Drainage and Above-Ground Discharge
This domain tests above-ground drainage: how waste leaves the building without letting sewer gas in.
Systems
BS EN 12056-2 governs gravity drainage. A primary ventilated stack uses a vent pipe to protect trap seals; secondary venting or air admittance valves protect long branches and additional appliances.
Falls and sizes
110 mm soil pipe is commonly laid at 1:40 to 1:80; 40–50 mm wastes at about 1:40. Too little fall causes solids to settle; too much lets water run away and leave solids behind. Rodding eyes and access chambers go at changes of direction and on long runs.
Venting rules
The stack should terminate well above roof level and at least 900 mm above any opening within 3 m. An AAV must sit at the highest point of the branch, be accessible, and must not replace the only open vent on a system.
Trap seal loss
Self-siphonage, induced siphonage, back pressure, evaporation, capillarity and momentum all break the seal. Diagnose by when the gurgle happens.
Worked example
A bathroom smells and the basin gurgles when the WC is flushed. The branch is long and unvented, so the WC discharge pulls the basin trap dry - induced siphonage. Reason it through: fitting a bigger trap only delays it. Correct answer: vent the branch, often with an AAV at the high point, and confirm every appliance retains its seal.
Traps that cost marks
Assuming a smell means a blocked drain; putting an AAV where it cannot be serviced; laying a long branch with no vent; ignoring the 900 mm vent termination rule.