
Rodding access outside building
alternative rodding access at A branch connection on to a
head of main drain when branch is acceptable if it serves
terminated inside building a WHB, bath or shower
Drains serving any wastes
WHB, bath or shower
WC, Kitchen Sink or
Utility (Connected to
branch drain via
roddable stub stack/
SVP)
45 degrees max
45 degrees
max
2 m max. between
access chamber Access chamber
Access chamber
and first bend
Diagram 5 Diagram 6
(See Technical Guidance 5.3/01 on drain connections to chamber)
NHBC
NHBC House, Davy Avenue, Knowlhill, Milton Keynes, Bucks MK5 8FP
Tel: 0344 633 1000 Fax: 01908 747 255 www.nhbc.co.uk page 2 of 2
