Adding a New Shower and Bathtub Drain
Planning to tie a new shower and bathtub into existing plumbing? Learn proper venting, pipe size, and fittings to avoid code violations and clogs.
Home remodels often bring new plumbing challenges. A common situation is adding a bathtub and shower drain to an existing setup. While this may look simple, venting, pipe size, and fittings are critical to get right. Poor choices can lead to clogs, gurgling drains, or even code violations.
In this case, the plan is to cut off the old p-trap and tie into it using a Fernco or similar coupling. From there, the idea is to pipe upstream to both the new shower and bathtub locations while sharing the vent pipe already in place. The alternative option is capping off the old shower plumbing and running a new pipe to an Air Admittance Valve (AAV).
Question:
Can you tie a new shower and bathtub into an old P-trap line and share the vent, or should you use a different method?
Answer:
No. You should not tie two new fixtures into an old P-trap connection. Each fixture needs its own properly sized trap and vent connection. For showers and tubs, use 2” PVC drains. The vent must connect correctly to prevent siphoning. An AAV can be used only if your local code allows it. Always follow IPC or UPC requirements for trap arm length and vent placement.
Offsetting through old p-traps may work temporarily, but it can create slow drainage and is not a code-approved method. The better solution is to run new 2” PVC drains for both the bathtub and shower and connect them with proper wyes and a shared vent or separate vents as required.
For details, see this plumbing vent diagram guide. You may also want to check the International Plumbing Code venting section for official requirements.
Getting this right the first time will save you future headaches with drainage and inspections.
I’m Chris Mayer, writing for Plumbing 101.
