How to Install a Bathroom Sink Drain and P-Trap

Begin by clearing everything from under the sink to give yourself ample working space. Place a bucket directly underneath the P-trap to catch any residual water and debris that will spill out when you disassemble the old drain. Use channel-lock pliers to loosen the large slip nuts connecting the P-trap to the sink's tailpiece and the wall's trap arm. These nuts may be tight, so apply firm, steady pressure. Once loosened, you should be able to unscrew them by hand. Carefully wiggle the P-trap free and empty its contents into the bucket. Next, unscrew the large mounting nut holding the drain body to the underside of the sink. You may need the pliers for this as well. If you have a pop-up drain, disconnect the pivot rod from the lift strap before fully removing the drain body.

After removing the old drain body, the sink's drain hole will likely be caked with old plumber's putty, soap scum, and grime. It is crucial to clean this area thoroughly to ensure a watertight seal for the new drain. Use a plastic putty knife to scrape away the bulk of the old putty from both the top and bottom surfaces of the drain opening. Be careful not to scratch the sink's finish. After scraping, use a non-abrasive scouring pad and a cleaner or mineral spirits on a rag to remove any remaining residue. The surface should be perfectly clean and dry before you proceed.

Take a small amount of plumber's putty, about the size of a large marble, and knead it in your hands until it becomes soft and pliable. Roll it between your palms to form a uniform rope about 1/4-inch thick. Press this rope of putty firmly and evenly around the underside of the new drain flange. Insert the drain flange into the drain opening from the top of the sink, pushing it down firmly. The excess putty should squeeze out evenly around the edges. Do not wipe away the excess putty yet; it will be removed later.

From underneath the sink, slide the large rubber gasket onto the threads of the drain body, ensuring the tapered side faces up towards the sink bottom. Follow this with the friction washer (if included) and then the large mounting nut. While applying gentle downward pressure on the drain flange from above the sink, thread the mounting nut onto the drain body and hand-tighten it. Use channel-lock pliers to give it a final half-turn to compress the gasket and secure the drain body firmly in place. Now you can wipe away the excess plumber's putty that squeezed out around the flange in the sink basin. Finally, thread the sink drain tailpiece onto the bottom of the drain body and hand-tighten.

The P-trap assembly consists of a J-bend (the U-shaped piece) and a wall bend (the L-shaped piece that connects to the wall). First, attach the wall bend to the drain pipe in the wall (the trap adapter). Slide a slip nut and a tapered washer (taper facing the trap adapter) onto the straight end of the wall bend and push it into the adapter. Hand-tighten the nut. Now, align the J-bend between the sink tailpiece and the wall bend. You may need to trim the sink tailpiece or the wall bend to achieve a good fit. Use a hacksaw or tubing cutter for clean, straight cuts. Slide the necessary slip nuts and washers onto the pipes, connect the J-bend, and hand-tighten all the nuts. The P-trap should have a slight downward slope from the sink tailpiece to the bend.

If you are installing a pop-up drain, drop the stopper into the drain flange from above the sink. Underneath, locate the pivot rod opening on the drain body. Unscrew the pivot rod retaining nut. Insert the pivot rod through the hole in the tail of the stopper, making sure the rod is angled slightly downwards towards the floor. Tighten the retaining nut. Now, connect the pivot rod to the faucet's lift rod using the provided metal clip and strap. Put the lift rod through the hole in the faucet and connect it to the strap. Adjust the position of the pivot rod on the strap to control the height of the stopper when it's open and closed. Test the mechanism to ensure it moves smoothly.

Go back and check all the connections you made: the mounting nut under the sink and all the slip nuts on the P-trap and tailpiece. Hand-tighten them as much as you can, then use channel-lock pliers to give each nut a final quarter to half-turn. Be careful not to overtighten. Now, it's time to test for leaks. Close the sink stopper and fill the basin with a few inches of water. Let it sit for a minute to test the flange seal. Then, open the stopper and let all the water rush through the drain. As the water flows, carefully inspect every joint and connection with your hand and a flashlight for any drips. Check the P-trap nuts, the tailpiece connection, and where the trap arm enters the wall. If you find a leak, tighten the corresponding nut slightly. If it persists, you may need to disassemble the leaking joint and check that the washer is seated correctly.