Sanitary and Plumbing Systems
This domain covers sanitary drainage and venting inside the building: fixtures, traps, drainage pipe sizing by fixture units, venting, slopes, cleanouts, grease and oil separation, sumps, materials and the tests that prove the installation is sound.
Core rules
Every fixture except a few listed exceptions needs a water-seal trap of 50–100 mm, and the trap must be protected by venting or an approved air-admittance valve. Horizontal drainage runs at 1–4%, with about 2% the working default for DN 65–110; flatter pipes clog, steeper ones leave solids behind. A stack needs a vent extended above roof level, a cleanout at its base and at every change of direction, and vent connections must be taken above the fixture's spill level. Separate grease interceptors for kitchens, silt traps for car parks, and sump pumps with high-level alarms for basements. Test drainage with water (typically a head of about 3 m or to the highest point) and with smoke for the vent system; test supply lines under pressure before concealment, then flush and disinfect.
Worked example
A DN 110 soil branch starts at invert −0.90 m and runs 28 m at 2% to the stack. Fall = 28 × 0.02 = 0.56 m, so the downstream invert is −0.90 − 0.56 = −1.46 m. If the site drain is shallower than that, you cannot keep the slope - you must re-route, raise the start or add a lifting station, and you must show the decision on the drawing.
Traps
Treating 1% as acceptable for small branches (it is not, for solids); forgetting trap primers for floor drains in plant rooms; connecting a vent below the weir of the trap; omitting a cleanout at the base of the stack; sizing by fixture count instead of fixture units; and burying pipe before the pressure or smoke test is witnessed and recorded.