AS/NZS 3500.2 Sanitary Plumbing and Drainage

Its four ways of venting a building's sanitary plumbing, what each asks for and the sizes to design to, and which edition a consent is read against.

AS/NZS 3500.2:2025
G13/AS1Second Edition Amendment 102 Nov 2023

ToolDrainage design. Sizes, vents and draws the plumbing on your own plans, to G13/AS1 and AS2 or to this standard's systems.Open →

G13/AS3 cites: 2021 + A1Changed at five clauses, an acceptable solution
Current edition: 2025Replaced 2021 at Standards New Zealand
A design to 2025: Alternative solutionFor the building consent authority to accept

Four systems

They differ in one thing: how air reaches the pipes, so that water leaving one fixture does not pull the water out of another's trap. The more venting a system has, the further a fixture can be from its stack; the less it has, the closer and fewer the fixtures have to be.

Fully vented

Section 8

Every trap has its own vent, joined into branch vents and up to the stack vent, with a relief vent beside a taller stack. Any fixture, any layout, any height.

Trap vent, trap DN 40 / DN 50 to 100DN 32 / DN 40Table 8.5.1.3

Where a trap vent joins, basin or bidet / any other fixture, from the trap75–600 / ≤ 1500 mm8.5.1.1

DN 100 stack, ≤ 3 floor levels, per floor / whole stack65 / 195 FUTable 8.2.2(B)

DN 100 stack, 4 or more floor levels, per floor / whole stack125 / 500 FUTable 8.2.2(B)

Most one floor may bring, short / tall stacks33 / 25 %8.4 (c)

Floors between a stack's highest and lowest branches that call for a relief vent1 or more8.5.3.1

Fully vented modified

Section 8

Fixtures grouped on one branch share a group vent instead of a vent each; every branch into the stack is still vented. Bathrooms and toilet rooms, with far fewer vents.

Group vents on a branch1 per 108.5.7.1

First group vent, from the most upstream fixture's trap≤ 1.5 m8.5.7.2 (a)

A fixture's own pipe to the branch, any / DN 100≤ 2.5 / 6 m8.5.7.5.5 (b) to (d)

Drop along the group-vented branch≤ 1.5 m8.5.7.5.2

Group vent, branch DN 65 / DN 100DN 40 / DN 50Table 8.5.7.3

The topmost connection takes its own trap vent beyond, WC / other6 / 2.5 m8.5.1.2

Single stack

Section 9

No trap or branch vents: each fixture runs its own short pipe to a stack of at least DN 100, whose own vent is the only air. Wet rooms stacked over each other, close to the stack.

DN 100 stack in a home: load, floor levels260 FU, ≤ 10Table 9.7.1(A)

Other buildings, DN 100 stack: load, floor levels60 FU, ≤ 4Table 9.7.1(B)

A fixture's own pipe, unvented, waste / WC DN 100≤ 2.5 / 6 mTable 9.5.1

Its grade, waste / WC2.5–5 / 1.65–5 %Table 9.5.1

Of each fixture, per floor per stack, in a home≤ 29.4.1

Vertical drop in a fixture's pipe, basin or bidet / other≤ 1.5 / 2.5 m9.5.5

The stack vent may be DN 50 on a stack this low, carrying this little3 floors, 30 FU9.6.1

Two fixture pipes may join this near the stack≤ 1 m9.5.6

Single stack modified

Section 9

Fixtures connect as in the single stack; a relief vent beside the stack, tied in by cross-vents, lets the same DN 100 stack take more load or more floors.

DN 100 stack in a home to 15 floor levels, cross-vents alternate / every floor290 / 390 FUTable 9.7.2(A)

DN 100 stack in a home, 16 to 20 floor levels, alternate / every floor320 / 500 FUTable 9.7.2(A)

Relief and cross-vent, to 15 / 16 to 20 floor levelsDN 50 / DN 65Table 9.7.2(A)

Other buildings, DN 100 stack: load, floor levels120 FU, 5–12Table 9.7.2(B)

Cross-vent joins the stack above the floor's highest flood rim50–600 mm9.6.2.2 (a)

Every system

The base of a stack is where water arriving from above meets the drain and pressure builds, so nothing may connect close to it. These zones, and the figures beside them, hold whichever system the building uses.

A stack of 3 or more floor levels. For 2 or fewer, 500 mm either way along the drain. AS/NZS 3500.2:2025 6.8.1 (a), 6.8.2.

Up the stack from its base, 5 floor levels or fewer above / more600 mm / 1 m6.8.2

No fixture on a branch this close to the stack, where the branch enters at grade500 mm6.7.1

Least grade, DN 40 to 65 / 80 to 100 / 1502.5 / 1.65 / 1 %Table 6.6.1

Least grade of a vent1.25 % (1:80)6.9.2

A trap's seal from the fixture's outlet≤ 600 mm6.5.3

A floor counts once more where the drain is this far below it> 2.4 m9.2.5

Air admittance valves

May vent a trap, a group, a stack or a branch drain; never a main drain's upstream vent or a boundary trap.6.10.1

The drain keeps at least one open vent to air of DN 50 or more.6.10.2 (a)

Not the stack vent of a stack through 10 or more floor levels.6.10.2 (d)

Set within 1000 mm (1 m) under the flood rim of the fixture it vents, never lower.6.10.4 (d)

Where it can be reached, with air to replace what it lets in.6.10.3

On a single stack modified stack, the relief vent then goes to open air on its own.6.10.2 (g)

Where it sits

G13's third acceptable solution, G13/AS3, is this standard: the 2021 edition with Amendment 1, changed at 2.2, 3.19, 4.4, 4.6.6 and Section 14. The 2025 edition has replaced 2021 at Standards New Zealand but no amendment to G13 cites it yet, so a design to 2025 is an alternative solution, read as the edition stands, without G13's five changes.

G13 Amendment 10, its references and G13/AS3 1.0

Where the standard parts from itself

A relief vent on every fully vented stack, or only some

The definition of a fully vented system includes a relief vent, but the clause that sets when one is needed asks for it only where at least 1 floor lies between the stack's highest and lowest branches. A two-storey house with its bathrooms on adjacent floors does not need one.

AS/NZS 3500.2:2025 8.2.2 (b), 8.5.3.1

A single stack sized from the modified system's table

The clause on single stack sizes points to the commercial table of the modified system, not the single stack's own. Read literally, a single stack in an office would take the modified system's loads without its relief vent.

AS/NZS 3500.2:2025 9.7.1, Tables 9.7.1(B) and 9.7.2(B)