What You Need to Know Before You Start

Installing a garbage disposal in a single sink is a plumbing project you can do yourself if you have basic tools and are comfortable working under the sink. The job takes two to four hours and involves disconnecting the old drain, removing the sink strainer, mounting the disposal unit, and reconnecting the drain lines. You do not need a plumber for a straightforward installation, though you will need to turn off the water and may need to call one if your sink has an unusual configuration or if you hit complications with the drain line.

The most common mistake is not turning off the water before you start, which can lead to spills and water damage. The second most common is forcing parts together instead of checking that they fit properly first. Take your time at each step, and do not tighten anything permanently until you have tested the fit of all the pieces.

Key Takeaways

  • Turn off the water at the shutoff valve under the sink before you disconnect anything, and have a bucket ready to catch water in the drain line.
  • The mounting ring holds the disposal to the sink from below, and you tighten it by turning the three tabs clockwise until the unit is snug but not forced.
  • The drain line from the disposal must slope downward toward the wall drain, or water will pool inside the disposal and cause odors.
  • Test the disposal with the stopper in place and water running before you seal any connections, so you can catch leaks before they damage the cabinet.
  • If your sink has a dishwasher, you will need to knock out the knockout plug inside the disposal inlet before connecting the dishwasher line.

Gather Your Tools and Materials

You will need a screwdriver (both Phillips and flathead), an adjustable wrench, a bucket, a flashlight, and possibly a hacksaw or PVC cutter if you need to shorten the drain line. Have towels on hand for spills. Before you buy the disposal unit, measure the space under your sink to make sure the unit will fit—most disposals are 8 to 10 inches tall and 5 to 6 inches in diameter, but check your cabinet depth.

You will also need plumber's putty or a rubber gasket (depending on your sink type), a new drain flange if your old one is corroded, and possibly a new P-trap or tailpiece if the old one does not fit the new disposal outlet. Most disposals come with a mounting ring and rubber gasket, but read the box to confirm. If you are replacing an old disposal, the new unit should have the same outlet size as the old one—usually 1.5 inches in diameter.

Turn Off Water and Remove the Old Drain

Locate the shutoff valve under the sink—it is the small knob on the water line coming from the wall. Turn it clockwise until it stops. Turn on the faucet to release any water still in the line, then turn it off again. Place a bucket under the drain trap (the U-shaped pipe under the sink) and loosen the slip nuts on both ends with your wrench. The trap will drain into the bucket as you remove it.

If you have an old disposal, unplug it from the outlet under the sink before you touch it. Loosen the mounting ring by turning the three tabs counterclockwise, and the disposal will drop out. If you have only a strainer basket, use a strainer wrench or a large flathead screwdriver to unscrew the locknut holding it in place from below. Once the locknut is loose, push the strainer up from above and remove it. Clean away any old putty or gasket material with a scraper.

Install the New Drain Flange and Mounting Ring

If your sink has a rubber gasket under the drain opening, remove it and inspect the opening. If the opening is clean and smooth, you can reuse the gasket. If it is corroded or damaged, replace it with a new one. Roll a rope of plumber's putty about the thickness of a pencil and press it around the underside of the drain flange (the part that sits on top of the sink). Press the flange into the drain opening from above, making sure it sits flat.

From below, slide the rubber gasket onto the flange, then thread the mounting ring onto the flange by hand. Do not tighten it yet. The mounting ring should sit flat against the gasket with no gaps. Once it is hand-tight, use your wrench to tighten the three tabs clockwise in a rotating pattern—tighten one tab a quarter turn, then the next, then the third, then back to the first. This keeps the ring even. Stop when the ring is snug and the flange does not move when you push it from above. Wipe away any excess putty from the top of the sink with a damp cloth.

Connect the Disposal to the Mounting Ring

Lift the disposal unit up to the mounting ring and align the three tabs on the disposal with the three slots on the ring. Push the disposal up and turn it clockwise until the tabs lock into the slots. The disposal should hang securely from the ring. If it does not, check that the tabs are fully seated in the slots and try again. Do not force it—if it does not turn smoothly, stop and check the alignment.

Once the disposal is locked in place, plug it into the outlet under the sink. Make sure the outlet is grounded (it should have three holes) and that the cord is not kinked or pinched. If there is no outlet under the sink, you will need to have an electrician install one—do not use an extension cord for a disposal.

Reconnect the Drain Line

The outlet of the disposal is usually 1.5 inches in diameter. Your old P-trap may not fit, so check the fit before you tighten anything. Slide the tailpiece (the short pipe that screws onto the disposal outlet) onto the disposal outlet by hand. It should thread on smoothly. Do not use a wrench yet—hand-tight is enough for now.

Attach the P-trap to the tailpiece by sliding the slip nut over the tailpiece and threading it onto the trap. Again, hand-tight is the goal. Now check the slope: the tailpiece should angle downward slightly as it moves away from the disposal toward the wall drain. If it angles upward, water will pool in the disposal and cause odors. Adjust the position of the tailpiece or the trap until the slope is correct, then tighten the slip nuts with your wrench—snug, not forced. A quarter turn past hand-tight is usually enough.

If your sink has a dishwasher, you will see a knockout plug inside the disposal inlet (the opening where food goes in). Use a flathead screwdriver to tap it out from inside, then connect the dishwasher drain line to the inlet with a hose clamp. If you do not have a dishwasher, leave the knockout plug in place.

Test Before You Finish

Turn the water shutoff valve back on by turning it counterclockwise. Place the stopper in the sink and fill it with a few inches of water. Remove the stopper and let the water drain while you watch under the sink for leaks. If water drips from a slip nut, tighten it a quarter turn with your wrench and test again. Do not turn on the disposal yet—just let the water drain through gravity.

Once the drain is clear and dry, turn on the disposal and let it run for a few seconds with no water. Then turn on the faucet and run water through the disposal for about 30 seconds. Listen for unusual noises and watch for leaks. If everything is quiet and dry, you are done. If you hear grinding or rattling, turn off the disposal when ready and check for loose parts or debris in the chamber.

Frequently Asked Questions

Do I need a plumber to install a garbage disposal?

Not necessarily. If you are comfortable with basic tools and your sink has a standard configuration, you can do it yourself in two to four hours. Call a plumber if your sink has an unusual drain setup, if you do not have an outlet under the sink, or if you run into complications you cannot solve.

Can I install a disposal in a sink that already has one?

Yes. Unplug the old disposal, loosen the mounting ring tabs, and it will drop out. The new disposal should have the same outlet size as the old one. If it does not, you may need a new P-trap or tailpiece.

What if water leaks from under the sink after I install the disposal?

Check the slip nuts on the P-trap and tailpiece first—they may just need to be tightened a quarter turn. If the leak is coming from the drain flange itself, you may need to reseal it with plumber's putty. Turn off the water, remove the mounting ring, and reapply putty around the flange.

Can I use the sink right away after installation?

Yes, once you have tested it and confirmed there are no leaks. Run water through the disposal for 30 seconds to make sure everything is working, then you can use it normally. Do not put hard objects or fibrous materials like celery or corn husks down the disposal.

What if the disposal makes a loud grinding noise when I turn it on?

Turn it off when ready. Something may be stuck in the chamber or a part may be loose. Never put your hand down the disposal. Use tongs or pliers to remove any visible debris, then try again. If the noise continues, the motor may be damaged and the unit may need to be replaced.