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My kitchen sink drains through an S-trap immediately below the sink to another trap (a P? a U?) in the basement below. The drain pipe then runs along the wall till it meets the main drain pipe shortly before it leaves the house.

Because this second trap is cast iron, I can't see any way to remove it and take it out. The sink has been draining poorly, and snaking the line seems to help for a bit, but I can't but think there's some real build-up in this trap.

I am also concerned because I have never seen any such double-trap arrangement elsewhere.

The trap in question:

Trap under kitchen sink S-trap

For images of the whole assembly, see this Imgur album.


After: I used this as an excuse to get a sawzall and used it with a carbide-tipped cast-iron cutting blade to saw through the cast iron pipe to the right of where it flared to join with the trap.

I removed the Fernco coupler that tied the PVC under the sink to the cast iron? lead? drain pipe that ran to the trap and pulled the whole assembly out. It turned out they'd run the smaller O.D. threaded 1-1/2" PVC down well inside the metal pipe; the PVC actually ended slightly in the basement.

I replaced the trap assembly with two lengths of Schedule 40 PVC and a 90-degree PVC elbow. A new 1-1/2" to 2" Fernco coupler joined the PVC to the existing cast iron drain pipe. I reused the old Fernco coupler to join the smaller sink PVC to the larger PVC I purchased; the smaller pipe is slightly inserted in the larger, but an inch rather than a foot. (They were both labeled 1-1/2" PVC. Go figure.)

The end result is a sink that drains fabulously and no leaks, and a sawzall I get to keep, for much less than it would have cost to hire a plumber just to snake the line, nevermind replacing the trap with an elbow. Yay DIY!

New PVC elbow

(full album)

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Is this the same drain as in this question - diy.stackexchange.com/questions/20700/…? If so then it's really the same issue and this information should be merged into the other question. –  ChrisF Nov 13 '12 at 10:10
    
@ChrisF Not sure I agree... they are seperate questions in my opinion because one is about a cast iron P-trap in the basement versus a plastic S-trap below the sink. –  maple_shaft Nov 13 '12 at 12:00
    
@maple_shaft - however, getting the "right" answer for each depends on knowing the other. The real question is something along the lines of "what, if anything, do I need to do with this drain under my sink?" –  ChrisF Nov 13 '12 at 12:03
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@maple_shaft That stuff looks gooey but doesn't feel it. I think maybe this is not the first time this trap has sprung a leak, and that mess is the result of prior patching. Anyway, the cast iron trap will be going away. :) –  Jeremy W. Sherman Nov 13 '12 at 20:40
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@JeremyW.Sherman Good job! Looks great! –  maple_shaft Nov 14 '12 at 11:54
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1 Answer

up vote 7 down vote accepted

No, there should not ever be two traps on a single drain line.

The point of the trap is to create a water barrier which prevents sewer gasses from coming up the empty pipe and into the house.

When you have two traps on the same line, you end up creating a kind of vapor block in the line which prevents it from draining properly.

What do you do?

Get yourself some 1 1/2" PVC (not CPVC, which is yellow, PVC which is white) with a 90 deg elbow, and also get two Fernco couplings (black plastic couplings with straps on each end that can be tightened) that will fit the pvc and iron drain pipe.

Use a metal blade on a sawz-all and cut out the p-trap in your basement. (Have a bucket handy!) Leave room to connect the Fernco coupling on both sides. I'd cut the right hand side out just past the coupling.

Measure and cut the PVC so that with the 90 elbow the ends line up with the newly cut drain lines. Glue the PVC together and confirm the fit, then connect to the old drains with the Fernco.

Viola - a proper drain line that will drain smoothly w/ no vapor lock.

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While you're at it, I'd consider just replacing the remaining metal pipe leading up to your PVC s-trap with new PVC. Then you only need one fernco coupling, but a bit more PVC pipe and glue. –  Shimon Rura Nov 13 '12 at 14:11
    
Entirely possible - no time like now to replace. –  The Evil Greebo Nov 13 '12 at 14:18
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And while you're at it, replace the S-trap with a proper P-trap and vent. If a proper vent is too difficult, install a mechanical vent as high as possible under the sink. And then while you're at it... :) just kidding, the P-trap and vent plus the above repairs will do it for you. You shouldn't have any more sink drainage problems. –  bcworkz Nov 13 '12 at 20:07
    
I'm going to patch the hole in the cast iron trap short-term just so we can (hopefully) wash dishes, then replace the cast iron trap entirely this weekend. Thank you for the advice. –  Jeremy W. Sherman Nov 13 '12 at 20:42
    
@bcworkz I'll leave the S-trap alone for now, unless it turns out to be a problem. I think there's a cast iron vent less than 6 feet to the right. A pipe goes up into the wall and then to I-know-not-where. I'll have to get way back and look up at the roof to see if there's a vent pipe coming out up there. A kitchen remodel is coming in the next 6 months or so, so I'll poke at it more then. –  Jeremy W. Sherman Nov 13 '12 at 20:45
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