Kitchen Sink Plumbing: What You Should Know

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Kitchen sink plumbing can be complicated, so contact a plumber if you're not comfortable.
Image Credit: Stephen Paul for Hunker

If you've updated your kitchen and added a new kitchen sink or moved the old one, you'll need to install new plumbing for the drainage and water supply under the sink. This can be a very intimidating project for many homeowners, particularly those with no prior plumbing experience. However, most people will find that with a little bit of knowledge, they can do most if not all of the necessary plumbing work themselves.

That's not to say that it will be simple. Plumbing often looks difficult, and it is. If it wasn't difficult, with the amount they charge, everyone would be a plumber. There are some problems that a homeowner may decide is best handled by a professional, like installing a garbage disposal. However, with a basic knowledge of the plumbing under your sink and how it works, installing the plumbing to a kitchen sink is a project that most handy homeowners can handle, saving some money in the process.

Basic knowledge of kitchen sink plumbing can help you tackle these projects alone.
Image Credit: Stephen Paul for Hunker

Where to Start

Of all the drainage plumbing required for your kitchen sink, the basket strainers are the only parts visible from the top of the sink. To many homeowners, the strainer is known as the sink drain. Of course, there is a lot more to the drain than the strainer, as anyone who has looked under a sink can tell you. The basket strainer, also known as a sink strainer, is a good starting point and is a very important part of the drain since it connects the sink with the rest of the plumbing pipes.

The strainer itself is the small slotted piece in the drain hole at the bottom of the sink basin. It is designed to allow water through while screening out and capturing larger items. It is also used to block off the drain and hold water in the sink. The strainer seats into a basket, which is why it's called a basket strainer.

Though installing the basket strainer to a sink is not too difficult, it's important to make sure it is installed properly. A basket strainer typically consists of the basket, a locknut, a rubber gasket and a cardboard friction ring.

Most plumbers like to work from the bottom up. This means that after installing the basket strainers and tailpieces or continuous waste pipes onto the sink, you do not continue working down to the branch drain pipe. Instead, you should connect the P-trap to the drain pipe and then connect the trap to the tailpieces.

A P-trap is an essential part of your kitchen drain because every plumbing fixture in the home is required to have a trap. The name is a little misleading and may lead to a little confusion — a P-trap is actually U-shaped. A P-trap is designed to stay full of water and to provide a seal that prevents dangerous sewer gas from the pipes from entering your home.

P-traps are an essential part of your kitchen plumbing.

How to Install Sink Plumbing

Step 1: Use Plumbers' Putty

Begin installing a basket strainer by running plumbers' putty along the underside of the basket rim and then press the basket firmly into the sink. You can soften the putty by running it between your hands and making a sort of rope that can be looped around the basket-strainer rim.

Make sure the basket is centered in the sink hole to avoid ending up with a leak. Then, from the bottom, wipe up any excess putty with a cloth or scrape it away gently with the edge of a screwdriver or putty knife. Add the gasket and cardboard friction ring and then turn the nut onto the basket threads until it is snug. Tighten the nut a bit further with tongue-and-groove pliers.

Step 2: Center the Basket Strainer

Remove any excess putty that has squeezed out from the lip into the sink basin. If the basket appears to be a bit off-center, you can loosen the nut, gently slide the basket until it is centered and retighten the nut. Some plumbers prefer to use silicone caulk instead of plumbers' putty because it is less likely to leak, but putty works well to prevent leaks and is less messy than silicone.

Step 3: Installing a Disposal

If you are installing a garbage disposal in your drain system, you can install it at the same time as a basket strainer. Instead of a basket strainer, though, you will find a flange fitting that goes into the sink-basin hole. Run a rope of plumbers' putty around the underside of the flange and then press the flange into the sink hole. Tighten the flange from under the sink with a mounting ring using a screwdriver, a snap ring and a lower mounting ring.

The disposal unit is then lifted and fit into the lower mounting ring, which is usually the most difficult part. This step may require two people to lift the unit. In the drain outlet of the disposal, you can fit a straight tailpiece or an elbow to hook up to the drain plumbing.

A disposal requires an electrical cable (for a hard-wired connection) or an outlet (for a cord-and-plug connection). If you are not comfortable with electricity, you may want to hire an electrician for this step. One thing to remember when considering a disposal is that, according to Old House Journal, sink-mounted disposals should not be used with a septic system. If you feel you must have one, you can purchase a septic-safe disposal.

Step 4: Attach a Tailpiece

Attach a 1 1/2-inch tailpiece to the basket strainer with channel-lock pliers using the provided threaded nut and gasket. These tailpieces are usually made of plastic (polypropylene) or brass. An average tailpiece length will be 4 to 6 inches, but they come in a variety of lengths and can be cut to fit. You may want to go with a plastic tailpiece because it is easier to cut. If the drain pipe comes from the wall, you'll want a shorter tailpiece so it will drain properly.

If you have a double-basin sink, you can use two tailpieces, but a better option is an end outlet waste tee, or continuous waste, that threads directly onto the basket strainers like a tailpiece. This fitting ties the two basins together, allowing them to drain into the trap through the same pipe. An adjustable side outlet waste tee can be adjusted using a nut located near the center of the waste, allowing it to fit basins of different sizes or spacing.

