Which document covers your plumbing?

Check the building height first.

Four documents can govern a New Zealand building's sanitary plumbing and drains. Height decides first, then whether the work is above or below ground, then which edition of AS/NZS 3500.2 you use.

It comes down to two routes.

G13 lists three acceptable solutions, but in practice they make two routes: G13/AS1 above ground with G13/AS2 below it, or AS/NZS 3500.2 for both, which is what G13/AS3 brings in. Follow either and the plumbing and drains are treated as complying. Anything else is an alternative solution, which the building consent authority has to be shown meets the clause.

• Ways to comply, on the G13 page

Close to 3500.2's fully vented system, with three differences.

G13/AS1 is the one most houses are designed to: gravity plumbing above ground, sized by discharge units. Set beside AS/NZS 3500.2's fully vented system:

• What is vented: G13/AS1 vents every discharge pipe but the stack's highest connection and the other cases Table 5 lists. Fully vented vents every trap but one discharging to a floor waste gully.
• Height: G13/AS1 stops at 3 levels. At ≤ 3 levels a DN 100 stack takes the same in both: 195 units, 65 from one floor. At ≥ 4 levels fully vented takes 500, 125 from one floor, with no height limit.
• Fixtures: G13/AS1 takes those its Table 2 lists and no specialised fixtures. Fully vented has no list limiting them.

G13/AS1 1.0.1 to 1.0.3, Tables 2, 4, 5 · AS/NZS 3500.2 8.2.2, Table 8.2.2(B)

• Who covers what, on the G13 page
• All five venting systems compared

The drains that go with G13/AS1.

Below ground, G13/AS2 sizes drains ≤ 150 mm from the same discharge units G13/AS1 counts above, whatever the building's height.

G13/AS3 is the 2021 edition of AS/NZS 3500.2, changed in five places.

G13/AS3 is one page: it points to the whole of AS/NZS 3500.2:2021 with Amendment 1, plumbing and drains together, and changes the five places listed here. It has no level limit, so it is how a taller building's plumbing complies.

• AS/NZS 3500.2, on its own page

Usable, but you have to show it complies.

The 2025 edition is the current one, but G13 has not cited it yet. A design to it as it stands is an alternative solution: usable, with the authority to be shown it meets G13.

Try it on your own plans.

Put your fixtures on your own floor plans and see what it sizes, vents and draws. The tool is still young, so treat it as a second pair of eyes and check what it gives you. Or start with the three-storey house: plumb it in 3D and watch the rules at work.

• Try Drainage design
• Plumb the 3D house