Although the drain pipes beneath the kitchen sink don’t need to be changed out so often, it is common when replacing the sink to also replace the drain assembly as well when renovating a kitchen.
The sink drain installation steps will be determined by the difference between the old and new sink.
Furthermore, you will need to take into account the other components to be installed.
Check the Height of the Drain Outlet
If you are changing from a standard sink to a deep-basin sink, the drain fitting inside the wall might need to be lowered to match the new sink drain height.
Before purchasing a new kitchen sink, make sure you check the trap arm height of the existing sink.
The trap arm is a piece sitting horizontally between the branch drain pipe within the wall and the P-trap.
It is connected to a tee fitting inside the branch drain and usually slopes downward at around a quarter inch per foot to the tee for adequate drainage.
Lower the Drain, If Needed
If the present drain opening is too high for the new sink, you (or an expert plumber) will have to open up the wall and reduce the sanitary tee position which connects the drain pipe within the wall.
This task can be rather complicated as the back of the cabinet and wall surface will need to be cut away.
Here, if a new sink doesn’t leave enough room to have a standard drain assembly connected to the drain outlet within the wall, then the sanitary tee fitting would have to be lowered.
Remember that the drain height must have sufficient room for removing and cleaning out the trap.
If the tee inside the branch drain needs to be lowered, it can help to first test-fit the other drain parts before permanently securing the fittings with glue.
At this point, install your new sink.
Here, we are installing a double-basin sink fitted with a garbage disposal on one basin.
Sink Drain Installation: Strainer and Garbage Disposal
First install the garbage disposal on the preferred side of the sink since this consumes much more space.
The basket strainer should be installed on the basin that doesn’t have the garbage disposal.
Now you can also install any other components underneath the sink such as hot water dispenser or water filter.
Sink Drain Installation: Waste Pipe and Tailpiece
The constant waste pipe connects the drain of the garbage disposal to the tee fitting.
The drain tailpiece is a straight pipe that runs from the sink strainer down to a tee. This is the fitting under the basin that doesn’t have the disposal.
Cut the waste pipe and tailpiece to length before installing.
Use a slip nut (and washer, if needed) to install the tailpiece temporarily to the sink strainer.
Also, use a slip nut and washer to connect the constant waste pipe’s curved end temporarily to the drain outlet of the garbage disposal.
Allow the pipe straight end to extend beyond the tailpiece, sloping downward to some extent towards it.
Hold the sanitary tee fitting up to the waste pipe and tailpiece, and make a mark on the two pipes to indicate where to cut to fit into the tee fitting.
Use a hacksaw or PVC tubing cutter to cut the waste pipe and tailpiece at the marked points.
Reinstall the waste pipe, tailpiece and the tee fitting, tightening the connections by hand.
Adjust the pieces as needed, ensuring the constant waste arm slopes slightly downward towards the tee.
Use channel-type pliers to tighten the slip nuts. Do not tighten too much so as not to damage the plastic threads.
Connect the Drain Trap
The drain trap configuration consists of a J-shaped trap arm and a trap bend that is U-shaped.
The trap arm goes all the way into the tee at the wall, while the trap bend goes into the bottom of the tee fitting at the tailpiece.
With a slip nut and washer, loosely connect the trap arm and trap bend together. Place a slip nut and washer on the trap arm’s straight end.
Push up the trap bend into the tailpiece tee while moving the trap arm into the tee at the wall.
Adjust both trap arm and trap bend as necessary to get a straight path to the wall tee from the sink.
Ensure the trap arm tilts downward slightly towards the tee at the wall.
If the arm is too long and will not fit in the area, mark and cut it as necessary, then re-install the trap assembly.
Use channel-type pliers to secure all slip nuts. Remember not to over-tighten.
Check that the trap arm and constant waste pipe slope downward slightly towards the flow of water, and confirm that all the drain connections are tight.
Ensure the trap is faced correctly with its sharp bend under the tailpiece.
You may mistakenly turn it the other way so as to fit a small space, but then it can leak and won’t work properly.
Test for Leaks
While running water in both basins, check for any leaks at the pipe joints. If you notice any leaks, tighten the connections.
Finally, fill up each basin and allow to it drain as you check beneath for leaks.
It’s quite common to experience leaks with slip-nut joints. It typically only needs little adjustments.
If this does not work, loosen the nut and adjust the washer. Retighten the nut and ensure it is not cross-threaded.