The size requirements for the drain pipes used for a tub are set and enforced by local plumbing codes, which may or may not be the same as any national code. If you are doing any new installations or a makeover involving the tub's plumbing, consult the local code first. Using the right size pipes helps you avoid future drainage woes and repairs and the need to install replacement pipes.
Tub Drain Pipe Size
The pipe size is determined by the inside diameter of the pipe, measured by extending a tape measure or ruler from one inside wall on the end of the pipe to the opposite inside wall. For a bathtub drain pipe, the correct size is almost always a minimum of 1 1/2 inches, as set by code. However, in other areas, codes set the minimum tub drain pipe size at 2 inches.
Importance of Correct Size
Installing tub drain pipes of the correct size is vital, because the tub drain, as well as the drain pipes from all the fixtures, are part of the home or building's overall drain, waste and vent system. Because of this, if the tub drain pipes clog easily because they are not large enough to handle the amount of water flowing through them, water can back up into other fixtures, especially those nearby, such as the toilet.
In addition to the straight sections of pipe the tub drain plumbing uses, it also uses a curved section, which is called a trap. All fixtures that drain, including toilets and sinks, have this curved section of pipe as part of the fixture's plumbing. The trap always holds an amount of water inside its curved portion, and it seals out smelly sewer gases that would otherwise travel up through the pipe and into the room via the fixture drain. The tub trap is the same size as the drain pipe, at either 1 1/2 or 2 inches.
In addition to the requirement that the tub drain pipes be correctly sized, correctly installed drain plumbing calls for the horizontal pipes to be sloped downward at the proper angle to prevent backflow. This is because drain pipes do not use the pressure that supply pipes use and instead rely on gravity to keep the water and other waste moving along inside them. The minimum slope required is 1/4 inch for every foot that the drain pipe runs.