1.3 Gully Traps, Water Seals & Overflow Relief Gullies (ORG)

Compliance note: Under G13/AS2 the gully water seal is at least 65 mm. The overflow level is at least 25 mm above paved ground or 75 mm above unpaved ground, the top of the water seal is no more than 600 mm below the dish, and housing gullies sit at least 150 mm below the lowest fixture overflow. Concrete support is required where physical damage is likely; otherwise use the specified compacted bedding.

Gully Traps, Water Seals & Overflow Relief Gullies (ORG)

In New Zealand sanitary drainage design, the Gully Trap and Overflow Relief Gully (ORG) serve two critical, interrelated functions: they isolate sewer atmosphere from the living environment using a liquid barrier, and they provide an external sacrificial relief point during hydraulic surcharge. These systems are governed by NZBC Clause G13/AS2 (Section 3) and AS/NZS 3500.2:2021 (Clause 4.6).

Without a functioning, properly positioned ORG, any downstream blockage in the private lateral or municipal reticulation will force raw sewage to back up into the lowest connected interior fixture—typically a shower tray, bathtub, or floor waste gully. The certifying drainlayer must understand the precise dimensional clearances, structural support requirements, and hydraulic mechanics required to ensure compliant operation.

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1. Operating Mechanism of the Overflow Relief Gully (ORG)

The fundamental operational principle of an ORG is hydraulic fail-safety:

+--------------------------------------------------------------+
|               HYDRAULIC SURCHARGE RELIEF MECHANISM           |
|                                                              |
|   Dwelling Interior                 Exterior Yard            |
|   -----------------                 -------------            |
|   Shower Waste Lip                                           |
|   [RL 12.450 m]                                              |
|         |                                                    |
|         | === 150 mm MINIMUM VERTICAL SAFETY DROP ===        |
|         |                                                    |
|         |                           ORG Grating              |
|         |                           [RL 12.300 m]            |
|         |                                 |                  |
|         |                                 +--- (SEWAGE EXITS |
|         |                                 |     HARMLESSLY   |
|         |                                 |     INTO YARD)   |
|         v                                 v                  |
|   ==================================================         |
|                   Private Sewer Drain                        |
|                   [BLOCKAGE DOWNSTREAM] <--- BACKUP OCCURS   |
+--------------------------------------------------------------+

When a downstream blockage occurs, sewage backing up the pipe reaches the elevation of the external ORG grate before it reaches the internal fixtures, provided the mandatory 150 mm vertical drop is maintained. The loose-fitting grating lifts or overflows onto external ground, relieving pressure and warning the occupants without contaminating indoor floor coverings, framing timber, or plasterboard.

ORG Grate Requirements

• Loose-Fitting Grating: The grating must never be screwed, cemented, or bolted down. It must be a lightweight, perforated plastic or cast-metal grid designed to dislodge freely under hydrostatic surcharge.
• Unobstructed Spillway: The area immediately surrounding the ORG must allow water to flow away by gravity into gardens or lawns. It must never be enclosed within an unvented porch, greenhouse, or beneath low decking lacking access panels.

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2. Mandatory Clearances: Vertical & Ground Heights

The positioning of the ORG grating requires balancing two opposing dimensional constraints: it must sit low enough to protect interior fixtures, yet high enough to exclude external surface stormwater.

+--------------------------------------------------------------+
|                 MANDATORY ORG CLEARANCE PROFILE              |
|                                                              |
|   Internal Fixture Lip (Lowest Shower/Bath Outlet)           |
|   ========================================================   |
|              |                                               |
|              | >= 150 mm Vertical Drop                       |
|              v                                               |
|         ORG Grating Lip                                      |
|         -------------                                        |
|              |                                               |
|              +-----------------+                             |
|              |                 |                             |
|              | >= 75 mm        | >= 25 mm                    |
|              | Unpaved Ground  | Paved Surface (Sloping 1:40)|
|              v                 v                             |
|          [Garden/Lawn]     [Concrete Patio/Driveway]         |
+--------------------------------------------------------------+

