How to Plumb a Basement Bathroom

Roughing-in the plumbing for a basement bathroom is a big job, but the savings are huge.

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A bathroom in the basement adds a lot of value to a finished basement. Here's how to plumb the bathroom yourself and save at least $1,000 on plumbing costs.

Tools Required

  • 4-in-1 screwdriver
  • Cordless drill
  • Hammer
  • Level
  • Miter saw
  • Rags
  • Reciprocating saw
  • Safety glasses
  • Shop vacuum
  • Sledgehammer
  • Socket/ratchet set
  • Spade
  • Stepladder
  • Tape measure
  • Trowel

Materials Required

  • 2-in. plastic and cast iron pipe and fittings
  • 2x4s
  • 3-in. plastic and cast iron pipe and fittings
  • 4-in. plastic and cast iron pipe and fittings
  • Band couplings
  • Concrete
  • Pipe glue

Adding a basement bathroom is a big, complicated project. But that doesn’t mean you can’t do it. Thousands of DIY plumbers successfully tackle the job every year, and so can you.

We will focus on installing the “DWV” system (drain, waste and vent), which is the most difficult part of plumbing a basement bathroom. The DWV system requires some hard labor—breaking up concrete—and enough know-how to construct it so that waste will be carried away without problems. You supply the labor and the basement bathroom ideas; this article will supply the know-how.

The materials for the DWV system shown here cost about $250. Plumbers’ labor rates vary a lot by region, but a basement bathroom cost would run between $1,200 to $2,000 for a pro job similar to the one shown here.

Here’s a sample floorplan and project plan, to give you an idea of the scope of this job:

Connect the basement bathroom plumbing to the existing drain and vent lines in the floor and ceiling to complete the rough-in plumbing.

Project step-by-step (13)

Step 1

Locate the Main Drain

  • Locate the “main stack,” the large (3 or 4-inch-diameter) vertical pipe that runs into the basement floor.
  • Look for a cleanout plug along the street-facing wall of the basement.
    • Note: If you find one, that’s most likely the spot where the line exits your home. The line usually runs straight from the main stack to the cleanout, but it may be diagonal.
  • Break through the concrete to verify that the main line is where you think it is.
  • Make sure it’s deep enough to allow adequate downhill slope in the new drain lines.

Step 2

Measure for Sufficient Slope

    • Note: Drain lines require a downhill slope of at least 1/4 in. per linear foot (see note below) so that waste flows smoothly through the pipes.
  • Measure the depth of the center of the main line (at the tie-in point) (A).
  • Measure the future depth of the horizontal pipe beneath the drain (B).
  • Do a little math. (A – B) x 4 = the maximum length (in feet) of the drain line, from the main to the end of the horizontal pipe under the drain.
    • Note: If A is 13 in. and B is 10 in., for example, the maximum length of the drain line is 12 ft. (13 – 10 = 3; 3 x 4 = 12).
    • Pro tip: If your main line isn’t deep enough, you’ll have to locate fixtures closer to the line or install a sewage ejection pump.

Step 3

Plan the System

  • With a pencil, mark out the whole bathroom on the basement floor: walls, toilet, sink, shower and finally, the drain lines.
    • Pro tip: This is a great way to try different layouts ideas for your bathroom.
  • Mock up sections of the system and lay them out on the basement floor, using sections of pipe and an assortment of fittings.
  • When the whole system is planned, mark it out on the floor.
    • Pro tip: We like to mark up the plan with bold lines of tape or paint. But simple spray paint is fine for drain lines.

Rent a Snapper

A cast iron pipe snapper works by tightening a cutting chain until the pipe cracks. They’re available at tool rental stores. Old cast iron pipe can crush rather than crack. If that happens, you’ll have to abandon the snapper and cut the slow way: with a reciprocating saw. If you have plastic pipe, cutting into the main is quick and easy with a reciprocating saw.

If the waste line is cast iron, cut it using a snapper.

Step 4

Trench the Floor

  • Start breaking up the basement floor at the main line with a sledgehammer.
    • Note: Breaking through at the tie-in point may take a few dozen whacks.
  • Pick out the larger chunks of concrete as you go.
  • Toss the dirt in a pile separate from the larger chunks of concrete.
    • Note: You don’t want big chunks in the soil you’ll use for backfill later.
Step 5

Break Out a Section of Drain

  • Cut into the main line so you can install a Y-fitting.
    • Pro tip: Make sure no one runs water (or flushes!) while the line is open.

