Sanitary Drainage, Soil, Waste and Vent Systems
What is tested
This domain covers the removal of soil (faeces) and waste (sullage) by gravity, and the vent system that protects trap seals. Expect questions on pipe sizes, gradients, stack arrangement, traps, inspection chambers and the difference between soil, waste and vent pipes.
Core rules
- Soil pipes carry WC discharge; waste pipes carry sinks, basins and baths; vent pipes carry air, not water.
- Drainage relies on fall. A common soil branch fall is about 1:60 to 1:80; mains may be 1:100 or flatter with larger pipes. Use the specified gradient, not guesswork.
- Every trap needs a water seal, commonly 50 mm for soil fixtures and smaller for some fixtures; the vent prevents siphonage.
- A soil stack should rise above the roof and be terminated with a suitable cowl or screen, clear of windows.
- Fit access for rodding at bends, junctions and inspection chambers; do not bury a junction without access.
- Keep vent pipes continuous and terminate them outside; do not combine a vent with a rainwater downpipe.
Worked example
A 110 mm soil branch runs 12 m at 1:60. Fall = length ÷ gradient denominator = 12 ÷ 60 = 0.20 m, or 200 mm. Set the upstream invert 200 mm higher than the downstream invert. If the available fall is only 100 mm, the run is too flat for that gradient; shorten the run, increase the pipe size if permitted, or revise the layout rather than forcing a backfall.
Traps
Do not use a 90° bend where a long-radius bend or two 45° bends are required. Do not omit a vent on a long branch. Do not connect a WC to a smaller waste pipe. Do not allow a trap to be inaccessible.