Key Points
Replacing old fixtures with new ones in the same location is fairly simple. But it’s best to leave more complex plumbing jobs to a professional, unless you’re an accomplished do-it-yourselfer. Either way, you should be aware of some basic rules and principles:

Each bathroom fixture must have a drain with a trap—a curved pipe that always retains a little water in it as a seal to prevent sewage gases from getting into the house.

Each drain must be connected to a vent pipe that either goes directly up through the roof or connects to another vent stack. Plumbing vents run from the lowest part of the system clear up to the roof. For cosmetic reasons, it’s best to run vent stacks up through the back side of the roof, not the front.