Lay out indoor plumbing as one supply line with a shutoff valve at the building entry, short branches to the kitchen sink and wash basin, and drains sloped at least 2%, which is 2 cm of fall per metre, to the greywater system. Pressure at a fixture depends on how far it sits below the tank’s water surface, not on how high the pipe is routed, so keep wet rooms on the ground floor and run pipes level or falling, with a way to release air at any high point.
Indoor plumbing layout is the arrangement of the supply pipes, fixtures and drains inside a building. In a gravity system the available pressure is small, so a poor layout wastes it.
A Correction To The Parent Page
The parent page advises: “Use the highest practical pipe routing to maintain pressure at all fixtures.” That is not how water pressure works. The pressure at a tap is the height of the water surface in the tank above that tap, less friction losses in the pipe (Calculating Head Pressure). Where the pipe runs in between does not add pressure. A pipe that rises toward the ceiling gives air a place to collect, and if a crest rises close to the pressure line the flow falls or stops. So route supply pipes level or falling from the entry to the fixtures, and treat any rise as a place where air must be released.
The Parent Page’s Layout Rules
- A single supply line into the building.
- Branches to the kitchen sink, wash basin and other fixtures.
- A shutoff valve at the building entry.
- All drain pipes sloped at a minimum of 2% (2 cm drop per metre) toward the greywater system or dry well.
Worked Example: Pressure On Two Floors
Inputs: the tank bottom is 10 m above the ground-floor level; the kitchen tap is 0.9 m above the floor and an upstairs basin tap is 3.4 m above the ground floor; friction in the supply is assumed to take 2 m of head at the flows in use. Using 0.0979 bar per metre (Calculating Head Pressure):
| Fixture | Height below the tank bottom | Static pressure | After 2 m of friction |
|---|---|---|---|
| Kitchen tap | 10 − 0.9 = 9.1 m | 0.89 bar | 7.1 m, 0.70 bar |
| Upstairs basin | 10 − 3.4 = 6.6 m | 0.65 bar | 4.6 m, 0.45 bar |
The upstairs basin falls below the 0.5 bar (5 m) that Pipe Sizing asks for at a tap. Each metre of height gained costs 0.098 bar at the fixture. In a low-head system, put the kitchen, basin and washing area on the lowest floor.
Drains
- Slope: 2% means 2 cm of fall per metre; a 6 m drain falls 12 cm. Check with a 1 m level and a 2 cm block under one end (Levels and Plumb Bobs).
- Traps: every fixture drain holds a U-bend of water to block drain gas from rising into the room. As I recall, traps are usually sealed about 50 mm deep. They are not backflow protection.
- Vents: the drain system needs an air path to open air so that water running down does not siphon the trap dry.
- End: the drain ends at the greywater filter, not the toilet; waste from the toilet never enters it (Plumbing Separation).
- Unions and cleanouts at each turn so a blockage can be cleared.
What You Need
- Supply pipe of one bore for the main and a bore matched to the flows for each branch (Distribution & Fixtures)
- A shutoff valve, unions and an air release at any high point (Air Valves)
- Drain pipe, traps and a vent
- A level and a measuring stick
Method: Plan, Fix, Fall
Step 1: Mark fixture positions on the plan, with the wet rooms stacked on one wall.
Step 2: Calculate the pressure at the highest fixture as in the example. If it is below 0.5 bar, move the fixture down, raise the tank, or accept low flow.
Step 3: Run the supply along the shortest level path, with a valve at the entry and a drain valve at its lowest point.
Step 4: Lay the drains to a continuous fall from each trap to the greywater inlet, checked with the level.
Step 5: Keep the pipes inside the building’s warm envelope or insulated, and keep them accessible for repairs.
Step 6: Test the system with water, check the air gaps and drains, and watch for low spots.
Common Mistakes
- Routing supply pipes high in the belief that it holds pressure.
- A fixture on an upper floor in a low-head system.
- A drain with a sag that holds water and smells.
- A trap without a vent.
- Burying pipes in walls without access.
Safety
Drain gas, burst pipes and cross-connections are the indoor hazards
Drains can carry foul gas and, in a sealed sump or tank, hydrogen sulphide, which deadens the sense of smell: do not rely on smell and never enter a sump or tank. A frozen pipe can split and flood; shut the entry valve if a pipe fails. Never join the drinking supply to a drain or a greywater line, even through a valve.
What’s Next
- Learn next: Hot Water Systems — add heated water to the supply
- Combine with: Greywater Recycling Systems — where the drains go