Drainage and sewerage
Drainage takes used water out of a building. Sewerage is the town-wide system of pipes (sewers) that carries it to a treatment plant. Drain pipes run mostly by gravity: the pipe is only part full and the water just flows downhill. There is no pressure, so the pipe is wide and sloped.
Kinds of waste water
- Soil water from toilets (heavy with solids).
- Waste water (grey water) from sinks, showers and washing machines.
- Rain water from roofs and yards.
Sewer systems
- Separate: one sewer for sewage, another for rain water. Common in new areas.
- Combined: one sewer for both. Cheaper, but heavy rain can overflow it.
Sewage then goes to a treatment plant: screens remove large things, tanks let solids settle, bacteria clean the water, and the water is disinfected before it returns to a river. Where there is no sewer, a septic tank and soak pit are used.
Design of drainage and vent installations
Slope (gradient)
A horizontal drain must slope so that the water carries solids along. A common rule is about 1:50 to 1:100 (1 to 2 cm of fall per metre). Too flat: solids settle and block. Too steep: the water runs ahead and leaves solids behind.
Pipe size
Pipe size comes from the load of the fixtures (counted in "fixture units": a basin is a small load, a shower more, a toilet the most). A toilet needs a pipe of about 100 mm. A drain pipe never gets narrower going downstream. The vertical stack carries soil and waste down to the building drain.
Traps and water seals
A trap (U-bend or P-trap) under each fixture holds a water seal (usually 50 mm or more deep). It blocks sewer gas, smells and insects.
Why vents are needed
When a lot of water falls down a stack, it pushes air ahead (pressure) and leaves low pressure behind (suction). Suction can pull the water out of a trap, or pressure can blow it back into the room. A vent pipe joins the drain system to the open air, usually through the roof, so air can enter or leave and pressure stays near normal. Vents end above the roof and away from windows.
Other design points
- Gentle bends and 45° joins, not sharp elbows.
- Cleanout (access) plugs at turns and at the base of stacks.
- Pipe materials: PVC, cast iron, HDPE.
- Test the system with water or air before use.
Water supply and drainage in houses
A house has two separate pipe systems that never connect:
- Supply: clean water, under pressure, in small pipes (about 15 to 25 mm). It goes to the taps, the toilet cistern, the heater and the washing machine. Valves can shut it off.
- Drainage: used water, no pressure, in big pipes (about 40 to 100 mm) that slope down. It has traps and vents.
At each fixture the supply comes in above (from a tap) and the drain goes out below (through a trap). An air gap between the tap and the water in the basin stops dirty water from being sucked back into the supply.
Indoor layout tips: keep pipes short and straight, put bathrooms and the kitchen close to the stack, give every pipe a way to be cleaned, and make sure the roof vent is open and clear.
Key formulas and definitions
- Fall = length × slope (for 1:50, fall = length ÷ 50)
- Slope (%) = fall ÷ length × 100
- Remaining seal = trap seal depth − loss by suction or evaporation (must stay > about 25 mm)
- Sewage ≈ 80 % of the water supplied
- Fixture units: add the loads of all fixtures on a branch
Worked examples
1. A horizontal drain 5 m long needs slope 1:50. How much lower is the outlet end?
Fall = 5 ÷ 50 = 0.10 m = 10 cm.
2. A pipe falls 4 cm over 2 m. Find the slope as a percentage and as 1 in n.
Slope = 0.04 ÷ 2 = 0.02 = 2 percent = 1 in 50.
3. A trap has a 50 mm seal. A flush sucks out 20 mm. What is left, and is it safe if at least 25 mm must remain?
Left = 50 − 20 = 30 mm. This is more than 25 mm, so it is still safe, but a vent would keep it at the full 50 mm.
4. A family of 5 uses 135 L per person per day. About how much sewage is made?
Water used = 5 × 135 = 675 L. Sewage ≈ 0.8 × 675 = 540 L per day.
5. A bathroom has one basin (1 unit), one shower (2 units) and one toilet (4 units). Find the total load in fixture units.
Total = 1 + 2 + 4 = 7 fixture units. This number is used to choose the pipe size from a table.
Common mistakes
- Thinking the trap is only to catch things. Its job is the water seal that blocks sewer gas.
- Leaving out the vent. Flushing can then empty the seals.
- Sloping too much (more than about 1:20). Water runs ahead and leaves solids behind.
- Joining the supply and drain pipes. They must stay separate.