How building plumbing works
Building plumbing is two separate systems that must never mix. The supply system brings clean water under pressure to every fixture. The drainage system carries used water and waste away by gravity to a sewer or septic tank.
Supply usually comes from the mains to an underground sump, is pumped to an overhead tank, and flows down by gravity to taps, or is pressurised by booster pumps. Pipes are sized so that fixtures get enough flow and pressure even when several run together, and hot-water lines are added for kitchens and baths.
Drainage starts at each fixture's trap, a water-filled bend that stops sewer gases entering the room. Branch pipes fall at a gradient to vertical stacks, which carry waste down to the building drain. Vent pipes let air into the system so that flowing water does not suck the traps dry, and let gases out above the roof. Inspection chambers allow cleaning.
Rainwater from roofs is drained separately and can be harvested. Fire-water systems, with tanks, pumps, risers and sprinklers, are a third separate network in larger buildings. In India these are covered by NBC Part 9 and Part 4; other countries use codes such as the IPC, UPC, EN 12056 and EN 806, AS/NZS 3500 and the National Plumbing Code of Canada.
Pipe flow Q = A V, so the required bore d = √(4Q / (π V))
Problem. A supply branch must carry 0.6 L/s at a velocity of 1.5 m/s. Find the minimum internal diameter.
- Q = 0.0006 m³/s.
- d = √(4 × 0.0006 / (π × 1.5)) = √(0.000509) = 0.0226 m.
Answer. About 23 mm internal, so pick the next standard size
Quick quiz
1What stops sewer gas entering a room?
The water seal in the trap.
2Why are vents needed?
To balance air pressure so traps are not siphoned, and to release gases.
3Should rainwater pipes connect to the soil stack?
No, they are kept separate.
- NBC 2016 Part 9 (Plumbing Services) (India): Water supply, drainage, sanitation and gas supply within buildings.
- IS 2065 (India): Code of practice for water supply in buildings.
- IS 1742 (India): Code of practice for building drainage.
- International Plumbing Code (ICC) (USA): Model code for plumbing systems in buildings.
- EN 12056 (series) (Europe): Gravity drainage systems inside buildings.
- AS/NZS 3500 (series) (Australia): Plumbing and drainage: water services, sanitary drainage, stormwater, heated water.
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- Bureau of Indian Standards, National Building Code of India 2016, Part 9, BIS, 2016.
- S. K. Garg, Water Supply Engineering, Khanna Publishers, latest edition.
Free and official sources
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