Sanitary plumbing and drainage systems
Sanitary plumbing and drainage systems
This domain tests the design and installation of above-ground sanitary plumbing and below-ground drainage under NZBC clause G13 and AS/NZS 3500.2. The exam probes trap seals, venting, grades, junctions, inspection openings, bedding and testing.
Core rules
- Each fixture must have a trap; WCs rely on their integral trap. The trap seal must be protected from siphonage, compression and evaporation.
- Venting is part of the system, not an optional extra: individual, common or relief vents, and air admittance valves where the standard permits.
- Size stacks and drains from discharge units and the tables in AS/NZS 3500.2; do not copy a neighbouring job.
- Lay drains to a self-cleansing grade. AS/NZS 3500.2 gives minimum grades; a common DN100 grade is 1:60.
- Use swept junctions in the direction of flow and provide inspection openings at stack bases, bends and long runs.
- Bed and support the pipe so the grade is maintained after backfill; test before covering.
Worked example
A DN100 drain runs 18 m at 1:60. Fall = 18,000 mm ÷ 60 = 300 mm. If the upstream invert is 10.450, the downstream invert is 10.150. Check that 10.150 matches the connection point. If it does not, adjust the alignment or seek a design change; never flatten the grade to make the connection.
Traps
Losing marks: using 1:100 “because it looks flat”, omitting a vent on a long branch, installing a junction backwards, leaving no inspection opening at a bend, covering work before inspection, and losing a trap seal through poor grade or venting.