Choosing Pipe

The first step in choosing pipe is to find out which type of pipe you have in your home. It’s often easiest to use the same type when adding on, but it’s not mandatory. To change type, you must purchase special adapter fittings to switch from one material to another in the middle of a pipe run. For supply lines—the pipes that carry pressurized water to your fixtures—the usual choices are copper and plastic. However if your home is old enough to have galvanized pipe and you need to install only a short run of pipe, it makes sense to continue with galvanized. In many localities, plastic supply pipe is not allowed. Keep in mind that you must learn how to solder before you can install copper pipe. When making final connections to a fixture or a faucet, usually it is easiest to install a flexible supply line. Use copper or plastic flexible tubing. Be careful to avoid kinks. Plastic pipe—either PVC or ABS—is now used almost exclusively for drains. If you have old cast-iron, galvanized, or copper drain pipes, make the transition to plastic. It is much easier to install and less expensive. Before you buy any pipe, check with the local building department to make sure you’re using material approved for use in your area.

Gas Lines

 ■ Gas lines are almost always made of black steel pipe. It has the same texture as galvanized pipe but not the shiny silver color. Black steel pipe is installed in the same way as galvanized pipe. Check for leaks by turning the gas on, pouring soapy water on all the joints, and looking for tiny bubbles.

■ Do not use copper pipe for long gas lines. A chemical reaction causes the inside of the pipe to flake, which can plug orifices and damage appliances. Contrary to some opinions, you can use galvanized pipe for gas, but it’s more expensive.

The Most Common Sizes - Here are the pipe sizes commonly used in residential plumbing in North America:

  • Main water-supply line entering a house: 3/4-1 inch.
  • Water-supply lines after the water heater usually are 1/2 inch, sometimes 3/4 inch.
  • Gas lines are most often 3/4 inch and sometimes are 1/2 inch.
  • Main drain pipes, called stacks, are 3 or 4 inches.
  • Kitchen, tub, and shower drains are almost always 1 1/2 inches.
  • Bathroom sink drains are almost always 1 1/4 inches.




Features and Joining Techniques



Hot and cold supply lines; rarely for DWV (Drain-Waste-Vent) lines

Sold in 10' and 20' lengths. The most widely used pipe for supply lines. Lightweight and durable, though a bit expensive. Once the soldering technique is learned, you can cut it on the spot and put it together quickly. Type M. is the thinnest, and is a good choice for home projects. Types L and K are used mainly in commercial projects.


Hot and cold supply lines, for short final runs to fixtures

Comes in easily bent 60' and 100' coils or by the foot. Can be soldered like rigid copper, but usually is connected with compression fittings.

Threaded Steel


Supply and occasionally DWV

Because it’s cumbersome to work with and tends to build up lime deposits that constrict water flow, it is not used widely anymore. It takes expensive equipment to cut and thread it, so you must buy precut pieces from your supplier. If you have a good selection of shorter pieces on hand, you can cut down on trips to the supplier.


Gas lines

Rusts readily, so it must not be used for water supply.



DWV only

Black in color, in 10' or 20' lengths. Lightweight and easy to work with, you can cut it with an ordinary saw and cement it together with a special glue. Check local codes before using.


Cold water supply and DWV

Cream-colored, blue-gray, or white, in 10' or 20' lengths. This has the same properties as ABS except that you must apply primer before cementing it. Do not mix PVC with ABS or interchange their cements.


Hot and cold supply lines

White, gray, or cream-colored, available in 10' lengths. Has the same properties as ABS and PVC.

Flexible PB (polybutylene)

Hot and cold supply lines, usually for short runs

White or cream-colored, sold in 25' or 100' coils or by the foot. Flexible. Expensive and not widely used, it is joined with special fittings.

Flexible PE (polyethylene)

Supply lines

Black-colored, sold in 25' or 100’ coils or by the foot. Same properties as PB. Used for sprinkler systems.




Cast-iron is extremely heavy and difficult to work with, so don’t try to install any new pipes of this material. Hub-and-spigot is joined with oakum and molten lead.



Joins with gaskets and clamps, but still is hard to work with. Make the transition to plastic.

Choosing the Right Fitting

The parts bins at a plumbing supplier contain hundreds of fittings that let you connect any pipe material in just about any way. To get the best pipe for your needs. Supply fittings connect the pipes that bring water to fixtures and faucets. When changing direction in a supply run, use an elbow (ell). The most common ones make 45- or 90-degree turns and have female threads on each end. A street ell has male and female connections to allow for insertion into another fitting. A reducing ell joins one size pipe to another. Use a drop ell to anchor the pipes to framing where they will protrude into a room.

