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Bathroom Plumbing

by Don Vandervort, © HomeTips

Every bathroom remodeling project inevitably involves dealing with bathroom plumbing. Whether you’re completely gutting a bathroom for a total makeover or simply replacing the flooring material, you’ll need to remove and replace plumbing fixtures such as the toilet, disconnect and reconnect water supply pipes, and often much more.

bathroom plumbing diagramBefore you begin any work that involves plumbing, it is important to understand how the plumbing system works and what’s involved in making various types of improvements and repairs.

Supply & Drain- Waste-Vent Plumbing

 Two separate plumbing systems serve a bathroom: the water supply system and the drain-waste-vent (DWV) system. (See the drawing at right for a closer look at these systems.) To prevent tainting water in the supply system, there is never a direct connection between these two systems.



The water supply delivers clean cold and hot water to the fixtures—sink, shower, tub, and toilet. The supply pipes serve the tub, shower, sink, and toilet (the hot-water supply pipe doesn’t connect to the toilet).


The DWV system sends waste water to the sewer or septic tank and exhausts sewer gases through vent pipes. Most fixture drains lead to branch drains that connect to the main drainpipe.

bathroom sink plumbing diagramBathroom Sink Plumbing

 A bathroom sink (also called a lavatory) is connected to its drainpipe by a curved section of pipe called a sink trap. The bottom of this curved section remains filled with water so sewer gases won’t enter the room. Drains that penetrate a wall have a P trap and those that go through the floor have an S trap. The water held by the trap is replaced each time the fixture is used. 



The trap is connected to a T fitting in the drain line. Just below the sink bowl, the drain’s tailpiece often contains a pop-up stopper. The tailpiece connects to the trap with slip-joint couplings. A flange at the top of the tailpiece is sealed inside the sink bowl’s drain hole with a bead of plumber’s putty.

The faucet body fits through a hole in the top of the sink or the countertop and is secured with large locknuts from underneath the sink or counter. Flexible supply tubes deliver water from water-supply shutoff valves at the wall to threaded tailpieces on the faucet base. These shut-off valves are connected to T fittings in the supply lines.

 Bathtub & Shower Plumbingbathtub drain plumbing diagram

A shower drain has several parts. Beneath the removable strainer cover in the shower floor, the drain leads to an under-floor trap that connects to a drain line and the waste/vent stack.

Tub drains have two legs, one to the main drain opening and the other to the overflow drain opening. To close and open the drain, two different assemblies are common: pop-up and plunger type. Both are operated by a trip lever at the overflow drain.

With a pop-up drain, linkage forces the drain stopper up or down by means of a rocker arm. With the plunger-type, a hollow brass plunger slides up and down inside the drain assembly to seal the drain opening.

Hot and cold water are delivered to the tub or shower faucets through pipes typically hidden in the floor and behind the faucets in the wall. Most modern bathrooms employ a pressure-balance valve to prevent scalding in the shower.


Toilet Plumbing

A toilet sits on top of a flange that centers over a large drainpipe called a closet bend. This large pipe connects to a T fitting in the main drain line. When the toilet is flushed, waste travels down through the flange and closet bend into the main drainpipe and onward to the sewer or septic system. A wax ring seals the connection between the toilet base and the flange.

The toilet’s tank holds fresh water that makes flushing the toilet possible. Water is delivered to the tank through a supply tube that connects to a small valve, called a stop valve, that is connected to the cold-water pipe. To get free recommendations for top-rated local contractors, call the most reliable and comprehensive referral service, HomeAdvisor, at 866-350-2983 (toll free).

Kitchen Remodeling Bathroom Remodeling Home Additions Home Renovations