If you have moved into an older home, you may have old drain pipes that are beginning to deteriorate from rusting. If your drain empties into a sink trap which is connected to a curved pipe that enters the drain line through the floor beneath your sink, you can replace the entire section with an "S" trap.
An S trap can replace both the sink trap and the curved pipe that connects the sink trap to the floor drain connection. If you want to avoid future rusting, use a PVC (Polyvinyl Chloride) S trap. It is completely compatible with the galvanized steel drain pipe that descends from your sink, and won't rust or corrode over time, causing you to need extensive plumbing services.
What will you need to install a PVC S Trap?
Large adjustable wrench or pliers
Paper towels or rags for waste water
PVC S trap (You will need an 1 1/4" diameter trap for a bathroom sink).
Removing the old pipes
Before you begin to disconnect the old pipes, you should place some paper towels or rags on the floor around the floor connection and on the bottom of the sink cabinet, because the sink trap will be filled with putrid water. When the floor is prepared, you will loosen the large compression nut at the top of the sink trap and the threaded nut at the bottom of the sink trap by turning the nuts in a counterclockwise direction. Hold the sink trap upright as you remove the second nut to minimize spillage of its vile contents.
When both nuts are completely loose, you can remove the sink trap and discard it. You will then need to remove the compression nut that secures the curved cane shaped pipe to the floor connection. When the nut is completely loosened, you will pull the old pipe from the floor. It will be inserted into the floor drain for a few inches, and you may encounter a slight resistance as you withdraw it from the floor drain.
Installing the PVC "S" Trap
Check the compression nut to be sure that the rubber washer is still inside it. That washer is essential for preventing leaks. If the washer is missing or looks worn, buy a new compression nut. Place the compression nut around the bottom portion of the trap, then insert the trap into the floor drain, but don't tighten the nut, because you will need to adjust the trap to connect the top portion to the sink drain pipe.
Twist the trap into position under the drain pipe that descends from the sink, then push the S trap upward so the sink pipe slides into the top portion of the trap. You will then tighten the compression nut at the top of the trap to create a watertight connection.
You can then tighten the compression nut at the floor connection to make that connection watertight as well. After the connections are secure, clean up your mess, and relax with the awareness that you avoided a possible breach of the old rusty sink trap and the mess that would have resulted except for your proactive response.