Overflow Relief Gully Requirements for Sanitary Drainage

What This Requirement Covers

An overflow relief gully (ORG) is a plumbing device fitted to sanitary drainage to prevent backflow of sewage into a building when the sewer or external drainage becomes surcharged. It provides a controlled, trapped outlet for wastewater from sanitary appliances or internal drainage where a high external sewer level or blockage could otherwise allow foul water to enter the building fabric and occupied spaces. The requirement exists to protect health, amenity and property by ensuring sanitary fixtures can discharge safely during sewer surcharge events while maintaining an effective trap seal to prevent sewer gas entry.

These requirements apply to designers, builders, plumbers and certifiers involved with sanitary plumbing and drainage for all building classes where an ORG is needed as part of the sanitary drainage arrangement. The Plumbing Code of Australia (NCC Volume Three) and related Deemed-to-Satisfy provisions set the design, installation and sizing expectations for sanitary drainage items such as traps, gullies, vents and ORGs; Australian Standards including AS/NZS 3500 series provide detailed methods and sizing guidance that must be followed.

Key Requirements

1. Purpose and location: An ORG must be installed where the external sewer level or point of discharge could permit sewage or effluent to back up into internal sanitary drains or floor wastes. Install an ORG at the junction between the internal sanitary system and the external sewer/drain run where surcharge risk exists, or provide an alternative approved overflow arrangement (NCC Volume Three - Plumbing Code of Australia).
2. Trap invert and connection: The invert level of a trap or gully weir must be at least 10 mm higher than the soffit of the pipe to which it connects, as specified in NCC Volume Three, C1D2 and C2D2 - Invert levels.
3. Trap weir and seal: ORGs incorporate a trapped outlet. Maintain an effective trap seal consistent with AS/NZS 3500.2 and PCC (Plumbing Code) requirements to prevent sewer gas ingress when the ORG is not discharging. The trap seal depth generally follows AS/NZS 3500.2 trap requirements for floor wastes and gullies.
4. Discharge capacity and sizing: Sanitary branch and stack sizing, filling degree and discharge units must be determined in accordance with the Plumbing Code of Australia (NCC Volume Three) using System 1, 2 or 3 principles and the relevant tables (for example Table C1V5a/C1V5b and Table C2V3a/C2V3b for capacities and discharge units). Where an ORG forms part of a branch or relief arrangement, ensure stack and relief vent sizing complies with clauses and tables in NCC Volume Three.
5. Relief venting: Where a separate relief vent is required, relief vents must interconnect with the stack vent at each floor and be sized per AS/NZS 3500.2 and NCC Volume Three clauses on ventilation (for example Clause 8.5.3 and 8.5.4 references in NCC Volume Three). Relief vents may terminate to atmosphere or use approved air admittance valves where permitted (minimum airflow rate for an air admittance valve is 8 times the wastewater design flowrate as set out in NCC Volume Three).
6. Materials and construction: Materials used for ORGs and associated drainage must comply with the material and installation requirements of the Plumbing Code and AS/NZS 3500 series. Use corrosion-resistant materials suitable for buried or exposed conditions. Install access covers or inspection openings to allow clearing and maintenance in accordance with AS/NZS 3500.3 and the Plumbing Code.
7. Accessibility and maintenance: Provide adequate access for rodding, inspection and maintenance - inspection openings should be provided at change of direction points and at the ORG location. Clauses in NCC Volume Three require access for maintenance and clearing blockages as part of surface and subsurface drainage design (see relevant state clauses and E4P3 style requirements).
8. References: Primary references include NCC Volume Three - Plumbing Code of Australia (C1D2, C2D2; ventilation and stack/relief vent clauses and tables), and the AS/NZS 3500 series (particularly AS/NZS 3500.2 for sanitary plumbing and AS/NZS 3500.3 for stormwater where interface with stormwater systems is relevant).

