Soil pipes and waste pipes together form the above-ground drainage system that carries foul water from bathrooms, kitchens, and toilets to the underground drainage or sewer. Getting the sizing, falls, and venting right matters: an undersized pipe blocks under load; insufficient fall causes solids to settle and build up; inadequate venting creates pressure fluctuations that siphon trap seals and allow sewer gas into the building.
The main soil stack in a domestic property is 110 mm diameter uPVC. Branch waste pipes to individual appliances are 32 mm, 40 mm, or 50 mm depending on the fixture. All drainage work that connects to a soil stack or underground drain falls under Part H of the Building Regulations.
Terminology
- Soil pipe / soil stack: carries waste from toilets (WCs). The main vertical stack is 110 mm. Horizontal branches from WCs are also 110 mm.
- Waste pipe: carries grey water (non-soil) waste from baths, showers, basins, sinks, and washing machines. Branch sizes: 32 mm (basin), 40 mm (bath, shower, sink, washing machine), 50 mm (larger commercial sinks; sometimes shower trays with large outlets).
- Single-stack system: the modern standard — all appliances, including WCs, connect into one 110 mm stack. Careful sizing and junction design prevent cross-contamination of traps.
- Two-pipe system: older pre-1960s arrangement — soil and waste carried separately. Still found in older properties; not installed in new work.
- SVP (soil and vent pipe): the stack continues above the highest connection and terminates above the roofline to vent the system. The open vent prevents pressure build-up that would push trap water out.
Pipe Sizes at a Glance
| Appliance | Pipe diameter | Minimum trap depth |
|---|---|---|
| WC (toilet) | 110 mm | 50 mm water seal (integral trap) |
| Bath | 40 mm | 50 mm |
| Shower tray | 40 mm (50 mm for large trays) | 50 mm |
| Wash basin (hand basin) | 32 mm | 75 mm |
| Kitchen sink | 40 mm | 75 mm |
| Washing machine / dishwasher | 40 mm (standpipe or trap) | 75 mm |
| Utility sink | 40 mm | 75 mm |
| Urinal | 50 mm | 50 mm |
| Main soil stack | 110 mm | — |
The 75 mm trap seal depth for basins is specified in Approved Document H because basins are prone to trap siphonage (the short 32 mm pipe and fast discharge can pull the seal).
Falls (Gradients) and Lengths
Correct gradient keeps water moving fast enough to carry solids without running so fast that it outruns them. Approved Document H gives the following guidance:
| Pipe diameter | Minimum gradient | Maximum gradient | Maximum length (no cleaning access) |
|---|---|---|---|
| 32 mm | 1:12 | 1:12 | 1.7 m |
| 40 mm | 1:48 | 1:12 | 3.0 m |
| 50 mm | 1:96 | 1:12 | 4.0 m |
| 75 mm | 1:150 | 1:80 | — |
| 110 mm | 1:150 minimum | 1:80 recommended | — |
A fall of 1:48 means the pipe drops 1 mm for every 48 mm of horizontal run — approximately 21 mm per metre. The 1:48 gradient for a 40 mm basin waste is the minimum; 1:40 is more practical and commonly used.
Common mistake: over-steep falls (steeper than 1:12 on 40 mm waste) cause the liquid to separate from solids, which then settle and block the pipe over time.
Vent Requirements
An unvented soil stack would build up positive pressure as large volumes (a flushed toilet) rush down, and negative pressure as they pass, siphoning the trap seals of connected appliances. Venting rules under Part H:
- The SVP must extend at least 900 mm above any openable window or ventilator within 3 m horizontally, or terminate at roof level if no windows are adjacent
- The SVP must terminate with a rodent-proof vent terminal (fly screen mesh minimum)
- The open end must be at least 200 mm above the eaves or any flat roof surface it penetrates
- Air admittance valves (AAVs) — also called Durgo valves — can replace an open SVP on internal stacks or branch pipes where extending to the roof is impractical. AAVs must be accessible for maintenance, used above the highest fitting in the system, and must not be used as the sole vent for a building — at least one open SVP must remain. AAVs must comply with BS EN 12380.