Step 6

Tie Into the Drain

  • Slip rubber couplers onto the main line.
  • Insert the Y-fitting.
  • Slide the couplers over the joints and tighten the bands.
  • Plug the inlet and tell your family it’s safe to flush the toilet again.

Step 7

Build the Drain System

  • Determine where the exact location of the shower drain will be after the walls are framed.
    • Pro tip: The location of the drains and vents is critical—check and double-check your work before you glue joints together.
  • Cap open pipes to keep sewer gas out of your home.
    • Pro tip: Don’t bury the lines until the building inspector has approved your work.

Step 8

Patch the Floor

  • Backfill the trench with soil and screed 3 inches of concrete over it.
    • Pro tip: Pack the soil firmly so it won’t settle later.
  • Patch the top of the trench with concrete smoothed with a steel trowel.

Tips to Help You Get it Right

  • Buy twice as many fittings as you think you’ll need and a few types that you don’t think you’ll need. Return the leftovers when the job is done.
  • If you don’t have a torpedo level, buy one. It’s the handiest tool for checking the slope of pipes.
  • When a section of pipe is complete, pack dirt under and around it to keep it from shifting as you build other sections.
  • Know the “rough-in” of your basement toilet (the distance from the wall to the center of the drain, most likely 12 in.). Don’t forget to account for the thickness of framing and drywall.
  • Backfill the trenches with care. You want to pack the soil tightly to prevent settling later, but be sure not to move the pipes as you tamp the soil.
Step 9

Build the Vent System

  • After framing the bathroom walls, assemble the vent lines.
    • Note: We ran our vent lines below the floor joists and later framed a lower ceiling to hide the pipes.

Step 10

Connect to an Existing Vent

    • Note: In most basements, you can tie your new vent system into the line that vents the laundry sink. 
  • Glue short sections of plastic pipe into a T- or Y-fitting.
  • Cut out a section of the existing vent pipe and make connections with rubber couplers.

Step 11

Position the Shower Drain

  • Set the shower pan in place and measure from the walls to determine the exact location of the drain.
  • Assemble the drain and trap without glue.
  • Set the pan in place again to check your work before you finally glue up the fittings.

Step 12

Waste Line Q & A

When Should I Use a T-fitting?

Use a T-fitting in drain lines to connect a horizontal pipe to vertical pipes. It can also be used to tie vent lines into horizontal drains or to join vent lines.

When Should I Use a Y?

In the drain system, use a Y-fitting to connect horizontal pipes (Photo 3). Along with a 45-degree “street” fitting, you can use a Y-fitting to run vertical drainpipes into horizontal pipes as shown. A Y-fitting can also be used in vent systems.

What Type of L Fitting Do I Need?

  • A standard L-fitting is used for horizontal-to-vertical flow in drain systems.
  • A “sweep” or “long-turn” L-fitting is OK for almost any situation and is required in two situations: horizontal-to-horizontal turns and vertical-to-horizontal turns (as shown). But it can be used in any situation where space allows.
  • Use a vent L-fitting only in vents.

What’s a Street Fitting?

Standard fittings have hubs that fit over pipes. A street fitting has a “streeted” end that fits into a hub, so you can connect it directly to another fitting without using a section of pipe. That saves labor and space.

What Size Drainpipe Should I Use?

  • The basement toilet requires 3-in. or larger.
  • Use 2-in. for the others.
  • Pipes smaller than 2 in. aren’t allowed beneath a concrete slab.
Step 13

Venting Q & A

Where Can I Use a Vent L-fitting?

  • A vent L-fitting can be used anywhere in the vent system, but only in the vent system—never where waste flows.
  • The other two types of L-fittings are OK for venting, too.

What’s the Vent For?

  • A plumbing vent is kind of like the air intake on a gas can; it lets in air.
  • Without venting, a slug of sewage racing through a waste line creates air pressure and vacuum in the pipe. That means noisy, gurgling drains. Even worse, the vacuum can suck all the water out of traps, allowing sewer gas to flow freely into your home. Yuck.

Can Vents Run Horizontally?

Yes, but horizontal vent lines must be at least 6 in. above the “spill line,” which is the level where water would overflow the rim of a sink, tub or basement toilet.

What Size Vent Pipes Do I Need?

  • A typical bathroom like the one we show (sink, toilet, shower or tub) requires a 2-in. vent.
  • You could run smaller pipes to the sink or shower, but it’s usually easier to use one size for the whole system.