How To Plumb A Basement Bath

Layout of plumbing on top of a concrete floor | Construction Pro Tips
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Installing a "DWV" System

This story will focus on the most difficult part of the project: installing the “DWV” system (drain, waste and vent). The DWV system requires some hard labor—breaking up concrete—and enough know-how to construct it so waste will be carried away without any problems. You supply the labor; this story will supply the know-how. The materials for the DWV system shown here cost about $250.

Digging down into the tie-in location | Construction Pro Tips
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Locate the main drain

You’ll have to connect new drain lines to an existing line under the basement. So before you can do any real planning, you have to find that line. First, locate the “main stack,” the large (3 or 4 in. diameter) vertical pipe that runs into the basement floor. From there, the pipe runs under the floor and out to the city sewage system under the street. But it may run at an angle rather than straight out to the street. Look for a clean-out plug along the street-facing wall of the basement. If you find one, that’s most likely the spot where the line exits your home. And usually, the line runs straight from the main stack to the clean-out.

If you have a private septic system, your main line will run toward the location of the drain field. If you’re unsure where the line is, you have a couple of options. You can punch through the floor where you think it is. You might end up enlarging that hole or breaking a second exploratory hole, but that’s not as bad as it sounds; all it will cost you is some wasted time and a couple of extra bags of concrete mix when you patch the floor. Your second option is to get a plumber to help. In most areas, a brief house call will cost you $75 to $150. Some plumbers have access to high-tech equipment that locates lines precisely, but expect to pay $200 for that service.

Plan the system

Once you’ve located the line, you’ll have to make sure it’s deep enough to allow downward slope in the new drain lines that will run from your future bathroom. Then grab a pencil and mark out the whole bathroom on the basement floor: walls, toilet, sink, shower and finally the drain lines.

Consider it all a preplan at this point. Chances are, you’ll have to make some changes as the plan develops. You may want to 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. For photo clarity, we marked out bold lines on the floor. But simple spray paint is fine for drain lines.

Cutting into the hub | Construction Pro Tips
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Trench the floor

A plain old sledgehammer will bust up a basement floor. Breaking through at the tie-in point may take a few dozen whacks. But once you have a starter hole, the job gets easier because the concrete has space to crack and break off. Within a few minutes, you’ll learn to aim your blows and bust out a neat trench line. Pick out the larger chunks of concrete as you go. Ideally, most of your trench will be just wide enough for your spade. When digging, toss the dirt on a pile separate from the larger chunks of concrete. You don’t want big chunks in the soil you’ll use for backfill later.

Break out a section of drain

After completing the trenches for the new lines, cut into the main line so you can install a Y-fitting. Our tie-in point was near an existing hub, so we cut out the hub. Make sure no one runs water (or flushes!) while the line is open.

putting the new plumbing together | Construction Pro Tips
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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. Then plug the inlet and grant your family the freedom to flush again.

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Build the drain system

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

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Patch the floor

Backfill the trench with soil and screed 3 in. of concrete over it. Pack the soil firmly so it won’t settle later. Smooth the concrete with a steel trowel.

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Build the vent system

After framing the bathroom walls, assemble the vent lines. The vent system is a lot simpler than the drain system. We ran our vent lines below the floor joists and later framed a lower ceiling to hide the pipes. If you want to preserve ceiling height by running pipes through the joists, you’ll have to bore some large holes, which can weaken the joists.

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Connect to an existing vent

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.

Measuring out from the framing to locate the shower vent | Construction Pro Tips
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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. Then set the pan in place again to check your work before you finally glue up the fittings.

Next, check out this guide to finishing a basement.