Different methods are used to plug single- or double-basin sinks.

Step 5: Locate the Drain Pipe

Locate the drain pipe stubbed into the cabinet. If it is a PVC pipe and comes out through the wall, cut it to the required length, which will allow your P-trap to be directly below the tailpiece or waste tee. You can cut PVC pipe with a hacksaw or specialized PVC saw or a PVC-pipe cable saw. If the pipe coming out of the wall is too short, glue a 1 1/2-inch coupling onto the pipe by first cleaning it with PVC cleaner and then swabbing it with PVC cement.

If the drain pipe comes out of the floor, cut it to fit the height you need or add a PVC coupling to raise it. A trap is typically located 6 to 12 inches below the sink, but this can vary. After cutting the floor pipe to the correct height, glue a 1 1/2-inch PVC elbow onto the pipe. Then cut a piece of 1 1/2-inch PVC pipe to the proper length to get the P-trap inlet directly below the tailpiece. This pipe should be at least 4 inches in length to prevent turning the P-trap into an S-trap, which can siphon water from the trap. Glue the piece of pipe into the elbow and glue an elbow for the P-trap on the other end of the pipe.

Step 6: Fit the P-Trap

Fit the P-trap on the elbow but do not glue it on or fully tighten if it is threaded. The tailpiece or continuous waste will drain into the inlet side of the trap, while the outlet side of the trap will connect to the drain pipe. Align the trap so that the tailpiece is directly above it.

One of the great things about P-traps is that they can be moved and adjusted, making installation easier. If everything lines up perfectly, your tailpiece will fit directly inside the trap and can be connected. Usually, though, you have to cut one or more straight sections of pipe to align the trap.

Step 7: Measure a Piece of Pipe

Measure and cut a piece of PVC pipe to fit from the trap to the tailpiece. Glue the piece of pipe into a desanco fitting. A desanco has a nut and gasket on one end, which is tightened onto the tailpiece. The benefit of a desanco and tailpiece is that if the length is off, the desanco nut can be loosened to move the fitting up or down the tailpiece until it fits correctly into the trap. Once the pipe is glued into the desanco, slide the nut, gasket and desanco onto the tailpiece and snug it hand-tight.

Step 8: Install the P-trap

Glue or tighten the P-trap onto the piece of pipe on the inlet side and the elbow on the outlet side. Some traps are connected with PVC cement, while others are connected with nuts and washers. A P-trap with nuts and washers is easier to disconnect and clean, but more nut joints can mean a higher chance of leaks. Tighten all the nuts with tongue-and-groove pliers and run water down the drain to check for leaks.

Once you have connected the plumbing, your new kitchen sink will be ready for use.
Image Credit: Stephen Paul for Hunker

Step 9: Connect the Dishwasher Drain

The dishwasher drain hose is run under the kitchen sink through a hole in the side of the cabinet. Connect the end of the hose with a clamp to a dishwasher drain fitting on the tailpiece or elsewhere above the trap, or, if you have a disposal, there is a similar fitting where it can be connected the same way with a clamp. Make sure you knock out the piece of plastic in the disposal fitting before running the dishwasher, or it won't be able to drain.

A dishwasher drain requires an air gap to prevent dirty water and waste from backflowing into the dishwasher from the drain or the disposal. A simple way to provide an air gap is to run a high loop in the dishwasher drain hose, making sure it is higher than your drain line or disposal. The loop will need to be secured with a clamp to the wall of the cabinet because the drain hose will move while in use and can drop down.

Alternatively, you can install an air-gap device on the sink or countertop and connect the dishwasher drain hose to the port on the air gap. Then, connect a second hose between the outlet of the air gap and the disposal or a tee on the drain line. Air-gap devices are required in some areas based on local code rules.

Step 10: Install Shutoff Valves

To connect the water supply to your faucet, the first step is to install shutoff valves on the hot and cold water pipes. The best way to do this is to use a 1/2-inch x 3/8-inch compression valve. If you are comfortable with soldering a valve, have at it. If not, you can use a SharkBite push-to-connect valve. With these valves, you simply push it onto your PEX or copper water-supply pipe. It's that simple.

Step 11: Attach Flexible Faucet-Supply Lines

Attach a flexible faucet-supply line to each of the faucet stems using either an adjustable wrench or a basin wrench if there isn't much room. These supply lines are usually braided stainless steel and come in a variety of sizes. These lines are flexible enough to be bent and looped, so longer is often better if you are unsure of what size you'll need. After connecting to the faucet, connect the 3/8-inch end to the shutoff valve with a wrench. It's important not to overtighten these connections. Snug is usually enough.

Step 12: Make the Dishwasher Water Connection

If you have a dishwasher, you'll need to cut the hot-water pipe and install a tee and a shutoff valve with a 3/8-inch compression fitting on the other side of the shutoff. You can use push-on fittings for this to make it easier. Be sure to install the dishwasher connection in the hot-water line and not the cold. Connect the dishwasher's flexible water line to the 3/8 fitting.

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Gary Sprague

Gary Sprague

Gary Sprague is a retired master plumber who now works as a writer. He lives with his family in Maine.