Last reviewed 16 Sept 2026 · 11 min read
Principles of house drainage
- Drains should be laid outside the building as far as possible, along boundary walls, and connected to the public sewer through an intercepting trap where required.
- Pipes laid at self-cleansing gradients, in straight lines between inspection points, with inspection chambers at every change of direction, gradient and junction.
- All fittings connected through traps with an adequate water seal to exclude foul gases.
- The system must be ventilated — soil and waste pipes extended above the roof as vent pipes.
- Pipes must be watertight and tested; joints durable; drains of adequate size (no over-sizing that prevents self-cleansing).
- Rainwater pipes should not normally be connected to soil pipes; sullage and rainwater may go through gully traps.
- The drainage system should be accessible for cleaning and not pass under living rooms where avoidable.
Types of pipes in building plumbing
| Pipe | Carries |
|---|---|
| Soil pipe | Human excreta from water closets and urinals (black water) |
| Waste pipe | Sullage from wash basins, sinks, baths and floor traps (grey water) |
| Vent pipe | Air — ventilates the system, releases foul gases above the roof |
| Anti-siphonage pipe | Air to the traps of fittings to prevent loss of seal by siphonage |
| Rainwater pipe | Roof rainwater to the storm drain or harvesting system |
Soil pipes are commonly at least 100 mm in diameter; vent stacks are carried above the roof well clear of windows and openings.
Plumbing systems
| System | Description | Remarks |
|---|---|---|
| Two-pipe system | Separate soil pipe and waste pipe stacks, each with its own vent pipe (four pipes in total when anti-siphonage pipes are provided) | Most expensive and space-consuming; safest; soil and waste kept apart until the inspection chamber |
| One-pipe system | A single stack carries both soil and waste; a separate vent pipe and anti-siphonage connections to all traps | Fewer pipes than two-pipe; traps need deep seals |
| Single-stack system | One pipe carries soil and waste and also acts as the vent; no separate vent pipe | Cheapest and simplest; needs careful design — deep-seal traps, short branches, fittings close to the stack; suited to low-rise buildings |
| One-pipe partially ventilated system | Single stack for soil and waste; a relief vent pipe ventilates only the traps of water closets | Compromise between one-pipe and single-stack; common in multi-storey buildings |
Traps
A trap is a fitting that holds a body of water (water seal) to prevent foul gases from entering the building, while allowing wastewater to flow.
- The depth of water seal (vertical distance between the dip and the crown weir) is generally at least about 50 mm; deeper seals (about 75 mm) are used in single-stack systems.
- A good trap is self-cleansing, has no internal projections, and has an access (clean-out) for cleaning.
Types by shape
- P-trap — outlet horizontal (like the letter P lying) — for water closets and wash basins discharging horizontally.
- Q-trap — outlet inclined at about 45° — upper floors.
- S-trap — outlet vertical — WCs on ground floors discharging into a floor drain.
Types by use
| Trap | Use |
|---|---|
| Floor trap (nahni trap) | Collects wastewater from bathroom and kitchen floors; grated inlet |
| Gully trap | Outside the building; receives sullage from kitchens and baths and rainwater; deep water seal; prevents gases from the drain and entry of rats and cockroaches |
| Intercepting trap (interceptor) | At the junction of the house drain and the municipal sewer; prevents sewer gases from entering the house drain; has a cleaning eye |
| Bottle trap | Under wash basins and sinks — compact, accessible |
| Grease trap | Hotels, restaurants, canteens — separates grease which floats and is removed |
| Silt/sand trap | Garages and workshops — retains grit and oil |
Loss of water seal
| Cause | Mechanism |
|---|---|
| Self-siphonage | Water discharging from a fitting flows full-bore in the waste pipe and sucks out its own trap seal |
| Induced siphonage | Discharge from another fitting in a common pipe creates suction that pulls out the seal of a trap connected to it |
| Back pressure (compression) | Air compressed ahead of a descending water plug blows the seal back into the fitting |
| Evaporation | Seal dries up in rarely used fittings |
| Capillary action | Thread or hair in the trap wicks water out |
| Wind effect / momentum | Strong winds across vent tops or sudden discharges disturb the seal |
Prevention: anti-siphonage (vent) pipes connected near the trap crown, adequate pipe sizes, deep-seal traps, correct gradients and lengths of branches, regular use of fittings.
Sanitary fittings
- Water closets: Indian type (squatting pan with footrests; Orissa pan), European type (pedestal/wash-down or siphonic), Anglo-Indian type; wall-hung types.
- Flushing cisterns — valveless siphonic cisterns (bell type, flushing by siphon action) and dual-flush cisterns to save water; flush valves for direct flushing from a pressurised supply.
- Urinals — bowl type, slab/stall type; automatic flushing; waterless urinals.
- Wash basins, kitchen sinks, bath tubs, showers, bidets, drinking fountains.
- Fixtures are made of vitreous china (sanitary ware), stainless steel, enamelled cast iron, plastics.
Building drains
Size and gradient
The load on drains is expressed in fixture units (discharge value of each fitting relative to a wash basin) for sizing stacks and branches.
FormulaMaguire's rule (self-cleansing gradients for house drains)
| Drain diameter | Gradient |
|---|---|
| 100 mm (4 inch) | 1 in 40 |
| 150 mm (6 inch) | 1 in 60 |
| 225 mm (9 inch) | 1 in 90 |
Roughly: gradient in .
Inspection chambers and manholes
Provided at junctions, changes of direction or gradient and at intervals, with benching and a cover; depth and size depend on the invert level.
Testing of house drains
- Water test — the section is plugged at the lower end and filled with water to a head; the level must not fall appreciably over the test period.
- Smoke test — smoke is pumped in; leaks show as smoke (also detects faulty traps above ground).
- Air test — air pressure; drop in manometer reading indicates leakage.
- Ball test — a ball slightly smaller than the pipe rolled through to check obstruction and alignment.
- Mirror test — light visible through the pipe from one inspection chamber to the next.