G13 foul water drainage, in plain English

NZBC clause G13 Foul Water covers carrying wastewater — from WCs, basins, showers, kitchen and laundry — safely to the sewer or an on-site system. If you’re building bathrooms, kitchens or laundries, this is the trade that has to run under your floors and through your walls, so it pays to know how it works.

The three things G13 hinges on

Foul water drainage works on three things, and every rule below comes back to one of them:

• Fall — gravity carries the flow along the pipe.
• Traps — a water seal that blocks sewer gas from coming back up.
• Vents — air so flows don’t siphon the traps dry.

All sanitary drainage must be done and certified by a Certifying Drainlayer (PGDB) — it is not builder-signed work.

Why the fall has a sweet spot

Fall isn’t “steeper is better”. Too flat and solids settle and block; too steep and the water races ahead, leaving solids stranded. That’s why G13 gives a target fall per pipe size — commonly around 1:60 for 100 mm and around 1:40 for 50 mm.

Every trap also needs a vent within range, or the next fixture’s discharge will siphon the seal and let sewer gas in. A few points worth knowing on the pipework:

• WC soil pipe is 100 mm DWV, min fall 1:60 (16.7 mm/m) — PVC orange, jointed solvent-weld (NZS 4404).
• Basin and sink wastes are 40 mm or 50 mm DWV at min 1:40, joining to an 80 mm or 100 mm collector.
• Shower waste is 50 mm DWV at min 1:60, with the trap below floor (P-trap or bottle trap).
• A branch drain of 80 mm or 100 mm at 1:60 combines waste from multiple fixtures.
• The vent stack (DPC) is 100 mm vertical, taken above roof level — min 600 mm above any opening within 3 m — to prevent siphon.
• A gully trap is 100 mm; the bathroom branch must vent through the gully (NZ standard), receiving the shower and basin via the floor.

The drainage rules that catch people out

• Drain falls are measured along the pipe centreline, not along the ground.
• No fall greater than 1:14 — steeper causes self-cleaning issues and gas escape.
• Vent stack to all WC branches — this is what prevents trap seal loss.
• Inspection points at every change of direction, and every 30 m of straight run.
• Gully trap on every branch before it connects to the main drain.
• Backflow prevention to the mains supply where cross-connection is possible.
• Keep foul and stormwater separate — cross-connections overload the sewer and breach most councils’ rules.

Floor waste sits at 50 mm or 80 mm falling 1:80 from the drain; tiled floors fall 1:60 to the drain (E3/AS1).