The typical plumbing setup for a bathroom sink & toilet

Updated February 21, 2017

"Water-in, water-out," plumbers say when asked to explain basic plumbing methods. In reality, it's not quite that simple. If you're planning to plumb a bathroom, you'll need a basic grasp of common plumbing techniques or you could end up with low water pressure, slow sink drainage and a toilet that gurgles and burps. In addition to charting water supply, you'll make provisions for venting the drain systems of both the sink and the toilet. In many communities, a licensed master plumber must install the plumbing.

Water Supply

When possible, locating the sink and the toilet along a common wall will reduce plumbing headaches because you only have to run the plumbing within a single wall. This is also true if you're installing a tub or shower, although putting all three fixtures on one wall is not always feasible.

If you live in a cold climate, resist running your water supply lines in an exterior wall to reduce the risk of frozen lines.

Water compression distribution units use PEX tubing to run an individual water supply line to every fixture. Standard tubing size is 1/2-inch for the toilet and 3/8-inch for the sink. Red and blue tubes differentiate between hot and cold water.

Drainage System

Your drainage system works through gravity. The individual sink and toilet drains will join a larger main drain and continue to flow downward until the waste flows out of the house through the main sewer drain.

Always check your local code before installing any type of plumbing. The standard fall for main drains is 1/8-inch to 1/4-inch drop per feet. That means your main drainpipe should slope downward at the same rate. Too little slope and solid waste from the toilet may not drain efficiently. Too much slope and the liquid contents from the toilet may outrun the solid waste, leaving it in the drainpipe.

Sink Plumbing

The sink requires both hot and cold water and a drain. The water supply tubes connect to the underside of the faucets and the drain features a U-shape trap beneath the sink, which holds a little bit of water and prevents sewer gases from backing up through the drain.


The water supply tube connects to the toilet tank to feed water into the tank after every flushing. Unlike the sink, the toilet contains its own drain trap, so you don't need to install one. You'll install a large, 3- or 4-inch drain beneath the toilet and you'll use a wax ring between the toilet and the drain to prevent leakage.


You must vent both the sink and toilet. Where the sink drain enters the wall, you will install a Sanitary Tee that directs water drainage downward while venting the drain from above. The configuration of the drain depends upon the location of the sink in relationship to the toilet and any other fixtures.

The toilet drain must also have a Sanitary Tee vent. The two vent pipes will join each other and they will tie into a main vent pipe that provides venting for all the fixtures in the home. This main vent pipe will extend upward through a wall and eventually exit through the roof.

Local Code

Plumbing is subject to local code. Not only must you comply with the correct slope of all drainpipes, there may be minimum distance requirements between the sink, the toilet and other fixtures. Most communities require pulling a permit before installing any type of plumbing in your home. Consult a professional plumber before installing any plumbing configuration.

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About the Author

Glenda Taylor is a contractor and a full-time writer specializing in construction writing. She also enjoys writing business and finance, food and drink and pet-related articles. Her education includes marketing and a bachelor's degree in journalism from the University of Kansas.