1. Vertical Safety Clearance to Lowest Internal Fixture

• The Rule: The top of the ORG grate must be located at least 150 mm below the flood level rim (overflow lip) of the lowest connected sanitary fixture inside the building (e.g., shower waste outlet, low-set bath rim, or floor waste gully).
• Slab-on-Grade Challenge: In modern concrete slab-on-ground construction, achieving this 150 mm difference is one of the most common site errors. If the concrete floor slab sits only 150 mm above finished ground, and a level-entry walk-in shower is installed, the shower waste lip may be only 50 mm above ground. In such cases, the external ground at the gully must be sculpted, battered down, or provided with an engineered retaining well to maintain the 150 mm clearance.

2. Clearance Above Ground Surfaces (Stormwater Ingress Prevention)

To prevent surface rainwater and garden wash from pouring into the foul sewer, the top of the gully rim must maintain strict clearances above adjacent finished surfaces:

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3. Water Seals, Evaporation & Trap Charging

A gully trap incorporates an integral P-trap or U-bend water seal that acts as an impermeable physical barrier between toxic sewer gases (hydrogen sulphide $H_2S$, methane $CH_4$) and the atmosphere.

Water Seal Dimensions

• NZBC G13/AS2: Mandates a minimum water seal depth of 65 mm for gully traps.
• AS/NZS 3500.2:2021: Specifies a minimum water seal depth of 65 mm for overflow relief gullies.
• Water Loss Mechanics: In arid, windy, or heated environments, water seals lose approximately 1 to 2 mm of depth per day through natural evaporation. Siphonage from downstream peak drainage flushes can also deplete an unvented water seal.

Trap Charging (Mandatory Upstream Fixture Connection)

An uncharged gully trap is an illegal installation under both New Zealand and Australian codes:

• Charging Fixture Requirement: A gully trap must receive the discharge of at least one frequently used, permanently plumbed fixture to maintain the liquid seal. Typical compliant charging fixtures include a laundry wash tub, a bathroom vanity basin, or a kitchen sink.
• Untrapped Waste Pipes: Waste pipes discharging into an open gully hopper must terminate above the water seal with a clear air break:
  • The waste pipe outlet must terminate at least 25 mm above the overflow level of the gully dish.
  • The outlet must terminate not less than 50 mm above the water seal surface.
  • This air gap prevents dirty water in the gully from siphoning back into the waste pipe and prevents bacterial slime from climbing into internal fixture wastes.

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4. Structural Support: Bedding Footings & Concrete Surrounds

Gully traps represent an interface between the building's fixed plumbing and external soil movement. Differential settlement, lawnmower impacts, or foot traffic can easily fracture the vertical riser or dislodge the trap.

+--------------------------------------------------------------+
|               CONCRETE SURROUND & FOOTING CROSS-SECTION      |
|                                                              |
|             Loose Grating [Removable]                        |
|                   +-------+                                  |
|   Finished Ground |       |  75 mm Wide Concrete Collar      |
|   ================+       +================                  |
|   |  Concrete     | Riser |    Sloping Concrete Surround     |
|   |  Apron        |       |    (17.5 MPa)                    |
|   +---------------+       +---------------+                  |
|                   |       |                                  |
|                   | P-Trap|                                  |
|                   +--   --+                                  |
|                      \ /                                     |
|             =====================                            |
|             100 mm Concrete Pad                              |
|             or Compacted Metal                               |
+--------------------------------------------------------------+

Construction Specifications (NZBC G13/AS2 Clause 3.3)

1. Base Support Footing: The gully trap fitting must rest on a stable bed of compacted granular bedding (crushed stone 7–14 mm) or a poured concrete bedding pad of at least 75 mm to 100 mm thickness (minimum 17.5 MPa compressive strength). It must never sit directly on uncompacted clay trench backfill.
2. Concrete Surround (Apron Collar): A continuous concrete collar must be cast around the gully hopper. The surround must be at least 75 mm thick and extend at least 100 mm laterally around the perimeter of the dish. The concrete surface must be trowelled to slope away from the grate rim to shed splashing water and exclude surface runoff.
3. Riser Pipe Extensions: Where deep drains require a gully riser exceeding 600 mm, the riser must be constructed from DN 100 uPVC pipe securely socketed into the P-trap, and the entire assembly must be backfilled with compacted gravel to prevent ovalization.

