What You Need to Know Before You Start

A double sink with a garbage disposal requires two drain lines that meet at a Y-fitting before connecting to your main drain. One sink drains directly to the Y-fitting; the other connects to the garbage disposal first, then to the Y-fitting. The key difference from a single sink is that you need to size your pipes correctly and position the Y-fitting so both drains slope downward toward it—if either drain slopes the wrong way, water and food waste will pool instead of flowing out.

This is a project you can do yourself if you have basic plumbing experience and the right tools. Most of the work happens under the sink, where you have room to move. The hardest part is usually removing the old drain assembly, not installing the new one. If you hit a corroded fitting that won't budge or discover your drain line is cast iron instead of PVC, you may need to call a plumber for that section—but the sink-level work remains the same.

Key Takeaways

  • The garbage disposal connects to one sink basin, and both drain lines meet at a Y-fitting before reaching the main drain line.
  • Use 1.5-inch PVC pipe for the main connections under the sink; both drain lines must slope downward at roughly 1/4 inch per foot toward the Y-fitting.
  • The Y-fitting sits lower than both sink drains so gravity pulls water downward into it, not sideways or upward.
  • You will need a slip nut wrench or adjustable wrench, a hacksaw or PVC cutter, PVC cement, and plumber's putty—most hardware stores sell a double-sink kit with the fittings already sized.
  • Test for leaks before you finish by running water in both sinks and the disposal, then checking every connection point underneath.

Gather Your Tools and Materials

Before you remove anything, collect what you need. A slip nut wrench (also called a basin wrench) fits into tight spaces under the sink and grips the large nuts that hold the drain assembly in place. If you do not have one, an adjustable wrench works, though it is harder to maneuver. You will also need a hacksaw or PVC cutter to trim pipes to length, a torpedo level to check your slope, and plumber's putty to seal the sink strainer.

For the pipes and fittings, buy a double-sink drain kit from a hardware store—these come with a Y-fitting, slip nuts, washers, and tailpieces already sized for standard sinks. The kit saves you from buying pieces separately and getting the sizes wrong. You will also need PVC cement (also called solvent cement) if your kit includes PVC pipes, or you can use compression fittings instead, which do not require cement and are easier to adjust later. Have a bucket and old towels ready—water will spill when you disconnect the old drain.

Remove the Old Drain Assembly

Turn off the water supply to your sink or straightforward stop using it while you work. Place a bucket under the P-trap (the U-shaped pipe under the sink) to catch any standing water. Loosen the slip nuts on both sides of the P-trap using your wrench, turning counterclockwise. The nut closest to the wall connects to the main drain line; the nut closest to the sink connects to the tailpiece coming down from the sink. Once both nuts are loose, the P-trap will slide out.

Next, disconnect the tailpieces from the sink strainers. If your sink has a garbage disposal on one side, you will see a large collar nut under that sink basin—loosen it with your wrench and the disposal unit will drop slightly, freeing the tailpiece. On the other side, loosen the slip nut where the tailpiece meets the sink strainer. Once both tailpieces are free, you can remove the old Y-fitting or tee if one exists. Keep the old P-trap and tailpieces in case you need to reference their size or reuse them.

Install the Garbage Disposal and Connect the First Drain Line

If you are installing a new garbage disposal, follow the manufacturer's instructions to mount it to the sink basin first—this usually involves tightening a collar nut underneath. Once the disposal is find, attach the tailpiece (the short pipe that comes out of the disposal) using the slip nut that came with the disposal. Hand-tighten it first, then use your wrench to snug it another quarter-turn; do not over-tighten or you will crack the fitting.

From the disposal side, run a 1.5-inch drain line to where your Y-fitting will sit. This line should slope downward toward the Y-fitting at roughly 1/4 inch per foot—use your level to check. If the line is perfectly horizontal or slopes upward, water will pool and smell. You can use PVC pipe with slip-nut connections (easier to adjust) or solvent-cemented PVC (more permanent). Measure twice and cut once; it is easier to add a small fitting than to cut a pipe shorter.

