A P-trap is a U-shaped section of pipe in a sink drain assembly (when viewed from the side, the assembly looks like a P lying face down). The trap is shaped that way, so it is filled with water at all times. The water prevents sewer gasses from the drain pipe from rising through the drain opening in the sink and out into the room.
A new P-trap assembly usually includes a straight piece of pipe with a 90-degree bend, called a trap arm. This piece fits between the trap and the pipe in the wall. The trap and trap arm typically are secured with short, large-diameter nuts called slip nuts. You can loosen these by hand or with tongue-and-groove pliers or even a pipe wrench.
Sink P-traps are commonly made of white PVC or black ABS plastic or with chromed metal (and sometimes even chromed plastic, if you can believe there's such a material). You can use the same material as the old parts or choose something different, but the new pieces must be the same size as the old ones.
How to Replace a Sink P-Trap
Because traps usually are full of yucky water, find a small bucket that fits under the trap, and have a few rags or towels ready for containing spills.
- Remove the old P-trap by unthreading the slip nut at both ends of the trap. Turn the nuts by hand, if possible, or loosen them with tongue-and-groove pliers or a pipe wrench, then unthread the nuts by hand. Pull the trap straight down to remove it, and carefully empty its contents into the bucket.
- Remove the trap arm by loosening the slip nut securing it to the drain pipe in the wall and pulling the arm straight out. If the arm is stuck, twist it back and forth while pulling.
- Remove the slip nut washer (a plastic beveled washer) and slip nut from the end of the sink tailpiece—the straight pipe pointing down from the sink.
- Test-fit the new P-trap and trap arm to make sure they will line up with the tailpiece and wall pipe. If necessary, cut the new trap arm to the same length as the old trap arm, using a hacksaw. Cut from the straight end of the arm, not the end with the threaded fitting.
- Prepare the new trap arm by sliding a slip nut onto the arm, so the nut's threads face down toward the trap connection. Add another nut onto the straight end of the arm, with the threads facing the straight end. Add a slip joint washer over the straight end, so the taper of the washer faces the straight end of the arm.
- Fit a slip nut and washer onto the end of the sink tailpiece with the washer taper pointing down.
- Fit the trap arm into the pipe in the wall, then fit the P-trap over the ends of the trap arm and the sink tailpiece. Slide the slip nuts against the P-trap and wall pipe, and loosely thread the nuts onto the joints.
- Adjust the P-trap and trap arm so the pieces are properly aligned (not at awkward angles) and the trap arm slopes down slightly toward the wall. Hand-tighten the slip nuts. If they are plastic nuts, you should be able to get them as tight as needed by hand; if they are metal, tighten them a bit further with tongue-and-groove pliers or a pipe wrench.
- Turn on the sink faucet full-blast and let it run while you check the slip nut connections for leaks. If any joint leaks, tighten the slip nut gently with tongue-and-groove pliers until the leaking stops. Be careful not to overtighten the nut, especially plastic nuts.