Clogged drains are always
a hassle, but some, like a
plugged P-trap under the
sink or a stopped-up toilet,
require only a wrench, a
plunger and a little elbow
grease to unclog. But
sometimes the clog is deep
in the drainpipe and
requires more work and
extra-powerful tools to
root out. We'll show you
how to find and clear out
these clogs, which are
often hidden in the drain
system under your floor.
Figure A: Under-floor drain system
Figure A: Under-Floor Drain System
The job of clearing clogs in the
larger drain lines found under the
floor isn't for everyone. In the first
place, you'll spend about $50 a day or more to rent
the large drain-cleaning auger
required, and you have to be strong
enough to heft the machine and to
wrench loose those old, corroded
cleanout plugs. Then there's the
mess, and the half-day you'll spend
running for rental equipment and
miscellaneous plumbing parts.
So why would anyone in their right
mind attempt to clear out under-floor
drains? Well, some of us thrive on
challenge and love the satisfaction of
solving a problem on our own. If this
isn't motivation enough, consider
that professional drain cleaners will
charge quite a bit more, depending on the
problem, and you may have to miss
work or waste time waiting around for
them to show up. Keep in mind, however,
that some clogs require the services
of a pro. Don't hesitate to call a
pro if you suspect that the main drain
to the street is clogged by tree roots or
For a large, printable version of Figure A, see Additional Information, below.
Finding the clog
The first step in clearing a clog is
locating it. This often takes some trial
and error, but here are a few pointers
to get you started. If only one fixture is
clogged, the problem is either in the
trap or drain line leading from that
fixture. If a group of fixtures is
affected, look for the clog in a location
downstream from where their drains
join. Fig. A shows the drain system
under the floor of a typical house.
Notice that a clog in the location
shown would affect the kitchen and
laundry drains, but not the upstairs
bath that drains into the main stack.
A clog in the larger main drain
would cause all the drains to stop
As many of us have discovered the hard way, a clog in the under-floor
drain system often results in
wastewater backing up onto the floor
through the floor drain. To prevent
this backup, many floor drains are fitted
with an insertable backflow preventer
that allows water down but not
up. Photos 1 – 4 show one method of
removing the ball-type backflow
preventer to gain access to the floor
drain trap for cleaning.
If cleaning the fixture trap doesn't
solve the problem, and you've determined
that the clog is in one of the
under-floor drains, then you'll have
to rent a drain-cleaning machine.
With it, you can punch through the
clog, snag and retrieve an obstruction,
or cut through roots or stubborn clogs.
Never attempt to remove a cleanout
plug from, or run a cable into,
a drain that contains chemical
drain cleaner. Call a pro.
Use a drain-cleaning tool to unclog the rest of the drain.
The first thing you have to do to work
on an under-floor drain is remove the
cleanout plug. Removing a plug from a
corroded steel or cast iron fitting is a
real chore. Try using a pipe wrench
with a steel pipe slipped over the handle to increase leverage. If this
doesn't work, you'll have to resort to
chiseling (Photo 5).
Note: Removing the cleanout may release a
flood of backed-up wastewater, so be
prepared with buckets and rags, and
After you've conquered the clog
and it's time to replace the cleanout
plug, use a plastic rather than metal
plug, and don't forget to use Teflon
plumbing tape to seal the threads. If
the cleanout fitting is too damaged or
corroded to use a threaded plug,
install an expansion plug (available through our affiliation with Amazon.com) (Photo
10). Plumbing suppliers also carry a variety of plugs and couplings that will solve the problem.
Renting a drain-cleaning machine
Before you head to the rental store, try
to determine the location of the clog,
or be able to describe the symptoms.
Smaller drain lines, from 1-1/2 in. to
3 in. in diameter, require a 1/2-in.
cable. Larger main drains require a
3/4-in. cable. Ask the rental agent to
recommend the correct machine and
show you exactly how to use it. Also
ask for safety instructions. Inspect the
machine to make sure the motor and
pulley are covered with a guard. Ask
the rental agent to test the built-in
ground fault circuit interrupter
(GFCI), check the cord for fraying or
wear, and make sure the cable is not
bent, kinked or tangled. Ask for an
assortment of cleaning tools (Photo
6) and a description of their use.
These machines are very heavy; a
large machine with 100 ft. of 3/4-in.
cable can weigh 215 lbs. You'll need
help getting it in and out of your car
and into the house.
Some rental machines use a cable
that's dual-wound and has a self-feeding
feature. Since we aren't demonstrating
the use of this particular
machine, ask your rental dealer for
safety and operating instructions.
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Operating a drain-cleaning machine
These machines are powerful, and
dangerous if safety precautions aren't
followed. Read and follow the instructions
provided with the equipment.
Position the machine 2 to 3 ft. from
the cleanout opening, plug it into a
grounded outlet or 12- or 14-gauge
grounded extension cord, and make
sure the switch on the motor is in the
“Forward” position. Put on heavy
leather gloves and safety glasses and
make sure you aren't wearing any
loose clothes, belts or jewelry that
could become entangled in the cable.
Position the foot-operated switch
where you can step on it while you're
feeding cable into the drain. Practice
starting and stopping the machine
with the foot switch to get the hang
Keep both hands firmly on the
cable and slowly feed it into the pipe
as you stop and start the motor with
the foot switch (Photo 7). Feel for an
increase in cable tension and listen for the motor to slow or the built-in
safety clutch to slip. All of these indicate
you've reached a clog. Stop
immediately when you sense a
change. Photos 7 through 9 show
how to operate the machine safely.
If you have the time and energy
after boring through the clog, clean
the sides of the pipe by attaching a finishing
tool to the cable and running
the full length of the cable down the
drain. Then use a hose to run water
down the drain as you retrieve the
cable. The water will flush debris
down the drain and rinse gunk off the
cable as you
reel it in.
Do not allow tension to build
up in the cable. This will happen if the
cutting head hits a snag and stops
turning, but the motor and its cage
continue to rotate. Torque builds
until the cable suddenly twists,
potentially wrapping around your
hand or arm like a steel boa constrictor!
This can happen quickly and
without warning, so proceed slowly
and carefully as you feed the cable
into the drain.