Materials
uPVC (standard, white or grey)
The overwhelmingly dominant material for new above-ground drainage in the UK. Available in all standard diameters; push-fit or solvent-weld joints. Push-fit (ring-seal) allows for thermal expansion — critical for long runs exposed to temperature change. Solvent-weld is used where a permanent, compact joint is needed.
- 110 mm soil pipe (3 m length): £15–£30
- 40 mm waste pipe (3 m length): £5–£12
- Push-fit 110 mm double socket: £5–£10
- 110 mm swept T-junction: £8–£18
ABS (acrylonitrile butadiene styrene)
Similar to uPVC in appearance and cost; slightly more impact-resistant. Less commonly stocked in builders’ merchants but available from specialist suppliers. Compatible with uPVC fittings when same-specification connections are used.
Cast iron
Found in pre-1970s properties. Heavy, durable, repairable. Still specified for listed buildings or conservation areas where uPVC is not permitted. Connections to existing cast iron with new uPVC work are made with rubber couplings (e.g. Fernco or equivalent).
HDPE and PP
High-density polyethylene or polypropylene drainage systems (e.g. Geberit, Wavin Tigris) are used in higher-specification work and in commercial buildings. More expensive than uPVC; superior resistance to chemical attack and temperature cycling. Uncommon in standard domestic work.
Building Regulations: Part H
Any new above-ground drainage connection to a soil stack or change to the drainage system is subject to Approved Document H (ADH) of the Building Regulations. Key requirements:
- Notification: drainage work must be notified to Building Control unless it is purely maintenance of existing drainage. Adding a new toilet, bathroom, or kitchen branch is notifiable.
- Competent Person Scheme: a plumber registered with APHC (Association of Plumbing and Heating Contractors) or the JIB Plumbing Competent Person Scheme can self-certify Part H work and notify on the homeowner’s behalf.
- Access for cleaning: branch pipes over 6 m long, or pipes with bends, must have rodding eyes or access caps. The stack should have a cleaning access port at the base.
- Rodding access points at every 45° or 90° change of direction in the underground drain run
Additional rules for loft conversions and extensions
Adding a WC or bathroom in a loft conversion requires a soil pipe route to the main stack or, if the stack is not accessible, a new independent stack. Planning permission is not normally required for soil pipe installations within the existing building envelope. However, an external soil pipe added to the front elevation of a property in a conservation area may require planning consent — check with the local planning authority.
Typical Costs
| Work | Typical cost |
|---|---|
| Adding a single 40 mm waste pipe connection to existing stack | £80–£200 |
| Adding a new WC branch (110 mm) to existing stack | £200–£500 |
| New soil stack installation (3-storey, internal) | £800–£2,500 |
| Replacing old cast iron stack with uPVC (2-storey) | £600–£1,500 |
| Adding an en-suite with new branch waste and vent | £400–£900 (drainage only) |
| Air admittance valve installation (internal stub stack) | £80–£200 |
Prices for plumbing labour in London and the South East are typically 25–40% higher than the national average. Emergency callouts outside working hours are charged at 1.5–2× normal rates.
Common Problems and Solutions
| Problem | Cause | Fix |
|---|---|---|
| Gurgling at basin after WC flush | Trap siphonage; insufficient vent | Check and restore trap seal; install AAV if not already present |
| Smell of drains from bathroom | Dry trap; cracked trap | Run water to refill; replace trap |
| Slow drain at multiple appliances | Partial blockage in stack or underground drain | Rod or jet-wash from stack access point |
| Leaking push-fit joint | Rubber seal perished or pipe not fully inserted | Replace seal; re-insert pipe fully and check locking ring |
| Overflowing at WC branch during heavy use | Undersized or blocked underground connection | Rod underground drain; CCTV survey to confirm capacity |
When to Call a Professional
Any connection to a soil stack, new WC installation, or substantial change to branch drainage should be carried out by or at least inspected by a registered plumber under a competent person scheme. Drainage errors are often hidden within walls and floors; failures that are only discovered when water tracks down a structural wall can be expensive to rectify and may create a damp problem. For a new bathroom or additional WC, always obtain a Building Regulations completion certificate — it will be needed when you sell the property.
