There aren't many reasons for changing out the drain pipes under a kitchen sink, but it's not uncommon to replace the drain assembly during a kitchen renovation when the sink is being replaced. The steps for installing the sink drain will depend on how different the new sink is from the old one, as well as what other components are being installed. For example, the new sink may have a different depth than the old one, so some alteration of the branch drain pipe fitting entering the wall may be necessary.
Equipment / Tools
- Tape measure
- PVC tubing cutter or hacksaw
- Channel-type pliers
- Kitchen drain trap kit
Check the Drain Outlet Height
If you're planning to replace a standard sink with a deep-basin sink, you may need to lower the drain fitting in the wall to match the lower height of the new sink drain. Before you buy a new kitchen sink, it is a good idea to check the height of the trap arm on the old sink. The trap arm is the horizontal piece between the U-shaped trap bend (the P-trap) and the branch drain pipe in the wall. It connects to a sanitary tee fitting in the branch drain and should slope down toward the tee at about 1/4 inch per foot for proper drainage.
Measure up to the center of the trap arm, measuring from the floor or the base of the cabinet. If the height is more than 16 inches, you may find that a new sink will not allow for the necessary downward slope for the trap arm.
Lower the Drain, If Necessary
If you find that the existing drain opening is not low enough to accommodate the depth of the new sink, you (or a professional) will need to open the wall and lower the position of the sanitary tee connecting to the drain pipe in the wall. This can be a somewhat complicated job that requires cutting away the back of the cabinet and the wall surface.
In this example, the new sink left insufficient room for connecting a standard drain assembly to the drain outlet in the wall (left photo). So, the sanitary tee fitting has been lowered (right photo). Keep in mind that the drain height must leave enough room for removing the trap and cleaning it out.
If you need to lower the sanitary tee in the branch drain, it is a good idea to wait until the other parts of the drain are test-fitted before gluing and securing the fittings permanently. Leaving the fittings loose will allow you some room for adjustment.
Before continuing to the next steps, install the new sink. In this example, the installation includes a double-basin sink with a garbage disposal fitted to one basin.
Install the Garbage Disposal and Strainer
Install the garbage disposal onto the desired side of the sink first, since this takes up the most space. Install a basket strainer onto the sink basin without the garbage disposal. This is also a good time to install any other under-sink components, such as a water filter or a hot water dispenser.
Install the Tailpiece and Continuous Waste Pipe
The drain tailpiece is a straight pipe that connects to the sink strainer and runs down to a tee fitting (below the sink basin without the disposer). The continuous waste pipe connects the disposal's drain to the tee fitting. The tailpiece and waste pipe must be cut to length for installation.
Temporarily Install the tailpiece onto the sink strainer, using a slip nut (and washer, as applicable). Temporarily connect the curved end of the continuous waste pipe to the garbage disposal's drain outlet, using a slip nut and washer. Let the straight end of the pipe extend past the tailpiece. The pipe should slope downward slightly toward the tailpiece.
Hold the tee fitting up to the tailpiece and waste pipe, and mark both pipes where they should be cut to fit into the tee fitting. Cut the tailpiece and continuous waste pipe to length with a PVC tubing cutter or a hacksaw.
Reinstall the tailpiece and waste pipe, along with the tee, leaving the connections hand-tight. Adjust the pieces as necessary, making sure the continuous waste arm has a slight downward pitch toward the tee fitting.
Secure the slip nuts with channel-type pliers. These do not need to be overly tight; the plastic threads can be damaged if you use too much force.
Connect the Drain Trap
The drain trap assembly includes the U-shaped trap bend and a J-shaped trap arm. The trap bend fits onto the bottom of the tailpiece tee fitting, while the trap arm extends into the sanitary tee at the wall.
Assemble the trap bend and trap arm together loosely with a slip nut and washer. Slide a slip nut and washer onto the straight end of the trap arm. Push the trap bend up into the tee fitting on the tailpiece while sliding the trap arm into the sanitary tee at the wall.
Adjust the trap pieces as needed to create the most direct path from sink to the wall tee. Make sure the trap arm has a slight downward angle toward the wall tee.
Mark and cut the trap arm, as needed, if it is too long to fit the space, then reinstall the P-trap assembly. Tighten all slip nuts with channel-type pliers. Again, don't overtighten.
Confirm that the continuous waste pipe and the trap arm slope slightly downward in the direction of the water flow, then check all of the drain connections to make sure they are tight.
Be sure to face the trap the correct way, with the sharp bend below the tailpiece. It may be tempting to turn it the wrong way in order to fit it into a small space, but it doesn’t work properly this way and can leak.
Check for Leaks
Run water in both sink basins and check for leaking at each pipe joint. Tighten up any connections that leak. Conduct a final test by filling up each sink basin and letting it drain while you check for leaks below.
It's not uncommon to get a little leaking with slip-nut joints. Usually it needs just some minor adjustment. If initial tightening doesn't stop the leak, back out the nut, reposition the washer and re-tighten the nut, making sure it's not cross-threaded.