Adding a basement bathroom
Adding a basement bathroom is a big,
complicated project. But that doesn’t
mean you can’t do it. Thousands of DIYers
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 waste will be carried away without problems.
You supply the labor; 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 most
licensed pros would charge $1,200 to
$2,000 for a job similar to the one shown here.
Figure A: Plumbing a basement bath
Figure A: Plumbing a basement bath
Connect the basement bathroom plumbing to the existing drain and vent lines in the floor and ceiling.
Find the main drain line
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
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
cleanout 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 cleanout.
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
(Photo 1). 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
locates lines precisely, but expect to pay $200 for that service.
Make sure you have sufficient slope for the waste line.
Slope Makes the Sewage Flow
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. To
determine if your plan allows for
that, take a few measurements:
A: The depth of the center of the
main line (at the tie-in point).
B: The future depth of the horizontal
pipe beneath the drain.
Now 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. 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).
If your main line isn’t deep
enough, you’ll have to locate fixtures
closer to the line or install a sewage ejection
Note: Some local codes allow 1/8 in. per foot with 3- or 4-in. pipe.
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.
Cut cast iron pipe with a snapper.
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
Trench the floor
A plain old sledgehammer will bust up a
basement floor. Breaking through at the
tie-in point (see Photo 1) 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.
Build the drain system
Begin the drain system by cutting into
the main line (Photo 2) and splicing in a
Y-fitting (Photo 3). We used a no-hub cast
iron Y-fitting to tie into our cast iron
main. But you can use a plastic Y-fitting
instead if you glue short sections of pipe
into the Y-fitting to accommodate the
rubber couplers. Use that same method
to tie into a plastic main.
For your DWV system, you can use ABS
plastic (as we did; Photo 4) or PVC. Both
are easy to cut and join. The hard part of
any underground pipe work is building
branches that end up exactly where you
want them while maintaining a constant
slope of at least 1/4 in. per running foot.
Here are 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 (see Photo 7). 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 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
- Backfill the trenches with care (Photo
5). You want to pack the soil tightly to
prevent settling later, but be sure not to
move the pipes as you tamp the soil.
Build the vent system
The vent system is a lot simpler than the
drain system. We ran vent lines under
the floor joists (Photo 6) and framed in a
lower ceiling later. 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. To avoid that, see “How to drill through floor joists.”
In most basements, you can tie your
new vent system into the line that vents
the laundry sink. Our plumbing inspector
allowed us to connect our new 2-in. vent
line to an existing 1-1/2-in. vent. Before
cutting a section out of the old steel vent,
we installed extra metal strapping to support
the pipe during and after cutting.
Note: Plumbing codes vary by locality.
The rules we give in this article generally
follow the strictest codes. Your
local rules may be more lenient about
issues like vent sizing, the choice of fittings, etc.
Waste line Q & A
What size drainpipe should I use?
The toilet requires 3-in. or larger. Use
2-in. for the others; pipes smaller than
2 in. aren’t allowed beneath a concrete slab.
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Venting Q & A
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, vacuum can
suck all the water out of traps, allowing
sewer gas to flow freely into your home.
Vent-to-trap distance—there’s a limit
Every drain needs a trap, and every trap needs a vent. The
maximum distance between the trap and
vent depends on the diameter of the pipe.
Memorize these distances for midterm exams:
For 1-1/4" pipe, the max. horizontal distance to vent is 30"
For 1-1/2" pipe, the max. horizontal distance to vent is 42”
For 2" pipe, the max. horizontal distance to vent is 5'
For 3" pipe, the max. horizontal distance to vent is 6'
For 4" pipe, the max. horizontal distance to vent is 10'
Note: A toilet has a built-in trap, so it doesn’t need one in the drain line. It still needs a vent, though.
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 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.