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New drain piping to fix broken drain under a kitchen sink.

Old House Plumbing Repairs:

Replacing Sink Drain Plumbing

Bringing It Up To Code With Proper Venting

In This Article: Very lousy sink drain pipes literally fall apart and are replaced with a new slip-joint fittings, P-trap and flexible coupling. An air admittance valve is added for proper venting.	Related Articles: Index of Kitchen Articles Index of Plumbing Articles Installing New Sink Drain Pipes Cutting PVC Pipe Gluing PVC Pipe Installing A Sink Basket
Skill Level: 3 (Intermediate)	Time Taken: About 3 Hours
Author: Bruce W. Maki, Editor	Project Date: July 2007

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One evening my neighbor told me about a plumbing problem he was experiencing. The kitchen sink in their 110-year-old house was backed up. His wife was using a plunger on the sink drain when suddenly the drain pipe fell off, dumping dirty water all over the bottom of the cabinet and kitchen floor.

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Really Stupid Plumbing:

Example of poor-quality kitchen sink drain piping.

The drain plumbing looked like this before it broke apart.

The whole arrangement of drain piping was done poorly. For a start, flexible drain pipes just don't work very well.

Point Of Failure:

The section between the two red arrows had fallen off from the agitation of plunging the plugged-up sink.

Kitchen drain showing section that had fallen apart.

Twin-basin kitchen sink installed over two base cabinets, with drain pipe that runs through cabinet side.

The kitchen sink was actually straddling two separate base cabinets, so the drain to the left basin went through a hole in the cabinet partition. This wasn't a problem... it's just a bit unusual.

I began by taking apart the old drain piping.

S-trap for kitchen sink drain, installed wrong.

The nut (lower arrow) was actually cracked, and barely held the trap together. Somebody had applied a heavy layer of pipe thread compound to this connection.

Note that this entire S-trap was upside down. The nut is normally on the top piece... but I'm not sure it really matters.

This part of the trap had been merely inserted into the end of the 1½" galvanized steel pipe. Since the steel pipe had been cut off, and was not threaded, the slip-joint nut didn't actually engage anything.

Surprisingly, this connection didn't leak, even when the drain was backed up.

Plastic drain pipe simply pulled out of metal drain pipe under kitchen sink, not fastened at all.

Cleaning up end of galvanized steel drain pipe with a file.

The proper approach is to use a flexible rubber coupling, commonly called a "Fernco".

I used a small fine file to remove the burrs and rough spots on the end of the steel pipe. I didn't want to take chances with the rubber coupler leaking.

A Hack Job:

On this fitting below the sink tailpiece (arrow), the drain pipe had been installed WITHOUT a slip-joint nut.

Or perhaps the nut had become cracked and was thrown away by an unwitting homeowner.

Surprisingly, this connection didn't leak badly... even when the sink was plugged. But water had been seeping out somewhere, leaving these brown stains.

Plumb wrong - sink drain tailpiece missing a slip-joint nut.

Badly corroded threads on bottom of kitchen sink drain basket.

After I removed the white plastic pipes I tried to remove the metal nut (red arrow) on the bottom of the sink basket.

I placed a basket wrench in the drain (from above) and had the homeowner hold the wrench to keep the basket from turning. Then I used a large pair of Channel-Lock pliers to remove this metal nut. The corrosion on the threads made it very difficult to remove.

Despite our efforts, the basket rotated a bit. That was a problem.

When the sink basket moves it almost always leaks. So I had to remove the sink basket on both basins. Of course one of the the large ring-nuts broke, so we had to buy another.

After the baskets had been removed, I cleaned up the sink.

Kitchen sink with basket strainer removed.

Hard water spots on stainless steel sink, from leaking sink basket strainer.

Hard Water Deposits:

Note spots around the rim of hole. Even rubbing alcohol wouldn't remove these spots, though I could scrape them off with a screwdriver.

I figured these spots were hard water deposits, so I applied some calcium and rust remover with a stiff bristle brush, and then rinsed thoroughly with water. This removed all of the spots.

Once the sink had been cleaned up, I installed the baskets. Instead of using plumber's putty, I used clear silicone between the underside of the basket and the sink. Read about installing sink baskets in an earlier article.

New Sink Drain Basket:

If I tried to attach anything to the basket while the silicone was soft I would probably squeeze out the caulking, creating a mess and possibly inviting a leak.

After the silicone around the basket rim had cured for an hour, I installed the tailpiece. I used a dab of TFE pipe thread compound on the metal threads and used pliers to tighten the metal nut.

Kitchen sink with new drain basket and tailpiece, bottom view.

Installing New PVC Drain Pipes:

Start Over With A New Drain Kit:

Kitchen sink drain kit made from PVC plastic.

This is a sink drain kit, which we got at Home Depot.

This set of pipes connects the two basins together and directs the drain water towards the trap.

New Drain Kit Installed:

After I installed the drain kit, I had two pipe ends (arrows) that needed to be joined together.

But... I also needed a trap and an air admittance valve between these two pipes.

New kitchen sink drain kit after being installed.

Applying pipe thread compound to cut end of metal pipe before installing rubber coupling.

Old Pipe Prep:

Since the end of the steel drain pipe was rather rough, I applied a thin layer of pipe thread compound and spread it around.

I was hoping that the pipe dope would harden and form a tight seal that would prevent leaks. It must've worked because four years later it still doesn't leak.

