Basement bathroom rough plumbing: how to identify each stub and test it

The rough plumbing in your basement floor is the set of capped pipes a builder left in the slab for a future bathroom. The toilet drain is usually a 3-inch pipe, and the shower and sink drains are often 2-inch lines.¹² You can often build a bathroom off it. First, identify each stub, measure it against your planned walls and test it before framing starts.

TL;DR

• Many builder rough-ins cover only the pipes that had to go under the concrete. Expect to add the vents above the slab.³
• The usual toilet figure is 12 inches from the finished back wall to the drain center.² A stub that sits closer needs an offset flange or a short concrete cut to move it.¹
• Some pipes in a basement floor serve something else. Backflow preventers, ejector pits and radon pipes can all look like bathroom stubs.¹³
• Trace every pipe and water test the drains before the floor gets covered. A leak found after tile goes down means opening finished work.
• HomeGuide puts rough-in plumbing for a new bathroom at $450 to $1,800 and a full basement bathroom at $10,000 to $25,000.⁴ Distance from existing drain and vent lines raises the plumbing cost.⁴

What is considered rough-in plumbing?

Rough-in plumbing is every pipe installed before fixtures and finishes go in. In a basement that means drain lines under the slab, vent pipes and hot and cold supply lines. The toilet, sink and shower connect later. Many basement rough-ins include only the pipes that had to go in before the concrete was poured.³

One experienced answerer on DIY Stack Exchange described a builder's rough-in as "put pipes before pouring concrete." Pipes above the concrete are easier to add later than pipes below it.³ So the builder sets the drains in the floor and leaves the vents and walls for whoever finishes the room.

On a new build, the rough-in stage ends with tests. The drain lines get a water test and the supply lines get a pressure test, so the work can pass inspection before the slab is poured.² When you buy a house with a capped rough-in, you usually can't tell whether those tests passed or how the pipes have held up since. Plan to test it again before you build on it.

Not every house has a rough-in, and one that exists may not fit the layout you want. One reply on DIY Chatroom said rough plumbing for future baths often gets too little forethought.¹

What does rough plumbing look like in a basement?

Rough plumbing in a basement looks like a few capped pipes sticking out of the concrete and a square pocket left open in the slab. A 3-inch pipe about a foot out from a wall line is usually the toilet.³ The open pocket is the shower or tub drain, and smaller pipes near the wall are the sink drain and vents.¹

Use this table as a field guide while you stand in the space.

The toilet drain

The toilet stub is usually the capped 3-inch pipe set out from a wall.¹ In a 2009 New Hampshire house, the owner only confirmed the toilet stub after taking the silver wrap off it.³ On a new rough-in the pipe is left long and cut after the concrete is finished. The closet flange, which the toilet bolts to, then goes on top of the slab.²

The shower or tub drain

The shower drain is often out of sight. Builders box out a square of the slab so the trap can go in later at the exact spot the shower pan needs. In one build that box measured 13 by 13 inches and was filled with sand.² A homeowner on DIY Chatroom dug about 4 inches into a 12-inch square hole and found no pipe. A reply said the pipe should be stubbed in there somewhere, deeper down, and the owner should plan to install the trap.¹

The sink drain and vents

Vents are the vertical pipes near the wall. They let air into the drain system so water flows and traps don't siphon dry. A sink drain often shows up as a short horizontal stub off those vertical pipes. The vents have to stay, even if your contractor cuts and re-couples them to frame around them.¹ Finding no vents above the slab is fairly normal on a builder's rough-in.³

Other pipes that look like stubs

• Backflow preventer. It keeps a street sewer backup from coming up into the basement. In the DIY Chatroom thread it was a black 5-inch pipe, and the advice was to cut it flush, cap it and leave it accessible for service.¹
• Sewage ejector pit. A sealed round cover may hide a pit. In the New Hampshire house, the stubs the owner could snake all ran to that pit.³
• Floor drain, laundry drain or radon vent. A lone pipe outside the bathroom may serve any of these.³ If it could be a radon pipe, have a radon mitigation professional look at it before anyone caps or cuts it.

