How To Plumb the Drain Lines from a Kitchen Double Sink

How to plumb kitchen sink drain pipes? Whether the sink is a double (the standard these days) or a single, the process involves three main steps:

    1. Connecting the drain line(s), including sink strainer(s) and water trap(s), plus connecting all that to the main line running to the sewer or septic system.

    2. Connecting the faucet and/or other through-sink goodies such as stretch hose sprayer, soap dispenser, etc.

    3. Running hot and cold water inlet pipes from the main supply lines to the appropriate faucet connections.

This page will focus on Step #1 only.

First, the sink strainers. Home Depot stocks quite a variety of these things. A very few are plastic (no way!); most are stainless steel except for the stopper mounted on the bottom side of the strainer. The stopper is made of rubber. Additionally, most currently use a kind of “snap-clip” to lightly snug the strainer down into the receiving body. One fancy schmancy model has a screw-down setup instead of a clip.

I went with the most common, the snap-clip, primarily because I could see my wife going nuts if she couldn’t get the screw threads started in the hole on one of her bad days.

The first photo (below) shows the sink strainer parts laid out on the counter top. We couldn’t figure out why the thin sort of gasket-looking “part” was included unless it was maybe designed to protect the rubber gasket during shipping. There was no mention of it in the instructions (such as they were), nor could I see where it might be useful.

We trashed it.

Yeah, I found out later it’s supposedly there for a purpose, specifically to help the locknut tighten down more easily (see comments), but there were no hints in the instruction sheet, so I ended up doing the job without it.

The sink strainer assembly parts, laid out on the counter top. The thin "gasket" looks like a "real part", but it's not. Trash it.

The sink strainer assembly parts, laid out on the counter top. The thin “gasket” looks like a “real part”, but it’s not. Trash it. Or not–Levi posted on a few months after this was published, advising that the gasket is just that, placed between the rubber washer and the locknut to let the locknut tighten more easily without “twisting” the rubber. How about that?

Installing the strainers

Installing the strainers is pretty straightforward, though not without its minor challenges.

The instructions said to use plumber’s putty as a sealant, just like the instructions for the sink itself (between the sink and the counter top) said to do. After a quick consultation with fellow Hubber aviannovice (whose outstanding bird photography Hubs reveal anything but a novice), I rejected that plumber’s putty advice out of hand.

Plumber’s putty is not all that easy to work with. It also cracks, given time, and leaks, thus defeating the very purpose for which it was born.

Instead, a bead of clear silicone sealant (utterly waterproof and extremely reliable stuff) is run around the rim of the holes in the first sink’s bottom, then the stainless steel strainer receiver body (what I labeled “strainer body” in the photo above–forgot the “receiver” part) gets pressed firmly in place.

A bit of excess silicone squooshes out around the edges and is “squeegeed away” with the tip of an index finger.

Then, from below the sink (inside the base cabinet, head and hands out of public view with rear end clearly visible–just like in a typical Dagwood plumbing cartoon) the round black rubber “O” gasket is placed atop the strainer “cup”. This combination slips carefully over the threaded strainer pipe, after which the appropriate nut screws onto the pipe and snugs up against the bottom of the strainer body.

Time for photo clarification?

Certainly. Here you go.

A bead of waterproof silicone sealant is applied...

A bead of waterproof silicone sealant is applied…

...around the hole in the bottom of the sink.

…around the hole in the bottom of the sink.

The strainer body is seated in place atop the bead of sealant.

The strainer body is seated in place atop the bead of sealant.

Rubber gasket, strainer cup, and pipe nut ready to go. [Note: The nut goes the other way up, with the wide side "up top" against the bottom of the strainer cup.]

Rubber gasket, strainer cup, and pipe nut ready to go. [Note: The nut goes the other way up, with the wide side “up top” against the bottom of the strainer cup.]

The strainer body, prior to the application of the gasket, cup, and nut.

The strainer body, prior to the application of the gasket, cup, and nut.

