Wyes › Wye vs tee: which fitting for which job
Wye vs tee: which fitting for which job
Both fittings let one pipe join another, but they behave very differently when something solid tries to get past. In drainage that difference decides whether a pipe stays clear.
What each fitting is
A tee joins a branch to a main pipe at 90 degrees, forming a T. A wye (or Y-junction) joins at about 45 degrees, so the branch flows in the same direction as the main pipe. In the UK a 45-degree junction is often called a Y-branch or an oblique junction, and a wye is the same thing.
Why the angle matters
Water carrying waste needs to keep moving. A tee forces flow to turn a sharp corner, and heavier material slows and drops out at the join. Paper and solids build up over time. A wye lets the flow merge smoothly. It is also easier to rod, because the rods follow the flow rather than hitting a wall.
What the rules say
Approved Document H (England) says connections between drains should be oblique or in the direction of flow. It says junctions between branch discharge pipes of similar diameter should be a 25mm radius sweep or a 45-degree angle. And when a branch of 75mm or more joins a stack of equal diameter, it should use a sweep of at least 50mm radius or a 45-degree angle. So for soil and drain pipework a 90-degree tee on a main run is not what the guidance points you towards.
Smaller waste branches (up to 40mm) joining a larger pipe of 100mm or more should where practicable connect to the upper part of the pipe wall, so that they do not flow back or become blocked by the main flow.
When is a tee acceptable?
- Vent pipes and air admittance connections, where nothing solid flows.
- Pressure supply pipes (water supply, not waste).
- Sweep tees: a tee with a curved branch is effectively a smooth junction and is the sort the guidance has in mind for stack connections.
A straight tee on a waste pipe is a poor choice, even if some old installations have them. If you find a tee in an existing system, you do not need to rip it out unless it blocks repeatedly, but you should not copy it in new work.
Single and double wyes
A single wye has one branch. A double wye has two, one on each side. Double wyes are used for a pair of back-to-back connections, though in drainage they are less common than the single. Also consider a wye with a reducing branch for joining a 40mm waste onto a 110mm pipe, if you can find one, or use a boss connector, which is a different approach.
Using a boss connector
A boss is a small branch fitting that clips onto a large pipe. You cut a hole in the pipe (often with a hole saw) and solvent-weld or push-fit the boss in place. It is common for connecting 32mm or 40mm waste to a 110mm soil stack. Follow the maker's instructions, and make sure the branch enters at the right angle and height.
Fitting tips
- Dry fit first and mark the pipe so you know how far it goes in.
- Cut square, deburr and chamfer the end.
- Use the right lubricant for push-fit rubber seals, and never grease with oil-based products that may damage the ring.
- Push the pipe fully home, then pull it back slightly if the manufacturer specifies an expansion gap.
- Support the pipe with clips at the right spacing.
See also falls and bends and 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.