Drain traps are required on almost every appliance and fixture in your home that drains into the sewer or septic system. Exceptions include draining two kitchen sink basins into a common trap. Drain traps are curving sections of pipe shaped like a letter “U.” When water runs down the drain, some of it is caught in the trap. The water fills the pipe, creating an air-tight barrier that prevents sewer gas from drifting up the drainpipe and entering your home through drains.
To be installed according to code, traps must be installed levelly. This ensures the water seal is air-tight. The trap must contain between 2 and 4 inches of water. Any less water and the seal won’t be effective; any more water and self-siphoning can be an issue. The trap must not be larger than the drainpipe to which it connects. Traps must be protected from freezing, either by being place in interior walls or by through the use of insulation.
Some fixtures, like toilets, come with their own traps installed internally. You should not install additional traps on them. Traps need to be accessible, so don’t surround them with concrete. For traps that are difficult to access, like those attached to bathtub drains, install access doors so you can reach them in the future. Do not install S-traps, box traps or traps designed for commercial buildings on any residential fixtures or appliances.
In general, the smallest trap you can install is 1 ¼ inches in diameter on kitchen sinks or bidets. Fixtures and appliances that use more water need larger traps; a clothes washer needs one of at least 2 inches and a shower can need one of up to 4 inches. Before beginning a plumbing remodel in your home, contact a local building inspector. She can let you know which specific code is enforced in your city and ensure that your intended changes meets all local requirements.