Use tees wherever two runs intersect. A reducing tee lets you join pipes of different diameters; for example, adding a 1/2-inch branch to a 3/4-inch main supply. A coupling connects pipes end to end. Reducing couplings let you step down from one pipe diameter to a smaller one. Slip couplings function the same way as unions, joining sections of copper or plastic line. Use a cap to seal off a line. A plastic-to-copper transition fitting is one of many transition fittings that connect one pipe material to another. (Do not make the transition from steel to copper without a special dielectric fitting, or the joint will corrode.)

In any run of threaded pipe, you’ll need a union somewhere. This fitting compensates for the frustrating fact that you can’t simultaneously turn a pipe into fittings at either end. Nipples—lengths of pipe less than 12 inches long—are sold in standard sizes because short pieces are difficult to cut and thread. Examine drainage fittings, and you’ll see how they’re designed to keep waste water flowing downhill. Sometimes called sanitary fittings, they have gentle curves rather than sharp angles, so waste will not get hung up.

Choose 1/4 bends to make 90-degree turns, and 1/8 bends for 45 degrees. Also available are 1/5 bends for 72-degree turns, and 1/6 bends for 60 degrees. All types of bends also come in more gradual curves, known as long-turn bends, which make for a smoother flow. Sanitary branches such as the tee and cross shown here, come in a variety of configurations that suit situations where two or more lines converge. These can be tricky to order, so make a sketch of your proposed drain lines, identifying all pipe sizes, and take it to your supplier when you order.

Toilet hookups require a closet bend, which connects to the main drain, and a closet flange, which fits onto the bend. The flange is anchored to the floor and anchors the toilet bowl. To connect a sink trap to the drainpipe, use a trap adapter. To make the transition from cast-iron drain to plastic drain, use a no-hub adapter.

Ordering or Finding Fittings - When ordering materials, organize your description of a fitting in this way: first the size, then the material, and finally the type of fitting. You might, for example, ask or look for a 1/2-inch galvanized, 90-degree ell. With reducing fittings, the larger size comes first, then the smaller.

Measuring Pipes and Fittings

Beginning plumbers often spend more time running back and forth to their supplier than they spend doing the actual work because it takes practice and experience to be able to figure out everything you need ahead of time. The first step in becoming an efficient plumber is to learn to correctly identify the pipes and fittings a job requires. Plumbing dimensions aren’t always what they appear to be. A plastic pipe with a 7/8 inch outside diameter, for instance, is actually called a 1/2-inch pipe because it has a 1/2-inch inside diameter and pipes are usually sized according to their inside diameter (ID). This dimension is also referred to as the nominal size, the size you ask for at a plumbing supplier.

To find out a pipe's size, measure the inside... If you have a pipe with an exposed end, simply measure the pipe’s inside diameter and round off to the nearest 1/8 inch. Some manufacturers indicate the size on the fittings.

...or figure from the outside. You also can determine pipe size by measuring its outside circumference. Wrap a string around the pipe, straighten it out, and measure it. Use the chart below to find the nominal size you’ll need to order.

If you are at all unsure about getting the right material, make things perfectly clear by specifying ID for most pipes. In a minority of cases—flexible copper lines, for example—pipe is ordered by using the outside diameter (OD). If you can measure the inside dimension, you’re home free. However often you won’t have a way of measuring the inside of the pipe. Holding a ruler against a pipe will give you only a rough idea of the outside diameter. Instead, use a string or a set of calipers for a more exact measurement. Once you find the outside dimension, use the chart, opposite, to find the nominal size.

Fittings can be just as confusing. Their inside diameters must be large enough to fit over the pipe’s outside diameter. A half-inch plastic elbow, for example, has an outside diameter of about 1 1/4 inches. As a rule of thumb, the OD of copper is 1/8 inch greater than its ID, the nominal size. For plastic pipe, measure the OD and subtract 3/8 inch. For threaded and cast-iron, subtract 1/4 inch. Another mathematical pitfall for a beginning plumber is measuring the length of a pipe running from one fitting to the next. Pipes must fully extend into fixture and fitting sockets (see illustration, right), or the joint could leak. Socket depths vary from one pipe size and material to another, so you must account for the depth of each fitting’s socket in the total length of pipe needed between fittings. The only times you don’t have to take socket depth into account are when you are using no-hub cast-iron pipes or slip couplings with copper or plastic pipe.

Add the socket depths. To figure the length of a pipe, first measure from face to face, as shown above. Because pipes have fittings on both ends, multiply by 2, and add the face-to-face length.

Measure copper or plastic in place. When working with copper or plastic—materials you can cut on the job—often the most accurate way of measuring is to insert the pipe into one fitting and mark the other end, rather than using a tape measure.

Don’t leave the household high and dry while you drive back and forth to the plumbing supplier. When buying fittings, invest in a handful of caps in different sizes. That way, if you’ve misread a dimension—as even experienced plumbers do occasionally—you can cap off the line and turn the water on.

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