Cinching down the pipe nut.

Cinching down the pipe nut.

The pliers in the sink

And…repeat the process in the other sink.

The instructions mentioned using a pair of pliers in the sink strainer body (from the top side) to keep that piece from turning while the pipe nut is being tightened. Curiously enough, that wasn’t even necessary on the first sink; the nut cinched right up, the strainer body never spun a millimeter, and everybody was happy.

Not so with sink #2. That one, without being held still by the pliers, spun like a top at the slightest twist of the nut.

Hm.

Pam tried holding the pliers in place. She’s no wuss, despite her disabilities…but this was beyond her, at least for the moment. She simply couldn’t keep the strainer body from turning.(Neither her vision nor her mental confusion were in good places at the time. I “released her from duty” pretty quickly.)

We wondered why that was so–why sink #2 had a sit-and-spin situation when sink #1 did not. It’s possible I’d applied just enough more silicone in the second case to create a “silicone skating rink” for the stainless spinner. Don’t know for sure, though, and in the end it doesn’t matter.

What matters is getting the job done.

Fortunately, once I figured out the right way to position my body, my notably long arms were able to “lock down” the pliers in the sink (left hand) and “tighten up” the nut beneath the sink (right hand) simultaneously.

Monkey Octopus Man strikes again.

The logo for Monkey Octopus Man’s superhero costume is not working out so well, though, due to the unfortunate acronym comprising his logo. It’s hard for the villains of the world to take Monkey Octopus Man seriously when he’s got MOM blazoned across his chest.

Left hand gripping the pliers that keep the sink strainer body from turning while the pipe nut (below sink) is tightened with the right hand. Monkey Octopus Man strikes again.

Left hand gripping the pliers that keep the sink strainer body from turning while the pipe nut (below sink) is tightened with the right hand. Monkey Octopus Man strikes again.

The flanged tail pieces

Flanged tail pieces (in kitchen sink plumbing parlance) are truly specialty items. I had no idea what the instructions were talking about. Had to ask a Home Depot guy 50 years my junior to show me the right pipes.

Interesting, these tail pieces. They had two lengths in stock; you could purchase either 6″ tail pieces or 11″ tail pieces. Unsure of just how “deep” in the cabinet I wanted to set the P-traps, I bought the longer versions–and then ended up cutting them down to 5 1/2″ when the time came.

Oh well.

The tail pieces, besides coming in different lengths, have the following unique characteristics:

    1. One end is, as the name indicates, flanged. The flange allows the “lower pipe nut” (different from the upper–study instructions carefully) to cinch said flange tight to a plastic coned washer, and both of them to the steel pipe.

    2. Tail piece plastic is thinner, shinier, and harder than PVC.

    3. Tail pieces are made to precisely fit a plastic slip-nut and washer combo that allows the plumber to connect the tail piece to the P-trap without glue.

Eleven inch flanged tail pieces as they came from Home Depot.

Eleven inch flanged tail pieces as they came from Home Depot.

A shortened flanged tail piece, now 5 1/2" in length.

A shortened flanged tail piece, now 5 1/2″ in length.

The tail piece assembly, ready to install in the order shown.

The tail piece assembly, ready to install in the order shown.

The sink strainer assemblies, including flanged tail pieces, now fully installed.

The sink strainer assemblies, including flanged tail pieces, now fully installed.

The P-traps

P-traps at Home Depot–at least at our local store–can be purchased fully assembled (premade), or you can more or less assemble your own. I went with “assemble my own” for the simple reason that doing so would let me spin the parts around (relative to each other) before deciding exactly how I wanted to hard-glue them together.

Why?

Because at the moment of purchase, I had no clear vision regarding exactly what angles I wanted to use to make the whole thing come together. A fair number of my projects (of which this was definitely one) start out with sort of a hazy idea in my head. It’s kind of like a sculptor who carves a stone lion by taking a solid block of stone and “cutting away everything that doesn’t look like a lion”.

