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Drainage pipe sizes: what goes where
A house uses a small range of drainage pipe sizes. Knowing which is for what saves you buying the wrong parts.
Why sizes are confusing
Pipe sizes are quoted in different ways. Underground and soil pipe are often sold as 110mm, which is the outside diameter, and the inside is roughly 100mm. Plastic waste pipe is 32mm, 40mm or 50mm, again usually the outside diameter. Approved Document H talks about internal diameters. So 110mm pipe meets a 100mm minimum, and that is the standard choice for WC drains.
Approved Document H minimums
| Use | Minimum, according to Approved Document H (2015) |
|---|---|
| Foul water drain | 75mm internal diameter |
| Drain carrying WC or trade effluent | 100mm |
| Sewer serving several properties | Normally 100mm for up to 10 dwellings, 150mm for more than 10 |
| Single-appliance pipe | At least the diameter of the trap |
A pipe serving one appliance should not be smaller than its trap. That is why a 40mm trap needs 40mm pipe, not 32mm.
Trap sizes
Approved Document H gives these trap diameters and seal depths:
| Appliance | Trap diameter | Seal depth |
|---|---|---|
| Bidet | 32mm | 75mm |
| Shower | 40mm | 50mm |
| Sink | 40mm | 75mm |
| WC (outlet under 80mm) | 75mm | 50mm |
| WC (outlet over 80mm) | 100mm | 50mm |
Washbasin and bath traps are normally 32mm and 40mm respectively; check your appliance and the Approved Document for the table. All traps must keep a minimum seal of 25mm under working conditions.
What goes where, in a typical house
- 110mm: toilet branches, soil stacks and underground drains.
- 50mm: sometimes used for larger shared waste runs or longer basin runs.
- 40mm: sinks, baths, showers, dishwasher and washing machine wastes.
- 32mm: basins and bidets.
- Rainwater: usually 68mm round or 65mm square downpipes, connected to a 110mm underground drain through a gully or shoe.
Branch length limits
For unvented branch discharge pipes, Approved Document H sets limits. For example, a WC with an outlet over 80mm can have a branch up to 15m with up to eight WCs, minimum 100mm. A washbasin or bidet can use roughly a 30mm pipe up to about 1.7m, 40mm up to 3m and 50mm up to 4m. Longer or bigger groups need a vented branch. The figures are from Table 2; read the table for your case.
Bigger drains
For long runs or several WCs, 150mm drains may be used. A 150mm drain can have a gentler fall (1:150) once at least five WCs are served. For a typical house the 110mm drain does all the work.
Compatibility
- Push-fit, solvent-weld and compression systems come in different sizes and are not interchangeable. Check the label.
- Use adaptors to move between systems rather than forcing a join.
- Do not mix underground drain pipe (usually orange-brown or sand) with above-ground soil pipe (usually grey or black) unless the parts are made to join.
Next: soil pipe vs waste pipe or falls and bends.
This is general information, not professional advice. The rules apply to England; Wales, Scotland and Northern Ireland have their own. Check with your local authority building control or a competent professional before starting work.