Understanding the Plumbing of the Bathroom Sink

Bathroom sink plumbing is easy to understand and simple to repair. There are very few parts, and most parts are accessible inside the sink cabinet. All bathroom sink plumbing parts are divided into either water supply parts or drainage parts.

Learn about bathroom sink plumbing, including water supply parts like supply tubes and drainage parts like tailpieces and P-traps.

Water Supply	Drainage
Shut-off valve	Drain tailpiece
Supply tube	P-trap
	P-trap adapter

Shutoff Valve

A shutoff valve is a metal or plastic valve located between the incoming water supply pipes and the supply hoses or tubes that connect to the sink faucet. Most have an oval, football-shaped handle that you turn to open or close the valve.

Also called stop valves, shutoff valves allow you to turn off the water supply at the sink, rather than shutting off the water to the whole house. They are seen in pairs: one valve controls the hot water; the other controls the cold water.

Many shutoff valves use compression fittings, so they can be installed onto the water pipes without soldering. But you will have to turn off the water to the house to replace a shutoff valve. 

Supply Tube

A supply tube is a braided wire or plastic mesh tube that supplies water to the bathroom faucet. They always come in pairs: one to supply the hot water, and another to supply the cold water.

They are connected to the bottom of the sink faucet and shutoff valves with attached nuts. These supply tubes sometimes fail, and it's not uncommon to have to replace them.

Drain Tailpiece 

The tailpiece, or sink tailpiece, is the straight section of pipe that connects to the bottom of the sink drain fitting.

If the sink has a pop-up drain, the lever rod of the drain assembly connects to a port on the back of the tailpiece.

Typically, the tailpiece attaches to the drain fitting with a slip nut: a threaded ring that can be tightened and loosened by hand or with channel-lock pliers. Underneath the nut is a tapered plastic washer that creates a watertight seal. 

P-Trap

The P-trap is a curved drainage pipe under the sink. The P-trap's curve is designed to hold standing water. The standing water prevents sewer gasses from rising from the sink drain.

The bottom of the U-bend is always filled with water so that gasses can't pass through. Each time you run water down the drain, the old water in the bend is flushed out and is replaced with new water.

The P-trap parts are assembled with slip-nut joints, so they can easily be disassembled and reassembled without having to cut or unscrew pipes. If you ever drop an item into the sink drain or encounter a sink clog, you can disassemble the slip-nut joint to retrieve your item or remove the clog.

P-Trap Adapter

The P-trap adapter connects the P-trap to the home's drain pipe. This typically 1-1/2-inch-diameter tube uses slip joints because both ends (the P-trap and the home's drain pipe) are unthreaded.

Tips for Plumbing a Bathroom Sink

As long as the home has a drainline and hot and cold water supply pipes installed, it's fairly easy to plumb a bathrooom sink:

• Pop-up drain: If you're not installing a new sink and the sink's tailpiece and pop-up drain are adequate, you can leave this in place. Switch it out if it's damaged or if you're replacing the sink.
• Check before installing flexible tubing: Corrugated, flexible plastic drainpipes are designed to help with spacing and placement restrictions. But flexible drains do not meet plumbing code in many areas.
• Drain bend height: The bend in the P-trap should always be below the level of the drain pipe that extends from the house.
• Don't strain the pipes: When drain pipes aren't reaching properly, don't force them together. The slip-joints will leak as a result of this stress. Instead, use correctly sized pipes.