Wyes › Soil pipe vs waste pipe
Soil pipe vs waste pipe
The two names get used loosely, but they describe two different jobs. Knowing which is which tells you what size, what slope and what connection to use.
The simple difference
Soil pipe carries waste from toilets (and sometimes urinals and bidets). It is the larger pipe, typically 110mm. Waste pipe carries used water from basins, baths, showers, sinks and appliances such as washing machines. It is smaller, typically 32mm to 50mm.
The older terms for the underground version are foul drain (carrying toilet waste and used water) and surface water drain (rainwater). Modern houses have separate foul and surface water drains in many places, but older properties may have a combined system.
The stack
In a two-storey house, a vertical soil and vent pipe (often just "the stack") takes the toilet branch and, usually, the waste from upstairs washing facilities. It runs down to a drain at the bottom, and its top is open to the air above the roof so air can move in and out and the water seals in traps are not sucked dry. Inside a house it is often boxed in. Outside, the stack is usually 110mm plastic.
How waste joins the stack
Small waste branches join the stack through a boss connector. Larger branches join through a junction. Approved Document H notes that branches of 75mm or more joining an equal-size stack should use a sweep of at least 50mm radius or a 45-degree angle. Small branches up to 40mm joining a 100mm or larger pipe should where practicable connect to the upper part of the pipe wall. See wye vs tee for why.
Do you need a vent?
Traps hold a water seal to keep drain gases out. If waste runs too fast or too far, the water can be pulled out of the trap (siphonage), letting smells in. For this reason, the length and diameter of branch pipes are limited unless they are vented. Approved Document H sets those limits. Longer or more complicated runs can be vented with a branch vent pipe or an air admittance valve (AAV), but any valve must be suitable for the building, installed to the maker's instructions and accessible. Not all areas or buildings accept them, so check with building control.
Stub stacks
A stub stack is a short stack, such as for a ground-floor WC and basin, connected to a ventilated drain. Approved Document H sets conditions: no WC floor level above 1.3m from the connection, and no branch centreline more than 2m above the invert of the connection. They are common in extensions but must meet those limits.
Soil vs waste in practice
| Soil pipe | Waste pipe | |
|---|---|---|
| Carries | Toilet waste | Basin, bath, shower, sink, appliance water |
| Typical size | 110mm | 32mm, 40mm or 50mm |
| Fall (branch) | Approved Document H range of 18 to 90mm per metre for WC branches | Similar range, depending on length |
| Joins stack with | Junction or sweep | Boss or small junction |
Common mistakes
- Running a small waste pipe too far without a vent, so the trap siphons.
- Joining a shower waste onto the toilet branch below the pan outlet in a way that backs up.
- Using too many 90-degree bends.
- Forgetting access points; see falls, bends and rodding access.
See sizes in drainage pipe sizes.
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.