Want to make that extra space in the basement useful by installing a bathroom? It's a job you can do yourself, and here we show you how.
Time Multiple Days
ComplexityIntermediate
Cost$101–250
Introduction
The First Rule of plumbing is that water flows downhill, but how can you get downhill from a basement? You must solve that problem if you want to put a bathroom there. All the fixtures you install have to drain somewhere, and according to the Rule, that somewhere has to be at a lower elevation than the fixtures.
One way to solve this problem is to do what I plan to do with my cabin — which is at a lower elevation than the septic system for the property — and use a macerating (up flush) toilet system. The one I intend to buy can handle waste from the toilet, shower and sink and feed it under pressure to a sewer pipe several feet higher in elevation.
If you have a sewer that runs underneath your basement floor, you can rely on gravity, but since most basement floors are concrete, it will take some extra work to bury your new pipes. They form what plumbers call the drain/waste/vent (DWV) system for your new bathroom, and installing them is the focus of this how-to. We won't cover the water supply system, because you install it the same way you would in any other part of the house.
The materials you need for this project shouldn't cost more than about $250, but you'll save a lot by doing the work yourself. A plumber would typically charge between $1,500 and $2,000. Even if you run into complications requiring a plumber's help, you'll save money by doing most of the grunt work yourself.
Here's a schematic of the floor plan for the project we describe. Note that the red lines are waste pipes, which connect to the sewer, and the green ones are vents that connect to the building's vent system.
Tools Required
- 4-in-1 screwdriver
- Cordless drill driver
- Hammer
- Jackhammer
- 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
- 3-in. plastic and cast iron pipe and fittings
- 4-in. plastic and cast iron pipe and fittings
- Band couplings
- Concrete
- Pipe glue
Project step-by-step (11)
Step 1
Choose a Point to Tie into the Main Drain
- Locate the main waste stack — the large 3 or 4-inch-diameter vertical pipe that runs into the basement floor.
- 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. The line usually runs straight from the main stack to the clean-out, but it may be diagonal.
- Choose a convenient tie-in point somewhere in the middle of the floor between the stack and the clean-out. Break through the concrete with a sledgehammer and dig down to the main line to verify that it is where you think it is.
Step 2
Measure for Slope
Drain lines require a downhill slope of at least 1/4 inch per linear foot so that waste flows smoothly through the pipes. The depth of the main line at the tie-in point determines the maximum length of the drain line for the bathroom. Here’s how to calculate it
- Measure the depth of the main line at the tie-in point (A).
- Measure the depth of the pipe beneath the new 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.
- Example: If A is 13 in. and B is 10 in., 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 have a few options. You can locate fixtures closer to the tie-in point, build the toilet on a pedestal to raise point B or install an upflush toilet or another type of sewage ejection pump.
Step 3
Lay Out the Bathroom and Drain System
- Mark out the whole bathroom on the basement floor: walls, toilet, sink, shower and finally, the drain lines. Use a pencil to allow changes. This is a great way to try different layout ideas for your bathroom.
- Mockup sections of the drain system and lay them out on the basement floor, using sections of pipe and an assortment of fittings.
- Mark the whole system out permanently on the floor when you’re finished planning. I like to use tape, but spray paint is fine.
Step 4
Trench the Floor
- Start breaking up the basement floor at the main line with a sledgehammer. That’s the best place to start since you’ve already broken through at the tie-in point.
- Pick out the larger chunks of concrete as you go and set them aside.
- Toss the dirt in a separate pile. You don’t want big chunks in the soil— you’ll use it later to backfill the trench.
Pro tip: This part of the job will go faster if you use a jackhammer. I prefer an electric one. It’s easier to use than a pneumatic one, raises less dust and costs about $100 a day to rent. You’ll only need it for a day.
Step 5
Tie Into the Main Drain
Cut into the main line so you can install a Y-fitting. If it’s plastic, you can do this with a reciprocating saw, but if it’s cast iron, you’ll need a pipe snapper. This tool works by tightening a cutting chain until the pipe cracks. You can find one at any tool rental store.
- Slip metal couplers onto both ends of the main line and slide them back to expose the ends of the pipes.
- Insert a Y-fitting of the same diameter as the main drain pipe between the exposed ends.
