DIY drainage fittings, explained for UK homes
Wyes is a straight-talking guide to the pipes and fittings that carry waste away from your house, written for homeowners who want to understand the job before they pick up a saw.
What this site covers
Drainage looks simple until you are standing in a trench with a box of fittings. A wye (also called a Y-junction), a tee, a long-radius bend, a rodding eye: each has a job, and using the wrong one can cause blockages that are expensive to fix. These pages explain the common choices for ordinary UK houses, in plain English, and point you to the official guidance so you can check for yourself.
- Wye vs tee: why a gentle angle beats a right angle for waste.
- Drainage pipe sizes: 110mm, 40mm and the others, and which appliance uses which.
- Soil pipe vs waste pipe: the difference and why it matters.
- Falls and bends: how much slope, and where bends are allowed.
- Approved Document H and building control: what the rules say and when to ask the council.
The basic idea
Domestic drainage is a gravity system. Waste leaves the appliance through a trap (the bend that holds water and stops smells), runs along a branch pipe with a gentle fall, joins a vertical stack or an underground drain and ends in the public sewer or a private treatment system. There is no pump pushing it along, so the layout matters. Pipes need enough slope to carry solids, not so much that water runs ahead and leaves them behind. Joins should point in the direction of flow, and the system must be accessible, so a blockage can be cleared without digging up the garden.
Why fittings matter
Most drainage problems come from a handful of causes: a tight 90-degree junction where debris catches, a pipe laid with the wrong fall, a join that was not pushed home, or no access point to rod the pipe. All of those are down to the choice and fitting of parts. Choosing the right fitting at the start costs nothing extra.
What you can do yourself
Plenty of homeowners replace a section of pipe, add a new sink waste or fit a new gully. Other jobs, such as new connections to a drain or a sewer, changes to the drainage layout or work under or near a building, may need approval. The rules are set out in our building control page. If you are in any doubt, phone your local authority's building control team before starting. It is free to ask, and much cheaper than taking out work that does not comply.
Tools and materials you may need
For most small jobs you need a hacksaw or pipe cutter, a deburring tool, a tape measure and marker, a spirit level and the correct lubricant for push-fit seals. Underground work adds a spade, bedding gravel, a string line and a rodding set. Buy fittings from the same system as your pipe, because push-fit, solvent-weld and compression parts differ slightly in size between makers.
Take photos of the existing layout before you start. They are useful if you later need to explain to building control, a plumber or a buyer what was there and what you changed.
Terms you will hear
- Wye: a branch fitting joining at about 45 degrees.
- Tee: a branch fitting joining at 90 degrees.
- Trap: the water seal under a sink, basin or bath.
- Stack: the vertical pipe that collects waste from upstairs.
- Invert: the lowest inside point of a pipe.
- Rodding eye: a capped access point for pushing rods along a drain.
- Inspection chamber: an access box where drains meet or change direction.
Spotted a mistake or have a question? Get in touch.
Frequently asked questions
What is a wye in plumbing?
A wye, or Y-junction, is a fitting that lets a branch join a main pipe at about 45 degrees, which helps waste flow smoothly.
Do I need building control approval for drainage work?
It depends on the job. New connections and changes to a drainage layout usually do; check with your local authority's building control team before starting.
What size is a normal toilet waste pipe?
Typically 110mm outside diameter, which fits the 100mm minimum Approved Document H sets for drains carrying WC waste.