A sanitary appliance is any fitting that discharges domestic wastewater directly into the building’s drainage system. Basins, sinks, shower trays, baths, WCs, washing machines and dishwashers all count. A rainwater outlet does not: rainwater is not domestic wastewater.
The term matters because each appliance has its own discharge rate. Adding up all appliances on a branch — corrected for the likelihood that they discharge at the same time — gives the required pipe size. That prevents an undersized collecting pipe from surcharging, and an oversized one from silting up because the flow is too slow to be self-cleansing.
Basic rules for connecting
- Always through a trap — every appliance is connected via a trap with a water seal of at least 50 mm. Without the seal, drain air enters the room
- One branch per appliance — do not tee two appliances into the same branch unless it has been sized for it
- Enter at 45 degrees — an angled entry into the collecting pipe keeps the flow calm; a square entry causes back-up and siphonage
- Big dischargers last — connect appliances with a large flush and high flow rate (WC, washing machine, bath) upstream, so their surge does not siphon the seal out of a basin or sink trap
- Ventilation — the soil stack must run through to above roof level. A long branch serving several appliances often needs additional venting
Common waste pipe sizes
| Appliance | Waste size (typical) |
|---|---|
| Wash basin | 32 mm |
| Sink, washing machine, dishwasher | 40 mm |
| Shower and bath | 40 to 50 mm |
| WC | 110 mm |
Common DIY mistakes
- Too little fall — for horizontal waste pipes inside a dwelling, around 1 cm per metre is a common rule of thumb. Too little fall means standing water; too much lets the water run away faster than the solids, so the pipe silts up
- Flexible hoses with sags — a drooping waste hose forms a second trap that holds debris
- A trap that dries out — in a rarely used outlet the water in the trap evaporates and drain air comes through. Flush such outlets periodically
- Forgetting the vent — a gurgling waste or a WC that glugs is nearly always a venting problem, not a blockage
Related terms
- Rainwater drainage
- Roof penetration
- Construction moisture
- Condensation
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