How to Unclog a Drain

Tough clogs

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

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 caved-in pipes.

For a large, printable version of Figure A, see Additional Information, below.

Learn how to unclog a drain line running under the floor.

Step 1: Find the clog
unclog drain

Photo 1: Clear a floor drain trap

Remove the backflow preventer to gain access to the floor drain trap for cleaning. Drive a chisel against one of the notches in the retaining ring, turning it counterclockwise to unscrew it.

unclog drain

Photo 2: Clean out the trap

Vacuum out sand and dirt with a wet/dry vacuum. Loosen stubborn dirt or snag and retrieve other yucky stuff with a coat hanger or small, hand-powered drain snake.

unclog drain

Photo 3: Remove the cleanout plug

Unscrew the cleanout plug in the side of the floor drain with a pipe wrench, slip-joint pliers, or as a last resort, by chiseling it out with a cold chisel.

unclog drain

Photo 4: Clear the drainpipe

Unplug the drainpipe leading from the floor drain by using a 1/2-in. cable inserted through the cleanout opening. Start with a general- purpose drilling or retrieving tool attached to the cable, and complete the job with a finishing tool.

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 working.

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.

Video: How to Unclog a Drain
The Family HandymanHow to Unclog a Drain
Step 2: Use a drain-cleaning tool to unclog the rest of the drain.
How to Unclog a Drain

Photo 5: Opening stubborn plugs

Drive corroded cleanout plugs counterclockwise using a cold chisel and heavy hammer after squirting them with penetrating oil. If this doesn’t work, break out the plug with the hammer and chisel, being careful not to let any pieces fall into the drain. Wear gloves and safety glasses, and be prepared for a possible flood of wastewater (yikes!).

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 stay clear.

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 (Photo 10). Plumbing suppliers also carry a variety of plugs and couplings that will solve the problem.

Step 3: Rent a drain-cleaning machine
How to Unclog a Drain

Photo 6: Drain-cleaning tools

Connect a cleaning tool to the cable. Use the starting drill to bore through a clog, the retriever to snag rags or other items, the grease tool to cut through grease and soap and clean up the sides of the pipe, and the root saw to cut through roots.

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.

Step 4: Operating a drain-cleaning machine
How to Unclog a Drain

Photo 7: Feed in the cable

Feed the cable into the drain with the motor off until you can’t push it any farther. Then start and stop the motor with the foot switch as you slowly feed cable into the drain. Let up on the foot switch immediately if the cable stops turning or you think you’ve reached the clog.

How to Unclog a Drain

Photo 8: Reverse to relieve tension

Flip the switch on the motor to “Reverse.” Then use the foot switch to run the motor in reverse for a few revolutions of the cable cage. This will relieve tension that may have built up in the cable when it hit the clog. Switch the motor back to “Forward.”

NOTE: The only other time you should reverse the motor is when the cable gets stuck and will not turn in the forward position.

unclog drain

Photo 9: Take your time

Chew slowly through a clog by following these steps:

Tighten the lock bolt that secures the cable.
Depress the foot switch, running the machine in “Forward” while you hold the cable.
Loosen the lock bolt, feed a little more cable into the drain and retighten the bolt.
Repeat this process until you cut through the clog.

How to Unclog a Drain

Photo 10: Replace the cleanout plug

Replace the cleanout plug. Use an expansion plug as shown if the fitting threads are damaged or for easier access in the future. Push the plug into the cleanout fitting, and tighten the thumbscrew or Allen bolt in the middle of the cover to complete the seal.

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 of it.

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.

Additional Information

Figure A: Under-floor drain system

Required Tools for this how to unclog a drain Project

Have the necessary tools for this DIY how to unclog a drain project lined up before you start—you’ll save time and frustration. Allen wrenchBucketCold chiselHammerPipe wrenchRagsSafety glassesShop vacuumSlip joint pliers

Required Materials for this how to unclog a drain Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list. Expansion plugHand cleanerPlastic cleanout plugTeflon tape

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