Bathrooms often contain the largest number of plumbing fixtures and drains. Each of these fixtures and drains is connected to a main water supply line and a drain, waste and vent system. Waste water from the bathroom goes into a large pipes, and then into the main underground drainage system.
The water that leaves your bathtub, shower, and sink are all carried through a special line until it meets waste that comes from your toilet. This line does not have any waste material, and once an ideal slope has been applied to it, no waste from the toilet will be allowed to back up in it.
The diameter of the pipe that takes the water from the sink, bathtub, and shower is considerably smaller that the one that takes the overall waste from both lines. This is primarily due to what is known as “flow.” The tube that will receive all the waste material has to have a diameter that allows everything to pass.