Scientific? Not hardly. It’s art, I tell ya. Pure art.

Well. There is a little science involved. It helps to remember that water does usually flow downhill. Keeping the traps higher than the rest of the plumbing run would be a really good, scientific idea.

But other than that….

One question did need to be answered: Would it be better to “Y” both sink drain pipes into a single discharge pipe, then add a single trap to that pipe…or would it be better to build a separate P-trap for each sink drain and go from there?

The double-trap arrangement “felt” right.

Two P-traps, with the white nuts just finger tight so that the black pipes can be freely turned and adjusted as the rest of the drain runs are brought together.

Two P-traps, with the white nuts just finger tight so that the black pipes can be freely turned and adjusted as the rest of the drain runs are brought together.

The pipe through the wall

The photos that follow (quite a few of them) are not up to Canon PowerShot quality, but the Canon was in the shop for a couple of days. The Panasonic camcorder had to pull a bit of double duty.

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Since way back when in 2010 (this being April 25, 2013), we’ve had a drain pipe stub ready to accept drain piping from the kitchen sinks. That stub is near the geographical center of the home, just inside the utility room, which means a three foot section of pipe has to run behind a bottom cabinet drawer and through one interior wall.

With the through-pipe in place, it needed to be “sort of” secured at the utility room end–lifted a bit above the metal drawer slide that could otherwise “saw” through the pipe over time and held more or less in place, but not too tightly.

It seemed like a really good idea to let that piece of pipe have a bit of play until the under-sink plumbing attachments were all…um…attached.

At the far end (utility room end), these goals were accomplished by adding one 90 degree elbow and using a bit of metal strapping, the latter being simply nailed to the wall.

The through-the-wall drain pipe is shown in the lower right corner of the photo.

The through-the-wall drain pipe is shown in the lower right corner of the photo.

The through-pipe needs to retain a bit of clearance above the metal drawer slide, as shown here, by...

The through-pipe needs to retain a bit of clearance above the metal drawer slide, as shown here, by…

The utility room end of the through-pipe has one 90 degree elbow added and is secured with metal strapping (which is simply nailed to the wall).

The utility room end of the through-pipe has one 90 degree elbow added and is secured with metal strapping (which is simply nailed to the wall).

The headers

What happened next was quite remarkable, once it was all done. By the numbers:

    1. The sink end of the through-pipe was “stacked” atop wooden blocking to (a) provide clearance between pipe and metal drawer slide on that side, and (b) ensure an elevation drop for outgoing to water to follow as it travels toward the utility room.

Note: I accidentally typed “futility” room just then, but that’s a thing more of politics than of plumbing.

    2. A “Y” elbow was added to the sink end of the pipe.

    3. A connecting pipe assembly was put together so that both ends–at the P-trap and at the through-pipe Y–could be glued and shoved together at one time.

Note: When relatively short sections of piping are being inserted “in the middle”, between two open pipe pieces, the “two at the same time” trick is a handy maneuver to have in your kit bag. It can save a lot of moaning, groaning, and generalized cussing.

    4. A long 90 degree elbow–the longest one we had handy–was added to the through-pipe Y.

    5. Another connecting pipe assembly was double-shoved together to finish the connection to the remaining sink.

When this “under-sink stuff” was all done, something seemed familiar. It took a moment…and then I got it. The final product looked a bit like the headers for the hot rod I wanted to build when I was sixteen (but never did, mainly due to lack of money).

Pam says she can’t see it, but that’s because she only counts headers if they’re chromed from stem to stern.

A "Y" elbow is added to the through-pipe, with wooden blocking providing elevation. The blocks are glued together, and to the floor of the cabinet.

A “Y” elbow is added to the through-pipe, with wooden blocking providing elevation. The blocks are glued together, and to the floor of the cabinet.

Yep, the height looks good.

Yep, the height looks good.

One cobbled-together connecting pipe assembly. This was checked for fit (between P-trap and through-pipe) before being hard-glued together. Duh.

