You are tying a toilet drain into a vertical soil stack.
The toilet is separately vented.
You keep reading two answers.
Some say use a sanitary tee.
Others say use a combo wye and 45.
So which one is “right”?
Quick Q&A
Q: What is the preferred fitting for a horizontal toilet branch into a vertical stack?
A: A combo wye and 45 is usually the best choice, but many codes also allow a sanitary tee for horizontal-to-vertical flow.
Think about flow direction first
Plumbing fittings are about steering waste.
The “best” fitting depends on the flow direction.
Waste falling down a stack is predictable.
Waste moving across a horizontal pipe needs guidance.
When a sanitary tee makes sense
A sanitary tee is a short-sweep fitting.
It is commonly used on a vertical pipe.
It can be allowed when the flow drops from horizontal to vertical.
Gravity helps the turn happen cleanly.
This is why you see it used on stacks.
But it has a tighter turn inside.
That can be less forgiving with heavy toilet discharge.
It can also be harder to snake cleanly.
Why do many plumbers prefer a wye plus 45
A combo wye and 45 is a long-sweep pattern.
It guides the flow into the stack wall smoothly.
It reduces splash and turbulence.
It tends to resist clogs better over time.
It is also friendlier for drain cleaning tools.
If you have room, it is a strong “default” choice.
This is especially true for 3-inch and 4-inch toilet lines.
Your vent detail matters
You said the toilet is separately vented.
That is good for this decision.
It reduces odd air issues at the stack tie-in.
Still, follow your local vent rules.
Keep vent takeoffs vertical or at 45 degrees.
One important warning
Do not use a sanitary tee on its back for drainage.
That means horizontal-to-horizontal, or vertical-to-horizontal drainage turns.
Those setups can block flow and trap solids.
If two fixtures hit the stack at one level
Back-to-back toilets need extra care.
A double wye or double combo is usually best.
A double sanitary tee can cause cross-flow problems.
Some codes limit the use unless the stack is oversized.
My practical recommendation
- Use a combo wye and 45 if space allows.
- Use a sanitary tee only if your code allows it.
- Avoid “flat” tees on drainage lines.
- Plan cleanout access for future service.
- Confirm the fitting pattern with your inspector.
Helpful external resources
- ASPE discussion of approved fitting patterns
- IAPMO UPC spotlight on horizontal-to-vertical fittings
- Charlotte Pipe DWV spec notes on fitting use
I’m Chris Mayer, writing for Plumbing 101.