Sanitary Fixtures and Drainage Systems
This domain tests fixture installation, trap and vent logic, gradients and the external drainage chain from gully trap to septic tank.
Core rules and concepts
- Fixtures: wash-down and siphon WC, squat pan, wash basin, sink, urinal, and their traps. Every fixture needs a water seal, normally 50 mm minimum, to stop sewer gas entering the room.
- Soil pipe carries sewage from WCs; waste pipe carries sullage from basins and sinks. They may be separate or combined, but both end in the drain.
- A P-trap or bottle trap sits under the fixture; a floor (nahani) trap takes floor water; a gully trap receives waste water before it joins the drain and prevents foul air backing up.
- Vent pipes and anti-siphon traps prevent self-siphonage and back pressure from emptying the seal.
- External chain: gully trap, inspection chamber, manhole, septic tank, soak pit or sewer.
- Gradients are kept between about 1 in 40 for small branches and 1 in 100 for 100 mm drains - steeper for smaller pipes.
Worked example
An 8 m branch drain is laid at 1 in 60. What is the total fall? Fall = 8000 mm ÷ 60 = 133 mm. Reasoning: the gradient is a ratio of fall to length, so divide the length in millimetres by the ratio denominator. Check the invert level at both ends so the next chamber connects properly.
Traps that cost marks
- Fitting an S-trap where a P-trap with vent is needed; S-traps self-siphon.
- Omitting the vent on a long branch.
- Discharging a sink into the soil pipe without a gully trap.
- Laying a drain too flat, so solids settle.