Drainage and Sewerage Systems
What this domain tests
Drainage and Sewerage Systems tests soil, waste and vent pipework, gradients, traps, inspection chambers and connection to the public sewer. You need to know how foul water moves by gravity and how gases are kept out of the building.
Core rules
- Soil pipes carry WC discharge; waste pipes carry sinks, basins and showers; vent pipes equalise pressure and protect trap seals.
- Drainage relies on a continuous fall. A common house drain gradient is about 1:60 to 1:100, subject to the adopted code and pipe size.
- Changes of direction should use swept bends, not sharp elbows, to reduce blockage risk.
- Every branch needs a trap or trapped fixture; vent the system to prevent siphonage.
- Inspection chambers and manholes need access covers, benching and safe depth/entry procedures.
- Keep stormwater separate from foul drainage unless the system is designed as combined.
Worked example
A 12 m drain run falls at 1:60. The total invert drop is 12 ÷ 60 = 0.20 m, or 200 mm. If the upstream invert is 100.00 m, the downstream invert is 99.80 m. The reasoning is gradient = fall ÷ length, so fall = length × gradient. A 1:100 gradient would give only 120 mm, which may be too flat for the pipe size and load.
Traps
Laying drain pipe flat to save excavation, using 90° elbows, forgetting venting, connecting a floor drain without a trap, and backfilling before testing are common mark-losers.