Sanitary Plumbing, Drainage and Fixture Installation
System principles
Sanitary plumbing carries soil and waste to the sewer while protecting occupants from sewer gas and contamination. The core principles are a water seal at every fixture trap (commonly 50 mm, sometimes 75 mm), venting to equalise pressure, and self-cleansing flow. Understand the difference between soil (toilets) and waste (other fixtures), and between above-ground and below-ground drainage.
Traps, venting and fittings
Know trap types (P, S, bottle, gully) and why S-traps siphon. Venting prevents trap seal loss from pressure fluctuations; a blocked vent causes gurgling and odour. Understand air admittance valves where permitted, and the role of inspection openings and rodding eyes for maintenance. Junctions should be swept in the direction of flow; reducers should be eccentric on horizontal runs to avoid trapping solids. Backflow prevention protects the potable supply from contamination.
Gradients, sizing and installation
Drains rely on gravity: typical minimum grades are around 1:60 for DN100 and flatter for larger diameters, with the exact figure set by the applicable standard. Supports, bedding, and jointing must suit the material. Fixtures must be set level, securely fixed, sealed, and accessible for maintenance. Test before concealment - water test or air test as specified.
Traps and a worked rule
Traps: assuming a trap seal is enough without venting; running a drain flat "to save cover"; connecting a fixture without checking the trap arm length; and forgetting that a floor waste gully needs its own seal. Rule of thumb: if a fixture gurgles or smells, suspect the vent or trap seal before the drain. Worked check: for a 12 m run at 1:60, fall = 12 ÷ 60 = 0.2 m, so the downstream invert must be 200 mm lower.