The basic setup: what goes where under your sink

A garbage disposal under a double sink connects to one basin, usually the one without the strainer basket, and the drain line runs from the disposal to the P-trap that serves both sides. The sink with the disposal gets a special three-bolt flange; the other sink drains through a standard strainer basket into a tee fitting that merges both drains before they reach the P-trap. You will need a disposal unit, a three-bolt flange kit, a P-trap assembly, a tee fitting, slip nuts, washers, and plumber's putty or silicone caulk.

The disposal side requires more parts because the disposal itself replaces the standard drain assembly. The non-disposal side keeps its normal strainer basket but connects to a tee instead of going straight down. Both sides then feed into a single P-trap under the sink, which is where the water seal sits and where clogs usually form.

Key Takeaways

  • The disposal connects to one basin through a three-bolt flange, while the other basin drains through a standard strainer into a tee fitting that merges both drains.
  • A P-trap sits under the sink after the tee and holds water to block sewer gases; this is where you will find most clogs and where you turn to shut off water flow during repairs.
  • Slip nuts and washers create watertight seals between pipes; hand-tighten first, then use a wrench to snug them without cracking the plastic.
  • The disposal side needs a three-bolt flange kit, not a standard strainer basket, because the disposal unit bolts directly to it.
  • Test for leaks by running water through both sinks before you consider the job finished.

Installing the three-bolt flange on the disposal side

Start by removing the old strainer basket from the sink you will use for the disposal. Clean out the putty or caulk around the drain hole. Roll a rope of plumber's putty about the thickness of a pencil and press it around the underside of the three-bolt flange where it will sit against the sink. Push the flange up through the drain hole from below so the rubber gasket and metal washer sit flat against the sink basin.

From above the sink, thread the three bolts through the flange holes and hand-tighten the nuts underneath. Use a wrench to snug each bolt evenly, working around in a circle so the flange sits level. Tighten until the putty squeezes out around the edge, then stop—over-tightening cracks the sink. Wipe away excess putty with a wet cloth. The flange is now ready for the disposal unit to bolt on.

Connecting the disposal unit to the flange

Most disposals come with a mounting ring that bolts to the three-bolt flange. Align the three tabs on the mounting ring with the three bolts sticking down from the flange, then slide the ring up and rotate it clockwise until it locks tight. The disposal hangs from this ring and can be removed by rotating it counterclockwise, which is useful if you need to clear a jam or replace the unit later.

Slide the disposal up into the mounting ring and rotate it clockwise until it stops. The disposal should hang level and not touch the sink basin. If it tilts or sits too high, remove it and adjust the mounting ring position. Once the disposal is locked in place, you can connect the drain outlet at the bottom of the unit to the P-trap assembly.

Running the drain line from the non-disposal sink

Install a standard strainer basket in the other sink basin using the same method as the disposal side: plumber's putty around the flange, push it up through the hole, hand-tighten the bolts, then snug with a wrench. This strainer basket will catch large debris and prevent it from clogging the shared drain line.

Below the sink, attach a tailpiece—a short straight pipe—to the strainer basket using a slip nut and washer. The tailpiece should be long enough to reach the tee fitting. Hand-tighten the slip nut first so you can adjust the angle, then use a wrench to snug it. Do not over-tighten; plastic threads strip easily. The tailpiece from the non-disposal sink will connect to one inlet of the tee fitting.

Assembling the tee and P-trap

The tee fitting has three openings: two inlets (one from each sink) and one outlet (down to the P-trap). Position the tee so both inlets point upward at roughly the same height. Attach the tailpiece from the non-disposal sink to one inlet using a slip nut and washer. Attach the drain outlet from the disposal to the other inlet the same way. Both connections should be hand-tight first, then snugged with a wrench.

The outlet of the tee points downward and connects to the inlet of the P-trap using another slip nut and washer. The P-trap is the U-shaped section of pipe that holds standing water; this water seal prevents sewer gases from backing up into your sink. The outlet of the P-trap then connects to the wall drain or floor drain with a final slip nut. Hand-tighten all connections first so you can adjust angles, then snug each one with a wrench.

Checking for leaks and adjusting connections

Before you consider the job done, run water through both sinks at the same time and watch underneath for drips. Water should flow smoothly down both sides and out through the P-trap without pooling or leaking at any slip nut. If you see a drip at a connection, place a bucket underneath and tighten that slip nut a quarter turn with a wrench. Retighten only the leaking joint; over-tightening others can crack the plastic.

Let water run for at least one minute through each sink separately, then together. Check the P-trap and all connections again. If a leak persists after tightening, turn off the water, loosen that slip nut, remove the tailpiece or trap section, and inspect the rubber washer. A worn or misaligned washer is the most common cause of a leak that does not stop with tightening. Replace the washer and reconnect. Once both sinks drain without leaks, the installation is complete.

Common mistakes that cause leaks or clogs

The most frequent error is over-tightening slip nuts, which cracks the plastic and creates leaks that get worse over time. Hand-tighten first, test for leaks, then tighten only as much as needed to stop the drip. Another common mistake is installing the P-trap too high or at an angle that does not allow water to drain fully; water should flow downward through the trap without pooling in a horizontal section.

Forgetting to use washers under slip nuts is another source of leaks. Every slip nut needs a rubber or nylon washer between the nut and the pipe end; without it, the nut tightens against bare plastic and does not seal. Finally, running the disposal drain line upward before it reaches the tee can trap air and cause slow drains or gurgling sounds. The disposal outlet should slope downward toward the tee fitting.

Frequently Asked Questions

Can I put the disposal on the side of the sink that already has a strainer basket?

Yes, but you will need to remove the old strainer basket first and replace it with the three-bolt flange. The disposal must connect directly to the flange; you cannot bolt it to a standard strainer basket. Either sink can have the disposal, so choose whichever is more convenient for your kitchen layout.

What if water backs up into the non-disposal sink when I run the disposal?

This usually means the tee fitting is clogged or the P-trap is blocked. Turn off the disposal and run hot water through both sinks to clear minor clogs. If water still backs up, you may need to remove and clean the P-trap. Place a bucket underneath, loosen the slip nuts on both ends of the P-trap, and pull it out. Empty any debris and rinse it before reinstalling.

Do I need a special P-trap for a double sink with a disposal?

No, a standard P-trap works fine as long as you use a tee fitting to merge both drain lines before they reach the trap. The P-trap itself does not know or care whether the water came from one sink or two. The tee fitting is what handles the merging; the P-trap just holds the water seal and connects to the wall or floor drain.

How tight should I make the slip nuts?

Hand-tighten until snug, then use a wrench to turn each nut about a quarter turn more. If water drips at that connection after testing, tighten another quarter turn. Stop as soon as the leak stops; further tightening risks cracking the plastic. If a leak persists after a half turn of wrench tightening, the washer is probably worn and needs replacement instead.

Can I use PVC pipe instead of the flexible drain lines that came with the kit?

Yes, rigid PVC works and is often cheaper. You will need slip fittings and couplings to connect PVC sections, plus slip nuts and washers at the connection points. PVC is less flexible than the included hoses, so measure carefully and plan your angles before you cut. Rigid pipe also requires more support to prevent sagging, which can trap water and cause slow drains.