Plumbing Basics: Installing PVC Kitchen Sink Drain Plumbing

By Bruce W. Maki, Editor

Kitchen sink drains normally use 1-1/2" diameter drain plumbing. There is also 1-1/4" diameter drain plumbing which is mostly used for bathroom sinks and other low-volume drains. Years ago all drain pipe were made from brass, which was usually chrome plated on the outside. Brass has a tendency to corrode and leak. PVC plastic, while seemingly cheap and weak is a much longer lasting material, truly corrosion-proof, for a lower price. Perhaps the only drawback of PVC is the possibility of over-tightening the threaded fittings and ruining the threads.

The starting point for this task is the sink basket. The smaller diameter threads are for the 1-1/2" diameter drain pipe.

The sink drain kit includes fittings for two basins and a "T" junction. The "S" trap fit includes the two-component trap and a piece of straight drain tube.

Note that there are some big differences between this PVC pipe and the PVC pipe that is glued together for building drain lines.

1. This pipe uses slip-joint connections, which allow the pipe to be adjusted in and out.
2. The outside diameter is the nominal size (the stated or the the named size), where the glued-together PVC pipe the nominal size indicates the inside diameter. This means that the slip-joint drain fittings will slide inside the glued-up drain pipe, and there are threaded-to-glued adapters where this happens.

This metal nut connects the drain tail-piece to the sink basket. This short tail-piece came with the kit, but I also bought longer tail-pieces because I prefer them.

The longer tail-piece actually came as one part that is sawed in half to make two tail-pieces.

I assembled the drain parts that I figured I would need. This layout shows the form that the pipes would follow... if the sink drain line goes downwards through the floor.

Most newer houses use a horizontal drain line that runs into the wall (and then turns downward, eventually). Horizontal drains are easier because there is no need to drill holes in the bottom of the sink base cabinet.

When a horizontal drain pipe is present (which is more common than vertical) then a P-trap would be used instead of an S-trap.

The metal nut (which came with the sink basket) was installed over the tail-piece, and the plastic washer was inserted. I'm not sure if the plastic washer is needed with PVC pipe, but it certainly is needed with metal pipe.

This assembly was attached to the sink basket. I used pipe thread compound to help seal the joint and allow for easy removal later.

Below the tail-piece is an elbow which leads to the other basin.

Completion: At the other basin there is a "T" junction fitting, which leads into the "S" trap, which leads into the building drain. All of these pipe are connected with slip-joint fittings, which can be removed easily for repairs or cleaning.

In fact, the ease of dis-assembly is perhaps the best feature of PVC drain fittings. If a sink gets clogged, I try the usual drain cleaner products first, and if that does not work then I just disconnect the fittings, take them outdoors and blast them with the garden hose to dislodge any blockage.

More Projects Worth Viewing:

Installing A Bath Lavatory Faucet and Drain.

Some Thoughts On Kitchen Sink Drain Repairs:

I have repaired many sink drains that leaked, in residential and hotel kitchens. From the perspective of an engineer,  if we can build systems that hold thousands of PSI without leaking, then making a non-pressurized drain water-tight should be simple. What makes systems secure enough to withstand high levels of fluid pressure?

Pipes and fittings that are physically strong enough to prevent bursting. This is not a problem with any modern drain plumbing.

Pipes and fittings that resist the corrosive action of liquids they contain. PVC is virtually corrosion-proof. Brass is not. Brass will corrode over time, and slight acidity of the water can accelerate this.

The connections between parts have precise mating surfaces. I believe this is the culprit behind many leaks. If pipes are not aligned properly, because someone tried to force a straight pipe into a slight bend, there is a serious risk that a leak will occur. (I've seen this too often.) Consider this analogy: all automotive engines have incredibly precise mating surfaces between the engine block and the cylinder head, carefully machined to within a few ten-thousands of an inch. These connections are designed to maintain fluid-tightness at several hundred PSI. That is the last place for sloppiness.

The connections between parts must be clamped securely together, otherwise no amount of precision will do. Again, consider the automotive analogy: the numerous bolts that clamp the cylinder head to the engine block are very tight, perhaps 100 foot-pounds, more than the lug nuts that hold the wheels on the car. Most of the sink leaks that I have seen were simply caused by slip-joint nuts that had come loose.

Able to withstand the abuse of normal activity. Under-sink drain pipes are going to get bumped from people storing items there. Children and rental-property tenants are more likely to abuse things around the home. I believe the casual bumping of drain plumbing explains why they often become loose over time, and this is worse with PVC than with metal, because PVC fittings cannot withstand as much tightening.

One other tip to help prevent leaks is to apply a little pipe thread sealant (pipe dope) to the slip-joint washers. This gooey material will fill small imperfections in the mating surfaces.

If all else fails, I clean the area around the leak and apply a bead of silicone.

Copyright © 2000, 2005  HammerZone.com

Written November 11, 2000
Revised January 5, 2005