Because there is a badly smell from the pipe and I have just learnt that it is probably due to lacking of trap. Here is my kitchen pipe. The waste output is shared with the dish washing sink.

The washing machine pipe is putting at the top of the vertical pipe.

Here is the picture of the current status: current status

And then I plan to do this, I have bought a p-trap like this one without extra valve and going to implement. It is because there is not enough space at the bottom of the vertical pipe to fit the p-trap. So I need to extend it by adding a 90 degree bend corner to redirect the flow to the output. And then suddenly, I find the shape looking like a s-trap. So I would like to ask experienced people that will it cause any problem or unexpected result/drawbacks?

plan to implement

20210725 Edit (credit to @ThreePhaseEel for his suggestion)

This is another arrangement idea by levelling the P-trap output to the waste output. enter image description here

20210726 Edit Finalise

Based on the consideration of principal, practical size and limitation of the space. Here is the final version implemented. finalised layout

Based on the following reference from pin-interest: I am not sure if I should paste their image or link because link could change but image attachment might not. If I violate anything, please don't hesitate to tell me and delete them.

distance calculation principal reference 1

S-trap and P-trap design reference 2

Because my mansion design is not a direct straight to go down flow, it bend to the right and then right again to the wall outside and then bend again to the vertical pipe. So I think a typical vertical assumption does not fit the situation. I am not good in physics and I don't know how test. And the sink water flow will most likely not that fast to create a siphon to the p-trap water(hopefully).

Our local renovation worker are really workers only, they seldom explain nor resolving problem in right way. They did this setup to share the sink waste water with the washing machine, actually, it shouldn't. All they do is based on time and cost efficiency. I found that there are 2 outputs during the modification. I will keep an eye to see if there is any problem or unexpected effect.(most likely siphon to the washing machine p-trap or bad smell) And I am not sure if there is AAV available for purchase in local store.

After all, I go back to the original purpose, no smell from the sewage output. I cleaned everything including the sink drain.

20210728 Edit

As soon as I saw @Nelson's friendly reminder, I have corrected the direction immediately. (I did not take the updated photo)

  • 3
    Is rearranging things so the crosspiece is level with the P-trap discharge an option? Commented Jul 24, 2021 at 16:11
  • @ThreePhaseEel Thanks for your suggestion, I have added another layout. Did you mean something like that? It looks more natural and sensible than adding double bend corners. (feeling safe :)) Commented Jul 25, 2021 at 3:14
  • You cannot use the laundry as a vent for the sink
    – P2000
    Commented Jul 26, 2021 at 17:31
  • @P2000 I think there is a problem of the setup by the renovation work 10 years ago. I will keep track on the problem and may change the design later when have time. The first thing is to keeping smell from the waste output back to protect my family member health first. Thanks for dropping. Commented Jul 27, 2021 at 2:34
  • Frame challenge. Are you SURE the smell is from the pipes? Washing machines have a filter that needs to be cleaned regularly (at least monthly, depending on debris). The residue can clog the hoses and create still-water to rot in the machine. This happened to mine. Took hours to take everything apart and found significant sludge build-up in internal hoses, blocking about 1/3. The filter was not cleaned for years.
    – Nelson
    Commented Jul 28, 2021 at 4:09

2 Answers 2


This trap has no vent, and so it is an S trap. It doesn't have to be a problem, but it could be.

Currently if a high volume of water is quickly drained, the traps can be siphoned dry because there is no air admittance before the vertical, other than the trap itself.

Add a AAV (air admittance valve, a.k.a cheater valve or studor vent, at the top of the vertical, near the left shutoff valve. Extent the vertical as high as you can, almost touching the counter top, and higher than the bottom of the sink.

enter image description here

  • Thanks for your time. It is right answer for the primary question. (installing in Layout 1 will convert to s-trap ) So I have chosen the layout as suggested by @ThreePhaseEel. But there is still other problem. Too bad that our local renovation worker is so messy, DIY is generally not an option, so it is quite difficult for me to gather information other than online, local people knowledge is so limited, they only eye for "working" and receiving the money rather than long-term and working in right way. Commented Jul 27, 2021 at 2:38

Given that you provided updated picture, there is a serious problem with your final result:

Your sanitary tee is upside down:

enter image description here

Given the high volume of the washing machine waste water, the water will not flow very well, possibly filling up and creating a vacuum.

At minimum, that upside-down tee will wear much faster than normal due to the upward lip all the water needs to flow through.

I'm not familiar enough with code to give advice on whether this will cause problems for inspection.

  • As soon as I see your message, I have corrected the direction immediately. Thanks for your kind reminder. :) Commented Jul 28, 2021 at 16:40
  • I guess it's a good thing they're not glued in... but... Hmm, do they have to be glued in? I'm not licensed for plumbing so I don't know if not gluing them is code violation :D
    – Nelson
    Commented Jul 28, 2021 at 16:41
  • If the connectors connecting to the washing machine outward, they need to be glued for both vertical and horizontal side. Since there is a screw for the p-trap output to the trap arm, it is not necessary to glue that. And the pressure is from top to down with the gravity also, so the vertical parts on the right side is also opened(not glued) by the workers before. So I follow the original idea. But the bend down part of the sink to waste output is glued already, I guess it is also the consideration of the water pressure from top to down is high and will make a leaking risk. Thanks again. Commented Jul 28, 2021 at 16:46

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