What are the standard rough-in dimensions?

The common toilet figure is 12 inches from the finished back wall to the center of the drain.² At the framing stage that becomes 12 1/2 inches.² Replies on DIY Chatroom put side clearance at 15 inches from the drain center to a finished side wall.¹ Your local plumbing code sets the actual minimums, and local adoption and amendments vary.

These are layout figures from working plumbers and builders. The clearances that bind your job come from the plumbing code your city or county adopted. Your contractor and your inspector check them.

The half inch between the two toilet figures is the drywall. The builder in the video measured 12 1/2 inches from the string line because that line marked the framing.²

Wall position is where old rough-ins go wrong. Builders often expect a wall framed out from the foundation wall to leave room for insulation, which is why stubs can sit well out from the concrete.³ Trouble starts when the stub was set for a different wall plan. In the DIY Chatroom thread, the toilet stub sat 9 to 10 inches from the future wall.¹ One reply put that at about the depth of a 2x4 wall too close.¹

That thread named two fixes. An offset flange can buy an inch or two with little concrete removal. A bigger miss means chipping out the concrete around the pipe, moving it and pouring new concrete.¹

For the shower, the pan decides the walls. The video's builder chose a 38-inch pan, so the side wall had to sit 19 inches from the drain center.² Pick the pan or tub before framing and check its drain position against the stub.² The DIY Chatroom owner found the shower hole centered on the back wall and judged it could work for a shower but not a bath.¹

What should you test before building on an old rough-in?

Test an old rough-in in five steps before framing: trace each pipe, measure every stub against the finished wall plan, find the vents, water test the drains and confirm whether the lines drain by gravity or into an ejector pit. A plumber with a camera snake and a tracer can map what the builder intended.³

1. Trace every pipe

Run a snake or camera through each stub and see where it comes out. The New Hampshire owner snaked the pipes and found the ones reachable all joined and ran to a sewage pump pit.³ One answer on that thread suggested paying a plumber for an hour with a camera snake and a tracer to learn the original plan.³ HomeGuide lists plumber rates at $45 to $80 per hour or more.⁵ Your estimate should price the camera visit.

A camera also shows the fitting right under the toilet. One homeowner on DIY Chatroom tried three toilet brands and all of them flushed badly. The plumbers found the elbow under the toilet turned almost at once. They said only a power-flush toilet or a raised platform would likely work. The homeowner added that this may not be common.¹

2. Measure each stub against the finished walls

Mark your planned walls on the floor before you measure. The builder in the video set a stake at each corner, squared the layout with diagonal measurements and ran a string line for the inside of each wall.² Measure every stub from the finished face of the wall and compare it with the table above.

3. Find the vents

Look for vertical pipes along the wall and follow them toward the joists. If there are none, the vent route becomes part of the job.³ Without proper venting, toilets can gurgle, sinks drain slowly and sewer gas can enter the room. Common routes tie into an existing vent stack, run a new vent up through the framing or use an air admittance valve where it's permitted.

4. Water test the drains

A water test plugs the drain lines and fills them to the brim of the tallest pipe. On a new build, this is the test that gets the rough-in ready for inspection.² Ask your contractor to run the same test on an old rough-in before the floor gets covered. Roughed-in supply lines get a pressure test. The video's builder held 100 PSI overnight and noted that some places require only 80 or 70 pounds.²

5. Check where the drains go

Find out whether the bathroom drains by gravity into the main sewer line or into an ejector pit. If the main line sits higher than the fixtures, the bathroom needs a pump to lift the waste up to it.³ Confirm the cleanouts are accessible. Keep any backflow preventer reachable after the walls go up.¹ One DIY Chatroom member had no backflow preventer when the home's sewer pipe broke, and raw sewage came up through the basement tub drain.¹

Can you put a bathroom in a basement without rough plumbing?

