Sanitary Drainage, Waste and Vent Systems
This domain tests how soil, waste and vent systems carry sewage and greywater away without letting sewer gas enter the building. Expect questions on traps, gradients, stacks, venting, inspection chambers and the one-pipe versus two-pipe system.
Traps and gradients
Every fixture needs a water seal, commonly 50–75 mm deep, to block sewer gas. A P-trap with a vent is preferred; an S-trap can siphon itself. Drainage relies on gravity, so falls must be continuous and adequate - typically around 1:40 to 1:80 depending on diameter, with larger pipes on flatter gradients. Reduce pipe size only in the direction of flow, never against it.
Stacks, vents and access
A soil stack carries WC discharge; a waste stack carries sinks and basins; a vent stack equalizes pressure and protects trap seals. Vent pipes must rise above the roof and stay clear of windows and air intakes. Provide inspection chambers, rodding eyes and access covers at changes of direction, junctions and long runs. A one-pipe system combines soil and waste with a common vent; a two-pipe system separates them.
Worked example
A 15 m branch drain is laid at 1:60. Fall = 15 ÷ 60 = 0.25 m, or 250 mm. If the invert at the start is 100.000 m, the invert at the end is 99.750 m. Check that the fall is continuous and that the branch enters the stack or inspection chamber in the direction of flow.
Traps that cost marks
Do not use an unvented S-trap; do not lay a drain flat or back-fall it; do not connect a vent pipe to a drain; do not omit access at a bend; and do not assume a bigger pipe always drains better - velocity matters.