Sanitary Fixtures, Drainage and Sewerage Systems
Fixtures and outlet sizes
Indian or European WC with 3/6 L dual flush, wash basin, kitchen sink, urinal, floor trap (nahani trap), drinking-water fountain. Typical outlets: WC 100 mm, wash basin 32–40 mm, sink 40–50 mm, floor trap 50–75 mm, urinal 32–50 mm.
Drainage principles
Gravity carries soil (from the WC) and waste (sinks, basins) to the sewer. Three things make it work: gradient, diameter and ventilation. Typical sizes are soil 100 mm, waste 75 mm, vent 50 mm, rainwater 75–110 mm; a 100 mm soil line is laid at roughly 1 in 60, steeper for smaller diameters, and never flatter than about 1 in 100.
Every fixture must discharge through a trap. The water seal is commonly 50 mm; it is broken by siphonage, evaporation or back pressure, which is why stacks are vented above roof level and the vent terminal is kept away from windows.
Below-ground components
Gully trap (takes waste water, retains grit), intercepting trap with fresh-air inlet at the boundary, inspection chamber, manhole at junctions, changes of direction and intervals along the run, rodding eye at the head of a run.
Worked example. Connect a kitchen sink 6 m from the stack. Sink outlet is 40 mm, so use 40–50 mm waste pipe with a 50 mm P-trap (bottle trap where it stays exposed), gradient about 1 in 40, and a 50 mm vent if the trap is far from the stack. Reasoning: small-diameter pipe needs a steeper fall, and a distant trap needs venting or it will gurgle and lose its seal.
Traps that cost marks
Fitting an S-trap where a P-trap is required; connecting a waste pipe to the stack below the WC branch without its own trap; discharging rainwater into the soil line; forgetting the rodding eye.