How to Install a Bathroom Sink Drain and P-Trap

Begin by clearing all items from the cabinet under the sink to create a clean workspace. Turn off the hot and cold water by turning the knobs on the shutoff valves clockwise until they stop. Place a bucket directly under the P-trap. Use channel-lock pliers to loosen the large slip nuts securing the P-trap to the sink's vertical tailpiece and the horizontal trap arm going into the wall. Once loose, finish unscrewing them by hand. Carefully lower the P-trap, emptying its water and debris into the bucket. Next, use the pliers to loosen the large locknut holding the main drain body to the underside of the sink. While supporting the drain body from below, have a helper unscrew the drain flange from inside the sink, or reach in and do it yourself. Remove the entire old drain assembly. Finally, use a putty knife and a rag to scrape and wipe away all old plumber's putty and gasket residue from the sink's drain opening, ensuring a clean surface.

Take a golf-ball sized amount of plumber's putty and knead it until soft. Roll it between your palms to form a uniform rope about 1/4-inch in diameter. Drape this putty rope around the underside of the new drain flange, pressing it on gently. Insert the flanged end of the drain into the sink's drain opening from above. Press down firmly and evenly to create a preliminary seal, causing some putty to squeeze out around the flange. From underneath the sink, slide the large rubber gasket up over the threads of the drain body until it sits flush against the bottom of the sink. Follow this with the friction washer (if provided) and then thread on the large locknut. Hand-tighten the locknut as much as you can. To complete the seal, use channel-lock pliers to tighten the locknut an additional one-half to three-quarters of a turn. This will compress the rubber gasket. Go back to the sink basin and use a rag or your finger to wipe away the excess plumber's putty that squeezed out.

Drop the pop-up stopper into the drain opening from above. From underneath, identify the pivot rod opening on the drain body's side. Unscrew the pivot rod nut. Insert the pivot rod through the opening, ensuring the plastic ball seats correctly in the housing. Critically, you must guide the end of the pivot rod through the loop on the bottom of the pop-up stopper. You may need to jiggle the stopper to align the loop. Once the rod is through, thread the pivot rod nut back on and hand-tighten. Use pliers for a final quarter-turn to prevent leaks. Attach the spring clip to the end of the pivot rod. Slide the clevis strap onto the spring clip. Drop the lift rod down through its designated hole in the faucet. Insert the bottom of the lift rod through one of the holes in the clevis strap and secure it with the setscrew. Test the action by lifting and pushing the rod; the stopper should move up and down smoothly. Adjust the rod's position on the clevis strap if needed.

The P-trap kit contains several parts: a J-bend (the U-shaped piece), a trap arm (the L-shaped piece), and two or three sets of slip nuts and beveled washers. Begin by connecting the sink drain to the P-trap. Slide a slip nut onto the vertical drain tailpiece, with the threads facing down. Follow it with a beveled washer, ensuring the beveled side faces down towards the J-bend. Align the J-bend with the tailpiece and hand-tighten the slip nut to connect them. Next, prepare the trap arm. Slide a slip nut (threads facing the J-bend) and a beveled washer (bevel facing the J-bend) onto the straight end of the trap arm. Insert this end into the outlet of the J-bend. Leave all connections hand-tight for now to allow for adjustments.

Align the trap arm with the drainpipe opening in the wall (the stub-out). In most cases, the trap arm will be too long and require cutting. To mark it, slide the arm into the wall pipe until it bottoms out, then pull it back slightly and make a mark on the arm where it exits the P-trap's J-bend connection. Or, more simply, hold the trap arm next to the stub-out and mark it for a 1 to 1.5-inch insertion depth. Use a hacksaw or a dedicated tubing cutter to make a straight, clean cut at your mark. It is crucial to cut straight. Use a utility knife or a piece of sandpaper to remove any plastic burrs from the inside and outside of the cut edge. Slide the decorative escutcheon (if provided) over the trap arm first, followed by the slip nut and the reducing washer (the washer is needed to adapt the 1 1/4" trap arm to the 1 1/2" wall pipe). Insert the cut end of the trap arm into the wall pipe, align all P-trap components so there is no strain on the joints, and hand-tighten all slip nuts.

With all components in place, perform the final tightening. Re-check and tighten each slip nut connection: where the tailpiece meets the J-bend, where the J-bend meets the trap arm, and where the trap arm enters the wall drain. First, ensure they are all fully hand-tightened. Then, use channel-lock pliers to give each nut a final quarter-turn. Do not apply excessive force. Place the bucket back under the newly installed P-trap. Open the water shutoff valves fully. Turn on the faucet and let lukewarm water run for at least a minute. Operate the pop-up stopper to ensure it functions correctly under water flow. While the water is running, carefully inspect every joint, nut, and seal for any sign of dripping. To perform a stress test, close the stopper, fill the sink basin completely, then open the stopper to release a large volume of water at once. This tests the seals under maximum flow. Check again for leaks. Once satisfied, wipe the entire assembly dry with a rag and place a dry paper towel underneath for 10-15 minutes as a final, sensitive check for slow drips.