One cobbled-together connecting pipe assembly. This was checked for fit (between P-trap and through-pipe) before being hard-glued together. Duh.

One sink drain, good to go.

One sink drain, good to go.

Long 90 degree elbow added to the through-pipe.

Long 90 degree elbow added to the through-pipe.

The second connecting pipe assembly, ready to go. Note the black ABS glue spots on my hand--I do tend to get into my work.

The second connecting pipe assembly, ready to go. Note the black ABS glue spots on my hand–I do tend to get into my work.

Under-sink drain pipe assembly complete--and by golly, those do look a bit like hot rod exhaust headers!

Under-sink drain pipe assembly complete–and by golly, those do look a bit like hot rod exhaust headers!

Connecting to the main drain line

Connecting to the main drain line was pretty simple, thanks to my foresight in having that capped pipe stub available from the get-go. It was going to end up with a bit of a Rube Goldberg look, but that’s nothing new here at the Border Fort.

As long as the bottoms of the P-traps under the sink remained higher than the highest point of that last hop-up-and-over the cold water line (red valve handle in photos), the sinks should drain well enough.

Venting?

Glad you asked.

There’s a small in-house one way negative pressure vent mounted right next to the utility sink. More importantly, the big honker, providing major venting (also in-house) for the toilets and the tub and even helping out the utility sink at times…that one is closer to the sink drain entry point than it is to the utility sink entry point.

In other words (to put this in plain English), I believed the sinks would drain just fine.

If not, I’d have to punt, but I was optimistic.

Before cutting off the capped pipe stub at the necessary height (with a small, fine toothed handsaw), we flushed both toilets a bunch of times. Sewer gases would have free access to the home while that line was open.

Wow. When the pipe was cut, you could detect…zero gas smell!

Either we’ve got a really jim-dandy septic system going, including the monthly addition of Rid-X bacteria, or our sh*t don’t stink.

Not betting on the latter.

As with the under-sink connecting pipe assemblies, this one was double-slammed together–wait a minute. I got the first assembly wrong. Had to set that aside (maybe find a use for it someday) and redo another entire pipe set with the “long” piece cut at 4 1/4″ instead of 4 3/4″. Fortunately, I’d grabbed a dozen short-coupled 90 degree elbows on a last minute Home Depot run. Thought I’d need one. Ended up using four (two temporarily “wasted” and two installed).

That did it, and…done.

The final double barreled pipe assembly.

The final double barreled pipe assembly.

And...done. The Sink drains are now plumbed all the way into the former washing machine drain line (which joins the 3" septic line another six feet "downstream").

And…done. The Sink drains are now plumbed all the way into the former washing machine drain line (which joins the 3″ septic line another six feet “downstream”).

Test results

The fine print on the ABS glue can says to let the glue set for 15 minutes for good handling and 2 hours for pressurizing up to as much as 180 psi. Which is pretty hilarious when you think about it, since no ABS pipe is rated to handle any pressure at all.

Nevertheless, we waited the requisite 120 minutes and than water tested the sink system. Each sink was tested three ways:

    1. First, a half gallon jug of water was poured down sink #1’s drain, then the process was repeated with sink #2. Both sinks drained a lot faster than expected, and cleanly, with no “backing up” effect in the neighboring sink. It was like they were drawing air right through the water traps somehow–and maybe they were. I say they must be able to do that, whether I’m right or not.

    At any rate, the drains work, they work well, and there’s not a leak in sight.

    2. The stoppers were put in, and an inch or so of water was poured into the sinks and let stand for a while to see if the stoppers really worked. They did.

    3. The stoppers were pulled so that both sinks were draining simultaneously. This went noticeably slower than when only one sink was draining at a time–but the process was still smooth, and it wasn’t that slow.

It took about 10 seconds (give or take a second) to drain the final inch of water from the bottoms of the sinks. Still no sign of any leaks, which is a good thing.

You can see the tests being done in the video, live on candid camera.

The sink drain plumbing at the Border Fort is a definite go.