Residential vs Commercial

1. Residential (Class 1 and 10): ORGs are typically required where dwelling floor wastes, laundry or sanitary fixtures connect below or near external sewer invert levels or where local conditions create a surcharge risk. Design commonly follows System 1 or System 2 sanitary arrangements in AS/NZS 3500.2 and the Plumbing Code’s Deemed-to-Satisfy provisions (NCC Volume Three, C2D2/C1D2). In dwellings, attention is often on protecting laundries, basement or subfloor spaces, and floor wastes.
2. Commercial and other non-residential (Class 2 to 9): Higher fixture densities and different usage patterns (frequent or congested use) require sizing to the appropriate frequency factor and discharge unit tables in NCC Volume Three (for example Class 5-9 use factors). ORGs in commercial buildings may need larger capacity, more robust access and dedicated relief venting, and are more likely to form part of designed System 3 (single-stack) arrangements or engineered solutions. Stack/vent sizing and relief provisions in the Plumbing Code (with reference to Table C1V5a/C1V5b and C2V3 tables) should be applied.

Exceptions and Exemptions

1. Where an alternative, equivalent level of protection is provided (for example pumped sewage ejector with appropriate backflow protection, sealed lifting stations, or compliant mechanical backflow devices) the use of a traditional ORG may be unnecessary. Any alternative must achieve the same performance outcomes as required by the Plumbing Code and be documented as a Performance Solution where it departs from Deemed-to-Satisfy provisions (NCC Volume Three - Performance Requirements and Deemed-to-Satisfy pathways).
2. Small, temporary or single-fixture installations may be addressed by other measures (for example isolating valves or localised pumping), but these must be demonstrated compliant with the Plumbing Code and referenced Australian Standards.
3. Some states include specific exemptions or additional controls in their schedules to NCC Volume Three; check the relevant state schedule and local plumbing regulations before omitting an ORG.

State and Territory Variations

1. The NCC Volume Three applies nationally for plumbing, but each state and territory may have schedules or additional requirements. For example:
2. Victoria: VIC E4D2 requires surface and subsurface drainage to be in accordance with AS/NZS 3500.3 and includes state-specific application notes for maintenance and connections to points of disposal (NCC Volume Three, VIC E4D2).
3. Tasmania: TAS E4D2/E4P3 specify surface drainage design concerns including prevention of stormwater entering sanitary drains and require compliance with AS/NZS 3500.3 or housing provisions (NCC Volume Three, TAS E4P3 & E4D2).
4. Other states have similar schedules within NCC Volume Three; always verify the state schedule (Schedules 4-12 in NCC Volume One/Three) for local amendments affecting ORG siting, materials, or alternative acceptable solutions.

Practical Compliance Tips

1. Locate ORG where accessible - position ORGs with clear access for inspection, rodding and maintenance; provide an inspection opening nearby. This avoids costly excavation for routine clearing.
2. Observe the 10 mm invert rule - ensure the trap or gully weir invert is at least 10 mm higher than the soffit of the connecting pipe to comply with NCC Volume Three C1D2/C2D2.
3. Follow system selection - select the correct sanitary system (System 1, 2 or 3) and apply the correct filling degree and discharge unit tables from NCC Volume Three when sizing stacks and relief vents.
4. Vent correctly - where a separate relief vent is needed, interconnect it with the stack vent at each floor and size per AS/NZS 3500.2 and the Plumbing Code; consider air admittance valves only where permitted and sized at 8 times the wastewater design flowrate if used (NCC Volume Three).
5. Document alternatives as Performance Solutions - if using pumps, ejectors or mechanical backflow devices instead of an ORG, prepare a Performance Solution demonstrating equivalence to the Deemed-to-Satisfy outcomes in NCC Volume Three.
6. Check state schedules early - consult the relevant state/territory schedule in the NCC for any additional requirements or limitations before finalising design or tender documents.
7. Use compliant materials and provide access - use materials and installation methods in accordance with AS/NZS 3500 series and provide rodding/inlet points per the Plumbing Code and AS/NZS 3500.3 to simplify future maintenance.

References: NCC Volume Three - Plumbing Code of Australia (see C1D2, C2D2, ventilation and stack/relief vent clauses and associated tables), NCC Volume One - Part F (F2 Wet areas and overflow protection), and the AS/NZS 3500 series (particularly AS/NZS 3500.2 and AS/NZS 3500.3) for detailed installation and sizing methods. Note: check the applicable state schedule within the NCC for any local amendments or additional provisions.