- Slide the couplers over the joints and tighten the bands.
Caution! Before you cut into the drain, instruct your family to refrain from running water or flushing until you say it’s OK. Once you’ve got the Y-fitting installed and plugged the inlet hole, you can give the all-clear.
Step 6
Build the Drain System
- Construct the drain system using ABS drain pipe. Run a 3-in. pipe to the toilet and a 2-in. pipe to the sink and shower. Tie these together with an adapter fitting and connect them to the Y-fitting in the main drain.
- Dry-fit the pipes to ensure that everything is correct before gluing them. Then, check the slopes with a torpedo level.
- Add vent risers for the toilet, sink and shower. Check your local codes for the distance each vent has to be from the drain and for the vent diameter.
- Cap open pipes to keep sewer gases out of the house.
Pro Tips
- Don’t bury the line until the building inspector has approved your work.
- Determine the exact location of the shower drain after the walls are framed, and you know exactly where the shower is.
- 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.
- When a section of pipe is complete, pack dirt under and around it to keep it from shifting as you build other sections.
Step 7
Patch the Floor
- Backfill the trench with soil and screed three inches of concrete over it. Be sure to pack the soil firmly so it won’t settle later.
- Patch the top of the trench with concrete and smooth it with a steel trowel.
Step 8
Build the Vent System and Connect to an Existing Vent
Frame the bathroom walls, then assemble the vertical sections of the vents inside the framing so you can hide them behind the wall covering. Run the horizontal sections along the floor joists. If you want, you can hide them later with a drop ceiling.
- Choose a location to tie the vent into an existing vent pipe. In most basements, you can use the vent for the laundry sink.
- Glue short sections of pipe to a sanitary tee (santee) fitting. A santee fitting is a tee with a short sweep that is used exclusively for DWV connections.
- Cut out a section of the existing vent and insert the fitting. Check the outlet with a level to ensure it’s vertical or points slightly downward. Secure it to the pipes with rubber couplers or plastic glue fittings.
Step 9
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 10
Know Your Fittings
DWV fittings are designed to facilitate waste water flow in one direction only, and the plumbing codes specify which ones to use in particular circumstances. If you use the wrong fitting, the inspector will flag it and require you to replace it. That can be a hassle, and I’m speaking from experience.
When should I use a T-fitting (santee)?
Use a T-fitting (santee) in drain lines to connect a horizontal pipe to vertical pipes. You can also use one to tie vent lines into horizontal drains or to join vent lines.
When should I use a Y-fitting?
In the drain system, connect horizontal pipes using a Y-fitting (wye fitting). Along with a 45-degree “street” fitting, you can use a Y-fitting to run vertical drainpipes into horizontal pipes, as shown. You can also use Y-fittings in vent systems.
What type of L-fitting do I need?
Use a standard L-fitting for horizontal-to-vertical flow in drain systems. You can also use it for vents.
A “long-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 you can use it in any situation where space allows.
What’s a street fitting?
Standard fittings have hubs (female connections) that fit over pipes. A street fitting has a “streeted” end (a male connection) that fits into a hub, so you can connect it directly to another fitting without using a separate section of pipe. That saves labor and space.
Step 11
Venting FAQs
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?
The vent lets air into the pipes to break the vacuum created by flowing water. They are like the air intake on a gas can. Without venting, water runs slowly (or not at all), and the resulting suction can drain water from P-traps, allowing sewer gases into your home.
Can vents run horizontally?
Yes, but horizontal vent lines must slope at least 1/4 inch per foot toward the drain. They must also 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?
It’s best to use 2-inch vent pipe for the sink, shower and tub in a bathroom, as described here. You could run smaller 1 1/2-inch pipes for the sink or shower, but it’s usually easier to use one size for the whole system.
Do DIYers need any permits when finishing a basement?
Yes, you generally need a building permit when converting your unfinished basement into a finished one, and that’s especially true when you’re installing plumbing or electrical wiring and devices.
Is adding a basement bathroom worth the investment?
That depends on factors too numerous to list here, but if you can keep your costs down by doing the work yourself, it’s generally worth your time and expense. After all, it’s a rare house that can’t benefit from an extra bathroom.
Originally Published: October 07, 2020
