Building Services: Water Supply, Drainage and Sanitary Systems
Core concepts
Building services in the water and drainage area turn on a small number of ideas that questions repeatedly test. You must understand the difference between a direct supply (mains pressure delivered straight to fittings, permitted only where pressure and flow are adequate) and an indirect supply (water stored in a tank, usually fed from a break tank to protect the mains from backflow and to buffer demand). Know the role of the cold water storage cistern, the feed and expansion cistern in open-vented heating systems, and the vent pipe that keeps the system at atmospheric pressure.
On drainage, learn the vocabulary precisely: soil (foul) drainage carries WC discharge; waste drainage carries sinks, basins and appliances; vent pipes equalise pressure and protect trap seals; traps hold a water seal against sewer gas. Understand fall or gradient (typically expressed as a ratio such as 1:40 to 1:80 for soil pipes, steeper for short runs), rodding eyes, inspection chambers, and the purpose of an interceptor or gully. Know backflow prevention categories and devices: air gaps, check valves, and reduced-pressure zone assemblies.
Traps in questions
Watch for distractors that confuse the function of a vent pipe with a drain, or that claim a trap seals by mechanical means rather than a water seal. Questions often test which fitting needs a trap (almost every appliance, with exceptions such as a bath waste combined with a basin where permitted) or the minimum trap seal depth (commonly 75 mm, with deeper seals where pressure fluctuations are severe). Another classic trap is mixing up secondary and primary circulation in hot water systems, or assuming stored water is always safer than direct supply without considering stagnation and temperature control.
Rule of thumb
Use this working rule: if a fitting can allow foul water or gas to travel back into the supply or room, it needs a trap, a vent, or an air gap. For a worked example, a new washbasin is added to an existing soil stack. You check that it has a 75 mm water-seal trap, that its waste connects via a branch with an adequate fall (say 1:40), and that the existing stack ventilation is sufficient; if the branch exceeds the permitted length without a vent, you add an air admittance valve or a re-vent.