Use of flexible joints in drains

Issue

This advisory note has been prepared for industry outlining the Deemed-to-Satisfy (DtS) requirements for the use of flexible joints in sanitary drainage systems as prescribed in Schedule 3 of the NSW Plumbing and Drainage Regulation 2017.

Regulation summary

In NSW some installations may not be required to comply with Clause C2P7(1) of the Plumbing Code of Australia when avoiding damage to a sanitary drainage system from ground movement by meeting the Deemed-to-Satisfy provisions set out in Schedule 3 of the Plumbing and Drainage Regulation 2017.

Schedule 3 prescribes:

• Flexible Joints must be used in a drain where it has been laid in ground that is:
  • Filled, or
  • Unstable, or
  • Water-charged, or
  • Soil class H1, or
  • Soil Class H2, or
  • Soil Class E, and
  • In the opinion of a recognised expert is likely to move and the movement would be likely to cause damage to the drain and surrounding structures.
• A design must be prepared for a sanitary drainage system by a recognised expert.
• Recognised expert qualifications and education.
• The sanitary drainage system must be installed in accordance with the design.
• The design must be supplied to the plumbing regulator or their delegate with a Notice of Work.

For more information, see the technical advisory note on drains in other than stable ground.

Regulation requirements

How to meet the Schedule 3 Deemed-to-Satisfy requirements

1. a site-specific design must be prepared by a recognised expert detailing the location of flexible joints and any additional methods of support for the sanitary drainage system;
2. the design must be provided to the Local Plumbing Regulator at the same time as the Notice of Work, and;
3. the installation must be in accordance with the design.

What should be included in the design?

The following should be included in the design: 

• Reason for design i.e. Soil Classification Type or Approval Conditions such as mine subsidence.
• Detailed plan of proposed drainage system including location and type of flexible joints¹ required by the expert for the in ground sanitary drainage internal and or external to a building.
• Legend and details sheets indicating types of joints to be used and specified joint arrangement.Recognised experts’ details and qualifications
• Any additional methods to support or secure the drainage.

Note¹ – Flexible Joint as defined in AS/NZS 3500.0 is a joint permitting movement, expansion, contraction and rotation. The Type of Flexible joints specified within a design must be a ‘WaterMark’ approved drainage fitting designed to be suitable to accommodate differential ground movement. 

Who can prepare a flexible joint design?

A recognised expert, as defined in Schedule 3 (Definitions) of the Plumbing and Drainage Regulation 2017 is required to prepare the design. 

A recognised expert means a person who—

1. holds a Diploma of Hydraulic Services Design, or
2. is recognised or registered as a professional engineer in one of the following areas by a professional body of engineers that operates with a professional standards scheme—
   1. building services discipline,
   2. civil engineering,
   3. geotechnical engineering,
   4. mechanical engineering,
   5. structural engineering, or
3. is recognised by the plumbing regulator as having appropriate education, qualifications or skills.

Regulatory commentary

AS/NZS 3500.2:2021 - Sanitary Plumbing and Drainage

Appendix G Drains in unstable soils, is of an informative and guidance nature only and is not a mandatory requirement of the standard.

Appendix G is for information on the installation of plastic pipe drainage systems in unstable soils to Residential (Class 1) buildings in Australia.

Appendix G does not provide design advice. Advice from suitably qualified engineer is recommend to ensure that the solution proposed is appropriate for the conditions of the site.

AS 2870-2011 - Residential slabs and footings  

AS 2870-2011 provides special design considerations for drains associated with residential slab or footing systems on moderately, highly or extremely reactive soils.

AS 2870-2011 is currently not prescribed as a normative reference document in the Plumbing Code of Australia or the AS/NZS 3500.2:2021.