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New Zealand Plumbers, Drainlayers & Gasfitters Association Inc

New Zealand Plumbers, Drainlayers & Gasfitters
Association Inc

Latest News / Top Tech Tip - Basic drainage and plumbing inspection points for domestic installations

Basic drainage and plumbing inspection points for domestic installations

The bullet points below do not purport to cover all aspects and requirements of the drainage and plumbing requirements under G12 and G13. They are a simple list (with examples) of the most common inspection failure points, which, if done correctly, will reduce non-compliance and provide at least a minimum degree of quality assurance.

In all cases AS/NZS 3500.1 (cold water), AS/NZS 3500.2 (foul water/ drainage) and AS/NZS 3500.4 (hot water) are used as the Means of Compliance (MoC).


·         Trenches to be minimum depth (as below) and minimum width 200 wider than drain dia., so normally 300mm.  

·         Compacted base material minimum 100mm deep, side compaction 100mm.


·         Maximum practicable fall, minimum 1.65% for 100mm pipe, 2.5% for 65mm.

·         Minimum separation distances from other services.

·         Note distance out from building line, and time exposed.

·         Minimum depth of cover, 500 subject to vehicles, 300 otherwise.

·         ORG as near to top end of drain as possible.

·         Minimum main drain vent 50mm up to 30 FUs, over that 65mm.

·         Main vent must be within 10m of top of drain, and below highest connection.

Underfloor (sub-slab) drainage (AS/NZS 3500.2 part 3)

·         Pipes should be laid under concrete footings, but if they do penetrate then they must be sleeved.


·         Minimum 65mm dia, including outlet of FWG.

·         Minimum 25mm overlay between slab and pipe.

·         Protected as it passes through slab.

Cold water supplies

·         PLV (normally 500 kPa) fitted in accessible position.

·         Separation of water pipe from other services, usually 100mm minimum.

·         Silicon where pipe penetrates timber, grommets on metal studs.

·         Tested to 1,500 kPa, PS3 required if inspector not present.

·         Protection of pipes from damage, including ultra violet.

·         If water harvested from roof, must have leaf strainer and first flush devices as  minimum. Pipes to be lilac colour, only connected to non-personal hygiene fixtures and outlets labelled.

Hot water system

·         HWC to have safe tray, 40mm waste. Provision for discharge.

·         TPR, relief and CWEV to discharge point.

·         Properly positioned and fastened seismic straps.

·         AS/NZS 3500.4 absolute minimum, lagging to sink and for first 2 m from HWC. (see H1)

·         Instantaneous WH positioned away from openings, valves on cold water and  gas, pipes protected.

·          Maximum temperature at personal hygiene fixtures.

Foul water plumbing.

·       Determine system, use following characteristics as guide, but ensure all specific rules applied.

·         G13 AS1. Fully vented;

·          Every fixture or appliance vented except single pipes to gully if less than 3.5m (uncommon).

·         Elevated drainage (Drainage Principles):

·         Minimum (horizontal) pipe dia, 65mm.

·         Maximum unvented branch length, 10m.

·         All connections on horizontal except top floor may connect to stack on vertical. Otherwise no connections on vertical.

·         Fixture discharge pipes to floor level at outlet size (normally 40mm) but change to minimum 65mm at floor level.

Single stack

·         100mm main stack, if 80mm used then severe restrictions on what may be discharged to stack, including no WC, laundry Tub or clothes washing machine.

·         All fixture discharge pipes individually connected to stack, NO COMBINED WASTES.

·         Maximum length of branch (fixture) pipe 2.5 m.

·         SS has both minimum AND maximum gradient for discharge pipes.

·         No vents on system except stack continues as vent.

Fully vented modified

·         Waste pipes can be combined.

·         No individual vents, but vents on branch.

·         Branch vent to be situated between top and next fixture.

·         Fixture pipes maximum 2.5m to where it joins combined branch.

Floor waste gullies.

·         Some fixtures, such as kitchen sinks, must not discharge to FWG.

·         Absolute maximum from fixture to FWG, 2.5m trapped, 1.2m untrapped.

·         Outlet size from FWG depends on FUs, except if Drainage Principles being used then min 65mm.

·         FWG is deemed to be a fixture trap, so distance to stack etc. taken from outlet of FWG.

·         FWG must be in same room as fixture discharging to it, except for tundish (from HWC for example).

General rules.

·         Except for main vents, vents can be replaced with Air Admittance Valves (AAVs), note rules on positioning and accessibility.

·         Trap incorporating AAV can only be used as fixture vent, not branch vent.

·         For roll over (horizontal or offset) sections, note restricted zones at change of direction, base of stack etc. these may differ between systems.



·         Eccentric taper to be fitted immediately downstream of trap, with soffit at top level with pipe.


·         When a junction meets a horizontal (graded) pipe, it must be fitted at an angle which ensures there is a minimum of 10mm fall from the invert of the entering branch to the soffit of the main branch.


Note: At a mid-floor joist it is usually easier to fit an equal junction slightly twisted to give fall, then fit an eccentric taper.

·         Ensure rules on holes and notches are observed (from NZS 3604 for wooden structures) or if oversized holes are drilled an approved stiffener is used for top plates, studs or floor joists.

·         Note there are different rules for solid timber members and Laminated Ibeams for example. Manufacturer’s instructions must be followed.

·         Clips and brackets required as per standard, depending on material, size and whether vertical or horizontal.

·         Pipe diameters will depend on loading (fixture units) and minimum gradients on pipe diameters, except for single stack systems which have both minimum and maximum gradients.

NOTE: All plumbing and drainage installations must follow the consented plans, using the system shown in the plans. While minor changes within the system can be dealt with by an on-site minor variation (MV), any change of system (say from single stack to fully vented modified, will require a full amendment.

It is expected that plumbers and drainlayers are fully competent and have a full understanding of the Building Code clauses and are able to interpret and apply both G12 and G13 acceptable solutions, as well as the AS/NZS 3500.series of standards.

It is a requirement of the Building Act that both minor variations and amendments must be applied for prior to the work commencing.

Garry Cruickshank

Auckland Council

Senior Specialist- Building

Targeted Initiatives Team

Licencing and Regulatory Compliance

Phone 021 805 870

35 Graham Street, Auckland 1010




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