Adding a Basement Bathroom: What the Plumbing Rough-In Involves

Everything starts with where your main building drain leaves the house. Plumbers look for it before any design decisions are made.

• If the main drain runs under the basement floor, the new bathroom can usually drain by gravity. We break the slab, tie in below the floor and slope the new lines to it.
• If the main drain exits through the wall above the floor, gravity will not work for fixtures at floor level. The bathroom needs a sewage ejector pump in a sealed basin that lifts waste up to the main line.

Many older homes in central New York have the drain leaving through the foundation wall a foot or two above the basement floor. That is not a problem, it simply changes the plan and the budget. Ejector systems are reliable when they are sized correctly, vented properly and installed with a check valve and a shutoff.

For a gravity installation, the rough-in usually follows this order:

1. 1Mark the exact fixture locations on the floor and walls using the plans. We use a laser level so the shower drain, toilet flange and sink drain line up with the finished walls, not the bare studs.
2. 2Saw cut and remove a strip of concrete along the drain path and at each fixture location.
3. 3Dig the trench to the depth needed to reach the existing drain with the right slope.
4. 4Install the new drain lines in PVC or ABS, sloped at a quarter inch per foot for most residential drain sizes.
5. 5Set a trap under the shower drain and under any floor drain, and stub up the toilet and sink drains to the correct heights.
6. 6Tie the new lines into the building drain with the proper fittings.

In the photo you can see the shower trap in place and a PVC riser that will be cut to final height once the shower base is set. The primer and cement cans next to the trench are part of every PVC joint. Each joint is primed and cemented so it seals permanently under the slab.

Every trap needs a vent. The vent lets air into the drain system so water flowing down does not siphon the water out of the traps. Without it, sewer gas can enter the room and drains will gurgle and run slowly.

In a basement, vents usually run up inside a new wall and connect to an existing vent stack, or they run up to the attic and out the roof. Where running a vent is difficult, code in some cases allows specific alternatives, but the right answer depends on the layout and on the local inspector. We plan the vent routes at the same time as the drains so there are no surprises when the framing goes up.

Compared to the drains, the water supply side is simpler. New hot and cold lines are run from the nearest main lines to each fixture, usually in PEX with a shutoff valve at every fixture.

This is also a good time to look at your water heater. A new bathroom with a shower adds real demand. If your family already runs short on hot water, our article on piping two water heaters in parallel explains one way to add capacity.

If your home is on a private well, adding a bathroom can also change how often your pump runs. A quick check of your pressure tank and switch settings is worth doing before the new fixtures go into service. Our guide on well pressure tank replacement explains what to look for.

Before any concrete goes back in, the new drain and vent system is tested for leaks, usually by filling it with water or pressurizing it with air and watching for a drop. Where a permit is required, the inspector looks at the rough-in with the trench still open.

After the rough-in passes, the trench is backfilled with clean fill, compacted and patched with new concrete. Only then do the framing, drywall, tile and fixtures go in. The finish plumbing, which is setting the toilet, the vanity, the faucet and the shower trim, happens at the very end.

Every basement is different, but the biggest cost drivers are predictable:

• Gravity drainage versus an ejector pump system.
• How far the new bathroom sits from the main drain and the nearest vent.
• Slab thickness and what is found underneath, such as ledge rock or old clay tile lines.
• The number of fixtures, and whether you want a tub, a walk in shower or a laundry hookup.
• The age and condition of the existing drain the new lines connect to.

The plumbing rough-in itself often takes a few days. The rest of the timeline depends on the finish work. We quote the plumbing clearly up front after we see the space.

If you are finishing a basement in Sherburne, Hamilton, Earlville or the nearby towns, bring us in early, before the framing is done. We can confirm whether you need an ejector pump, lay out the drain and vent routes, and coordinate with your contractor on timing.

Learn more about our bathroom and kitchen renovation plumbing, see finished bathrooms in our project gallery, or call (607) 373-6421 to set up a walkthrough.