Plumbing standards, such as the minimum size for the trap on a shower drain, are designed to foster sanitary plumbing systems, free from disease. The specific regulation you should follow depends on the local plumbing codes where the shower drain is installed. These local codes are usually based on major codes, such as the National Plumbing Code, the Uniform Plumbing Code and others.
Trap Size
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The usual size for a shower drain trap is 2 inches, which is the diameter of the trap. This size is the minimum size that is allowed, which means that you cannot install a trap with a smaller diameter, or the trap is not within code. Also, the drain pipe that the trap is connected to must also be 2 inches to comply with the code.
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Importance of Trap Size
The size of the drain's trap is important because it has to be large enough to keep up with the amount of water that will drain through it to prevent unnecessary clogs. Because the shower drain is also tied into a home's overall drain-waste-vent system that connects to other fixtures, such as the toilet and sink, improperly sized pipes in the shower plumbing can cause clogging problems in the drains of the other fixtures. The demand on the shower's trap is part of the total demand needs of the overall plumbing system.
Identifying the Trap
The trap is the curved section of pipe that the shower drain connects to. It is normally called either a "P-trap" or an "S-trap" due to its shape. Every plumbing fixture, including the toilets and sinks, have a trap. The trap itself is important because it prevents sewer gases from entering the room through the shower drain. After all the water drains out of the shower, a small portion of water remains in the trap's curved section, acting as a seal against the sewer gas. This seal of water also protects against insects crawling up through the drain.
Finding out Local Codes
Consult a local licensed plumber, and check with your local government's building department to learn the local regulations on what size a shower drain trap should be. You must follow these specific local regulations to avoid having to do the work over with pipes of the correct size. These local plumbing codes, adapted from major codes to fit local needs, are enforced by local inspectors for the plumbing systems in homes, schools, hospitals and other buildings.