32mm and 40mm Waste Pipe Fittings — Under-Basin, Bath, and Shower Drainage for UK Plumbers

Every domestic bathroom and kitchen install depends on waste pipework that nobody sees but every plumber knows. The 32mm and 40mm solvent weld plastic waste system handles everything from the handbasin draining into a tee to the bath outlet dropping into a 40mm branch — and getting the fittings right first time saves call-backs. Here is a practical guide to the key components, sizing rules, and installation best practice for UK trade plumbers.

When to Use 32mm vs 40mm Waste Pipe

UK Building Regulations Approved Document H sets minimum trap and waste pipe sizes based on the appliance:

• 32mm — washbasins and handwash basins (max 1.7 l/s discharge rate). Also used for bidet waste.
• 40mm — baths, showers, kitchen sinks, urinals, and washing machine standpipes. Standard for any appliance discharging more than 0.8 l/s.
• 50mm — standpipes for washing machines on longer horizontal runs, or where multiple 40mm branches combine before joining the soil stack.

Grey solvent weld is the standard material for internal waste runs in the UK. It is lighter than copper, requires no heat to join, and resists the dilute household chemicals that flow through domestic drains. Black versions are available for more exposed or visible installations. Both conform to BS EN 1329-1 for thermoplastic pipework.

Key Fittings You Will Need on Every Job

Branch (92° Tee) — the core junction fitting

A 92° branch is used wherever you need to connect a waste run into another pipe or change the flow direction into a main line. The 92° angle gives a swept entry that reduces blockage risk compared to a sharp T-joint. On a stacked bathroom layout, 40mm branches typically take the bath and shower waste before connecting to the 110mm soil stack via a boss adaptor. 32mm branches are used for back-to-back basin layouts or where multiple handwash basins share a run.

Solvent Weld Bend (135°) — directional changes without the risk

The 135° bend (sometimes referred to as a 45° change-of-direction) is the preferred fitting for horizontal waste runs where you need to divert around a joist, pipe, or structural element. It provides a gentler curve than a tight 90°, which is important on long horizontal runs where gravity is the only driving force. Avoid 90° knuckle bends on horizontal waste — they create turbulence and trap debris. Reserve 90° bends for vertical-to-horizontal transitions only.

Access Plug with Screw Cap — clearing blockages without cutting out pipework

Every waste run of more than about 3 metres should incorporate an access point. A 40mm access plug screws into a standard socket and gives a clean access point for a drain rod or flexible clearing tool. Building Regulations require that rodding access is available to every length of buried or concealed pipework. Fitting an access plug at the furthest point of a run is cheap insurance against a future call-out to cut out concealed pipe.

Pipe Clip and Bracket — secure the run properly

Horizontal waste runs must be clipped at 500mm maximum centres for 32mm pipe and 600mm for 40mm, to prevent sag (which causes pooling and blockage). Use the correct size clip — a 40mm clip on a 32mm pipe provides no support and will allow movement that eventually pulls fittings apart. Grey and black clips are available to match the pipe colour.

Solvent Weld Jointing — Get It Right First Time

Solvent weld is a chemical joint, not a mechanical one. The solvent softens the outer surface of the pipe and the inner surface of the socket, and the two materials fuse as they re-harden. A correctly made solvent weld joint is stronger than the pipe itself and completely leak-proof. Follow this sequence:

1. Cut the pipe square — a mitred or angled cut will not achieve full socket engagement.
2. Deburr and chamfer the pipe end at about 15° to help it enter the socket without dragging.
3. Dry-fit the joint to check depth of insertion. Mark the insertion depth with a pencil.
4. Apply solvent cement to the outside of the pipe and the inside of the socket simultaneously using the supplied brush. Work quickly — the cement starts to cure within seconds.
5. Push the pipe firmly into the socket with a quarter-turn, hold for 15–30 seconds, and remove excess cement with a dry cloth.
6. Allow minimum 5 minutes before testing and 30 minutes before exposing to flow.

Do not use push-fit fittings on solvent weld pipe — the tolerances are different and the joint will leak under flow. Keep solvent cement tins sealed when not in use and replace them if the contents have thickened.

Connecting to the Soil Stack

Where 32mm or 40mm waste runs connect to a 110mm soil stack, use a boss adaptor (also called a strap boss). Drill a hole in the stack using the appropriate hole-cutter, push the boss through, and secure it with the strap clamp. The waste pipe then solvent welds into the boss. Maintain a minimum 200mm separation between boss entries on the same face of the stack to prevent siphonage. Boss connections must enter the stack at 35–70° to the vertical to ensure a swept entry and avoid turbulence.

Common Mistakes to Avoid

• Fall too steep — for 32mm and 40mm, the correct fall is 18–90mm per metre (approximately 1:40 to 1:12). Steeper than 1:9 causes the liquid to race ahead of solids, leaving residue in the pipe.
• Long horizontal runs without fall — waste pipe is not pressure-fed. Without adequate gradient, it will block.
• Undersized trap — always match trap seal depth to the ventilation strategy. Unvented systems require a minimum 75mm seal; vented systems can use 50mm.
• Mixing grey and black fittings — grey and black are usually from the same manufacturer and are dimensionally compatible for solvent welding, but mixing colours in visible locations looks unprofessional. Pick one and stick with it per run.

Featured Products