I slipped the flexible coupling (a.k.a. "Fernco") over the end of the pipe.

Placing a Fernco flexible coupling over the end of a metal drain pipe.

Tightening the band clamp on a Fernco flexible pipe connector.

Then I tightened the screw on the band clamp.

The instructions say to tighten the band clamp screw to 50 inch-pounds of torque. That's pretty firm.

The Final Section Of Drain Pipe:

This is an air admittance valve, which we bought at Home Depot for about $24. While this part added to the cost of the repair, it also improved the flow of the sink drain.

This air valve came in two parts, which need to be screwed together hand tight only.

Note: There needs to be an inch or two of room above the air valve, in case the top section needs to be removed for maintenance or replacement.

Air admittance valve, for venting branch lines in residential plumbing drain systems.

P-trap kit for kitchen sink drain, made from PVC plastic.

The basic P-trap kit.

The other important drain component is a tee fitting. There are several types of tee fittings available. I used a sanitary tee fitting (seen in the picture below) which has a curvature to help direct drain water down into the vertical pipe.

With all the necessary pipe and fittings in my hands, I laid out the drain line connection from the pipe below the sink to the old steel pipe that poked up through the floor.

Laying Out PVC Pipe Fittings For A Kitchen Sink Drain:

When figuring out the layout of plumbing systems, I always do a "dry fit" without glue before committing to anything.

First I assembled the P-trap (1 and 2) and installed them on the vertical drain pipe (below the sink) making sure I had at least one inch of overlap.

Then I established the height of the sanitary tee fitting (3) which gave me the length of the first section of 1½" pipe (P1).

Next I determined the distance of the slip-joint adapter (4) from the tee fitting, giving me the length of the short lateral pipe P2. I could have made piece P2 a couple of inches longer... and just cut fitting #2 a bit shorter.

Pipe layout for kitchen sink drain plumbing.

The air vent (the blue-striped part above #5) needed to be at least 4 inches above the water level of the trap, so I cut pipe P3 accordingly. I placed the air vent connector hub (5) on top of this pipe to make sure everything would fit below the sink basin.

Warning About Dry-Fitting PVC Pipe And Fittings:

When dry fitting PVC pipe and fittings, the pipe won't slide into the fitting's hub all the way. But when PVC cement is applied the liquid acts like a lubricant and then the pipe can easily slide all the way in. This fact means that the glued-together assembly of pipe and fittings will be a little shorter than the length of the dry-fit assembly.

It seems that during dry fitting the pipe will enter the hub about 2/3 to 3/4 of the actual overlap amount. You can force the pipe in farther but it will be difficult to remove.

I simply compensate by making the dry-fit section of pipe a little bit longer than the desired final length. With this size of pipe I'd estimate that I add a quarter-inch per pipe-to-hub connection.

Also: It's important to verify that the trap can be assembled and taken apart. It's possible to glue up the drain pipe and then be unable to get the slip-joint fittings into place because supply pipes or cabinet walls are in the way (though most sink base cabinets don't have this middle partition).

Since I was using a rubber coupling, which is removable, these concerns did not apply.

Read about cutting PVC pipe...

Gluing The PVC Pipe Together:

A Section Of Drain Pipe Prior To Assembly:

Once I had cut the PVC pipe to the required lengths and done a trial fit-up, I took the PVC parts outside to glue together.

The PVC primer and pipe cement gives off some strong vapors, so it's best to do the gluing where the ventilation is good.

PVC sink drain pipe and fittings laid out before gluing together.

Applying purple primer to PVC pipe fitting.

I applied some PVC primer to the inside of each hub (female part of the fittings) and let it dry for a minute or two. Then I applied primer to the ends of the pipe, and after a brief drying time I glued everything together.

For more information read Gluing PVC Pipe and Fittings...

The completed drain assembly, ready to be installed in the rubber coupling.

Section of sink drain pipe glued together.

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Vertical section of sink drain connected to old galvanized steel pipe.

I pushed the drain assembly into the rubber coupling and tightened the band clamp.

Then I assembled the downstream portion of the trap. (Red arrow)

First I inserted the straight section into the slip-joint adapter (which goes directly into the tee fitting). Then I aligned the curved part with the J-shaped piece of the trap and connected the pieces together with the slip-joint nut..

P-trap installed between sink drain and vertical drain line.

Re-Plumbed Sink Drain:

Rebuilt kitchen sink drain with P-trap and air admittance valve for proper venting.

After I made sure all the slip-joint nuts were firmly hand-tightened, I was done.

Using a wrench or pliers on these plastic nuts is not recommended.

To test for leaks, I filled each sink basin with water and then pulled the drain stopper. I examined each connection for signs of water, but everything was dry.

Now the water drains from kitchen sink much faster than before, thanks to the air admittance valve.

More Info:

Tools Used:

Channel-Lock Pliers, 12", 18"
Flat Screwdriver
File, Fine
Miter Saw (Optional)

Materials Used:

1½" Sink Tailpieces (2)
Kitchen Sink Drain Kit, 1½"
P-Trap, 1½"
Slip-Joint Adapter, 1½"
Sanitary Tee Fitting, 1½"
Air Admittance Valve
1½" PVC Pipe
1½" Flexible Rubber Coupling ("Fernco")
Pipe Thread Compound
Teflon Pipe Tape

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