Connect the Second Sink Drain and Install the Y-Fitting

On the other sink basin (the one without the disposal), install a new sink strainer if needed, sealing it with plumber's putty around the underside. Attach a tailpiece to this strainer using a slip nut. This tailpiece should also slope downward toward the Y-fitting at 1/4 inch per foot. Both tailpieces—one from the disposal, one from the regular sink—will connect to the Y-fitting.

Position the Y-fitting lower than both tailpieces so gravity pulls water downward into it. The Y-fitting has three ports: two inlets (where the tailpieces connect) and one outlet (where the line goes to your main drain). Attach the disposal-side tailpiece to one inlet and the regular sink tailpiece to the other inlet using slip nuts. Tighten each nut by hand first, then use your wrench to snug it another quarter-turn. Do not force it; if a nut is hard to turn, check that the pipe is seated straight in the fitting.

Connect to the Main Drain and Check Your Slope

From the Y-fitting outlet, run a 1.5-inch line to your main drain (the line that goes into the wall or floor). This line must also slope downward at 1/4 inch per foot. Use your level to verify the slope before you tighten everything. If you need to adjust the height of the Y-fitting or any tailpiece, loosen the slip nuts, move the pipes, and re-tighten. Once the slope is correct, reconnect your P-trap to the outlet of the Y-fitting using the slip nuts you removed earlier.

Check that the P-trap itself slopes correctly—the end going into the wall should be slightly lower than the end coming from the Y-fitting. If the P-trap is level or slopes the wrong way, it will trap water and create a siphon problem. Adjust the position of the Y-fitting or the main drain line if needed. This is the time to get it right; once you tighten everything and test it, you do not want to take it apart again.

Test for Leaks and Adjust as Needed

Before you consider the job done, run water in both sinks and turn on the garbage disposal. Watch underneath for drips at every connection point: the tailpiece slip nuts, the Y-fitting inlets and outlet, and the P-trap nuts. A small drip that stops after a few seconds is usually just air escaping; a steady drip means the nut needs to be tighter. Use your wrench to tighten the leaking nut another quarter-turn, then test again. Do not over-tighten—you want snug, not crushed.

Run water for at least one minute in each sink separately, then run both at the same time. The disposal should drain quickly; the regular sink should drain without backing up into the other basin. If water backs up into the non-disposal sink when the disposal runs, the Y-fitting may be positioned too high or one of the tailpieces may be sloping the wrong way. Turn off the water, loosen the connections, and adjust the slope. Once everything drains cleanly and no leaks appear after five minutes of running water, your installation is complete.

Frequently Asked Questions

Can I use 1.25-inch pipe instead of 1.5-inch under the sink?

No. Building codes require 1.5-inch pipe for double sinks because the combined flow from both basins is too much for a smaller line. Using 1.25-inch pipe will cause slow drainage and clogs. Stick with 1.5-inch for all drain lines under the sink.

What if my main drain line is cast iron instead of PVC?

Cast iron is older and harder to work with, but you can still connect to it. Use a rubber coupling and stainless steel hose clamps to join your PVC line to the cast iron. These couplings are sold at hardware stores and require no cement or solvent. Tighten the clamps firmly but do not over-tighten, or you will crack the old pipe.

Do I need to use PVC cement, or can I use compression fittings instead?

Compression fittings (slip nuts) are easier for a first-timer because you can adjust them if something is not quite right. Solvent cement is permanent and faster, but if you make a mistake, you have to cut out the bad section and start over. For a double-sink installation, compression fittings are the safer choice.

Why does water back up into one sink when I use the disposal?

This usually means the Y-fitting is too high or one tailpiece is sloping upward instead of downward. Water from the disposal is pushing into the other sink instead of flowing down and out. Check your slope with a level and lower the Y-fitting if needed. The disposal-side tailpiece should slope down at least 1/4 inch per foot.

How tight should I make the slip nuts?

Hand-tighten first, then use your wrench to turn each nut another quarter-turn. If you hear water dripping after testing, tighten another quarter-turn. If you over-tighten, you will crack the plastic fitting or strip the threads. Snug is the goal, not vice-grip tight.