Yes. Without a rough-in, your contractor cuts trenches in the slab, lays new drain lines sloped toward the main sewer line and patches the concrete. If the sewer line leaves the house above the basement floor, the bathroom needs a lift system such as a sewage ejector pump.³ The slab work adds cost and time.

The easiest spot sits below an existing upstairs bathroom, where new lines can tie into the plumbing stack. A bathroom far from the main sewer exit needs a longer drain run. HomeGuide notes that rough-in plumbing costs more when the new bathroom is far from existing drain and vent lines.⁴

The plumbing goes in the same order as the new-build rough-in in the video. The lines are dug out and the drains are laid at a steady slope. The shower trap gets a boxed pocket, and the drains are water tested before concrete is poured.² Your contractor pulls the permit and books the inspections.

The guide to adding a bathroom to your basement covers layout and budget for a bathroom starting from bare concrete.

Can a shower and toilet share the same drain?

Yes. A common layout runs the shower and sink on 2-inch branches into the toilet's 3-inch drain line. Each fixture needs a trap, and the group needs venting. Whether a new bathroom can join an existing 3-inch line depends on how much already drains into it.²³

In the video build, the shower drain joins the 3-inch main line through a combination Y that steps down to 2 inches.² The sink gets its own 2-inch line.² The toilet connects to the same main. The vent branches off with another Y and runs up the wall to exit through the roof.² The shower trap sits under the slab at the drain.²

Some layouts let one pipe work as a vent and a drain at once. A DIY Stack Exchange answer describes a stub that could vent the shower and also carry the sink drain as a "wet vent."³ Venting rules are highly code-specific, so your plumber and inspector settle them.

Capacity is the other limit. Drain load is counted in drainage fixture units. An experienced answerer on that same thread said a new bathroom group could join an existing 3-inch pipe unless the fixtures already on it meant it should join after the line grows to 4 inches.³

How much does it cost to rough in plumbing for a basement bathroom?

Rough-in plumbing for a new bathroom runs about $450 to $1,800, and it costs more the farther the bathroom sits from existing drain and vent lines.⁴ A full basement bathroom runs $10,000 to $25,000 and a half bath $3,000 to $10,000.⁴ Slab cutting and long drain runs add cost on top.

These are HomeGuide's national figures from its October 2025 cost guide. Its fixture prices include materials and labor.⁴ The guide to basement bathroom cost covers the full budget.

HomeGuide notes that converting space with plumbing already in place saves money, which is what a working rough-in gives you.⁴ Set aside another 10% to 20% of the budget for hidden plumbing, wiring or foundation problems found once work begins.⁴ Your estimate should price any stub moves, ejector system or new trenches.

What should a quote for finishing a rough-in show?

A good quote for finishing a rough-in names which stubs stay and which move. It says how the bathroom gets vented and whether it needs an ejector pump. It lists the drain and pressure tests, the permit and the inspections. If a line is missing, ask about it before you sign.

Questions to ask your contractor at the estimate:

• Which stub is which, and how did you confirm it?
• How far is the toilet stub from the finished back wall in my layout? Will you use an offset flange or move the pipe?
• Where will the vent run? Is an air admittance valve allowed here?
• Will you water test the drains and pressure test the supply lines before the floor is finished?
• Does this bathroom drain by gravity, or does it need an ejector pump?
• Is there a backflow preventer, and will it stay accessible?
• Who handles the exhaust fan wiring? HomeGuide notes a new bathroom needs an electrician for outlets, lighting and the exhaust fan.⁴

Basement Remodeling Co. matches you with one vetted, licensed basement contractor for your metro. That contractor gives you a free on-site estimate and a written, itemized quote, and nothing starts until you sign. When your rough-in checks out, get a written quote for your basement bathroom.

If the bathroom is one part of a bigger build, the basement finishing service page covers the whole project. You can also find a basement contractor by ZIP code. In Colorado, start with basement finishing in Denver, Colorado Springs basement finishing or basement finishing in Fort Collins. In Georgia, see basement finishing in metro Atlanta.