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Changes to branch drain connections

One of the changes to the NCC 2022 includes adoption of the revised edition of AS/NZS 3500.2:2021 - sanitary plumbing and drainage.

It will require plumbers and builders to pay close attention to the design and installation for the branch drain connections for sanitary plumbing installations for two storey houses.

There is potential if you have not designed the sanitary drainage system with this change in mind, that the first-floor ceiling joists may not be deeper enough to accommodate the added grade of the pipe. 

What are the changes?

The revised edition of AS/NZS 3500.2 amends Clause, and, Clause, and These clauses have been brought into place to add clarity around when to apply figure 4.9.1(a). It applies to the typical connection arrangements for graded branch drains entering the main drain.

The changes to AS/NZS 3500.2 relates to junction connections where the main drain and the branch drain are both DN100mm.

Under the new edition of the standard, which will be adopted alongside NCC 2022. It will require that the entry level of branch drain be elevated at an incline of no less than 15 degrees (a minimum grade of 1:3.732) above the horizontal.

This is to prevent back wash and strandings at the junctures and in the branch drain from upstream connection, which have been found to cause blockages. 

Figure 4.9.1 typical arrangement for graded branch drains entering main drain

When do these requirements apply? 

The new requirements will apply to new connection/junctures where the main drain and the branch drain are both DN100.

This does not alter the requirements for a fixture connected to a graded discharge pipe (as per clause or junctures installed at grade (as per clause and the installations must follow each applicable requirement.

For installations where the main drains and branch drains are not DN100; the work is repair work; or where WC pans are not installed up stream, the requirements are only recommended. The standard still allows the branch drain to be connected on grade.

Requirements for an unequal junction (where the branch drain is smaller than the main drain) with a branch drain having a 10mm minimum invert level above the soffit of the main drain (in accordance with figure 4.9.1(b)) will still apply. However, they do not need a 15-degree inlet as per figure 4.9.1(a). 

How will these changes affect your design? 

The change to entry level (grade) will cause an increase to the depth of main drain. This depth will be directly proportionate to the distance from the change in direction in 45-degree bends to the junction.

The below table shows how the distance from the change in direction at the 45-degrees to the point of junction will affect the increases depth of the main drain.

Distance from the 45-degree (bend) to the juncture Change in depth from the 45-degree (bend) to the main drain
200mm 54mm
300mm 81mm
400mm 108mm
500mm 134mm
600mm 161mm
700mm 188mm
800mm 215mm
900mm 242mm
1000mm 374mm

All measurements have been rounded up to the nearest whole millimetre.

When the plumber is setting out their work, they will have to be aware of the added grading/depth requirements to ensure it will not affect the design. Furthermore, they will need to ensure that these depths do not affect the angle of repose/angle of influence of the building or the adjoining properties structures.

If you have a main drain (DN100) within a ceiling space and there is a branch connection of DN100mm you will have to assess if the ceiling space will be deep enough to accommodate the added grade of the pipe.

What can I do when I have limited space?

In scenarios where ceiling space is limited you may wish to consider.

  • Using a male to female 45-degree junction to connect into the Y junction at the main drain.
  • Locating the juncture (100DN to 100DN) as closes as possible to the stack, this will reduce the area of the roof space effected by the increased fall.
  • Increase the number of stacks, which may cut the need for a juncture or reduce the length of pipes in the ceiling space.
  • Changing the orientation of the joists and or posi-struts to better accommodate the plumbing and which will in turn reduce the impact of the added fall on the ceiling space.
  • Change the location of the fixtures and/or the stacks to better accommodate the new requirements.

What action should you take?

It is recommended that you make your plumber aware of the changes and discuss how these changes may affect your design. This will limit issues with plumbing inspections your build.

To find out more, contact HIA’s Building Services team.

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