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5. Worked Numerical Example: Slab-on-Grade Elevation Check

A certifying drainlayer is setting out the drainage for a modern single-storey residential dwelling on a concrete raft slab.

+--------------------------------------------------------------+
|                ELEVATION CHECK & WORKING WINDOW              |
|                                                              |
|   Finished Floor Level (FFL) = RL 100.500 m                  |
|   Shower Drain Invert        = RL 100.480 m (Lowest Fixture) |
|                                                              |
|   Maximum ORG Grate Level    = RL 100.480 - 0.150            |
|                              = RL 100.330 m [CEILING]        |
|                                                              |
|   Finished Garden Level      = RL 100.220 m                  |
|   Minimum ORG Grate Level    = RL 100.220 + 0.075            |
|                              = RL 100.295 m [FLOOR]          |
|                                                              |
|   => PERMISSIBLE WORKING WINDOW: RL 100.295 m to 100.330 m   |
|      TOLERANCE BAND = EXACTLY 35 mm                          |
+--------------------------------------------------------------+

Site Survey Data

• Finished Floor Level (FFL) of concrete slab: $\text{RL } 100.500\text{ m}$.
• Lowest internal sanitary fixture: An ensuite walk-in tiled shower with its floor waste lip set at $\text{RL } 100.480\text{ m}$.
• Finished ground level outside the bathroom wall (unpaved lawn): $\text{RL } 100.220\text{ m}$.

Step 1: Calculate Maximum Permissible ORG Grate Elevation

To satisfy the 150 mm vertical safety drop:

$$\text{Max Grate Elevation} = \text{Shower Lip Level} - 0.150\text{ m}$$ $$\text{Max Grate Elevation} = 100.480 - 0.150 = \text{RL } 100.330\text{ m}$$

Step 2: Calculate Minimum Permissible ORG Grate Elevation

To satisfy the 75 mm clearance above unpaved lawn:

$$\text{Min Grate Elevation} = \text{Finished Ground Level} + 0.075\text{ m}$$ $$\text{Min Grate Elevation} = 100.220 + 0.075 = \text{RL } 100.295\text{ m}$$

Step 3: Determine the Operational Compliance Band

$$\text{Tolerance Window} = 100.330 - 100.295 = 0.035\text{ m} = 35\text{ mm}$$

Engineering Assessment

The certifying drainlayer must set the top of the ORG hopper between $\text{RL } 100.295\text{ m}$ and $\text{RL } 100.330\text{ m}$—a narrow operational window of only 35 mm. If the landscaper later adds 50 mm of topsoil and bark mulch to the garden, the ground rises to $\text{RL } 100.270\text{ m}$, reducing clearance below the mandatory 75 mm threshold and causing non-compliance. The drainlayer must advise the site supervisor in writing to install a concrete retaining border around the gully apron.

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6. Trade Traps in Gully Trap Installation

+-------------------------------------------------------------------------+
|                        DRAINLAYER TRADE TRAPS                           |
|                                                                         |
| [!] THE CONCRETE ENCASEMENT BOLT DOWN                                    |
| Concreting an external path around an ORG and screwing a brass or       |
| heavy iron grate into the surround to keep it neat. During a surcharge, |
| the grate cannot lift, forcing raw sewage to erupt through the indoor   |
| shower waste.                                                           |
|                                                                         |
| [!] THE DRY GULLY TRAP                                                  |
| Directing all internal bathroom wastes into stacks and installing an    |
| external ORG that receives zero charging fixtures. Within weeks, the    |
| water seal evaporates, venting raw sewer gas beside bedroom windows.    |
|                                                                         |
| [!] THE LANDSCAPE BURIAL                                                |
| Cutting an ORG riser flush with rough clay subgrade before landscaping. |
| The landscaper spreads topsoil and turf over the gully, burying the     |
| grate below ground level where it acts as a stormwater catchpit.        |
+-------------------------------------------------------------------------+