Plumbing Fittings Explained

Updated March 20, 2018

Just as a plumber's world is replete with water and drain pipes made from plastic, copper and other metals, it is just as full of fittings made from the same materials to join the pipes. Piping systems can be complex, and the fittings must be capable of bending them around gentle curves as well as sharp ones. They can join two or several pipes together. Fittings make pipes useful.

Pipe and Fitting Materials

Plumbing pipes serve two basic functions: they supply pressurised water or compressed gas and they remove liquid waste. Because water and gas lines have to withstand pressure, they must be made from durable materials. The most common are PVC plastic, copper and galvanised steel. Unpressurized waste lines are usually ABS plastic or cast iron. Each material has its own joining method, and the fittings must be constructed to conform to that method. With a few exceptions, copper and plastic fittings slip onto the ends of the pipes while galvanised fittings have threads. Some cast iron fittings must be bolted together.

The Function of Fittings

While joining pipes together is the main function of plumbing fittings, that function is usually combined with another, such as changing the direction of the line or connecting pipes of dissimilar material or size. Waste fittings must also incorporate a third function -- that of keeping the waste water flowing in the proper direction. In addition, there are specialised fittings that perform particular functions. An in-line shut-off valve, for example, provides a way to control water flow, while a P-trap stores water in its inverted "P" to prevent the incursion of sewer gases.

Types of Fittings

A coupler connects two pipes in a straight line and is the most basic fitting. When a plumber needs to change the direction of a line, he installs a 90-, 45- or 22 1/2-degree elbow, and uses a tee or cross to branch off from a water or waste line. Adaptors are fittings that allow the plumber to connect pipes of different materials, and bushings or bell reducers allow him to connect pipes with different diameters. A plug or cap is the fitting he uses to terminate a waste or water line.

Method of Connection

Plumbers join copper pipes with solder in a process called sweating; copper fittings are often called sweat fittings. They are also known as slip fittings because they are unthreaded and slip onto the ends of the pipes, as do most plastic fittings. Threaded fittings for steel pipes are sometimes called NPT fittings, in reference to the thread standardisation on steel pipes. It is also possible to join copper and plastic pipes with screwable fittings. They have a nut that compresses a flange around the pipe opening to form a seal; thus, they are known as compression fittings.

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About the Author

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.