Sanitary Plumbing, Drainage and Waste Systems
This domain covers the removal of soil and waste from fixtures to the sewer: trap design, stack and vent arrangements, drain gradients, access for rodding and the tests that prove the system is sound. Questions often hinge on trap seal loss and on correct falls.
Terms and systems
Soil carries WC discharge; waste carries basins, sinks and baths; a drain runs to the sewer; the vent protects trap seals. In the one-pipe (single-stack) system, fixtures connect to a ventilated discharge stack; the two-pipe system uses separate soil and waste pipes with their own ventilation.
Core rules
- Every appliance needs a trap; the WC pan has an integral trap. Water seal at least 25 mm, typically 50 mm, with deep-seal traps at gullies.
- Vent the stack and terminate it clear of windows and openings; an air admittance valve must be accessible and fitted above the flood level of the highest appliance.
- Falls: 40–50 mm wastes at 1:40 to 1:60; 110 mm drains at 1:60 to 1:110, never flatter than 1:110.
- Use a rest bend at the foot of the stack; avoid 90° elbows on horizontal drains; lay pipes straight between access points.
- Provide rodding eyes and inspection chambers at changes of direction, at junctions and at intervals not exceeding about 15 m.
- Test with an air test of 38 mm water gauge held for three minutes, or a water test, before backfilling.
Worked example
A 110 mm drain leaves a building with an invert 600 mm below ground and runs 18 m to a chamber at 1:60. Fall = 18 ÷ 60 = 0.30 m, so the downstream invert is 900 mm below ground. Check: 1:60 is within the acceptable range for 110 mm. If the same run were 36 m, the fall would be 600 mm and the chamber 1.2 m deep - deep enough to need steps and a stronger cover, which is why long runs are often split.
Traps
- Laying a drain to a fall in the wrong direction, or too flat, so solids settle.
- Using a 90° bend instead of a rest bend at the stack foot.
- Fitting an AAV below flood level or inside a sealed duct.
- Dry traps in unused rooms causing sewer smells.
- Leaving open pipe ends unplugged so mortar and rats enter.