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As I need to redo the p-trap system under my kitchen sink and I am all but an expert in plumbing matters, I would like to understand which one of the two alternative solutions shown below is the preferred one.

Solution 1 is proposed in this YouTube video and shown in the first picture.Solution 1: Cemented ABS p-trap assembly

Past the initial slip joints that connect to the tailpieces of the dual sink, only ABS pipes and ABS fittings cemented together are used, resulting in a fixed, highly leakage-resistant assembly.

Solution 2 is proposed in this other YouTube video and shown in the second picture.Solution 2: PP and slip-joints p-trap assembly

Despite the plumber in the video keeps very confusingly calling it an ABS p-trap, this system has no ABS in it but it is fully made of polypropylene (PP) parts assembled together by slip joints.

This is my non-expert assessment of pros and cons of the two solutions.

Solution 1 has two advantages: a lower probability of leakage thanks to the cemented joints and a lower probability of blockage due to the larger inner diameter of the 1.5" ABS pipes compared to that of the 1.5" PP tubular.

Solution 2 has two advantages as well: an easier installation thanks to the larger number of degrees of freedom and a much easier maintenance, with many options to take it apart without damaging it.

On the other hand, maintenance of solution 1 is not that painful as it can be taken apart without damage as well, albeit less conveniently, by unscrewing the two slip joints up at the tailpieces and the nut at the tail end of the p-trap. In the worst case, if needed, just one cut at the pipe before the p-trap allows disassembling and the use of a slip joint or a shielded coupling when putting it back together makes it more maintenance friendly. However that nut at the end of the p-trap that makes solution 1 easier to maintain makes it also more prone to leakage as that screwed joint is always under water by design.

At the end, is it basically a wash and just matter of personal preference or is there a cogent reason that makes one solution definitely the superior choice?

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    You seem to be asking several different questions. Stack Exchange prefers to address one question at a time. Please narrow your focus and/or rephrase to give us one question we can answer. (And no, not all black plastic is ABS, nor is all ABS black.)
    – keshlam
    Feb 1, 2023 at 7:09
  • Thanks for your input. Your answers to possibly the only two questions on the subject that I did not ask make the the need to reformulate really clear to me. Unfortunately, I can't get to it immediately though.
    – MarcoD
    Feb 1, 2023 at 9:50
  • I doubt that "everyone" says anything, but yeah, this question is both broad and subjective, and the assertion that poly pipe is glued is faulty.
    – isherwood
    Feb 1, 2023 at 13:45
  • Your question title does ask the one I answered, actually. I will confess to a certain amount of TL;DR.
    – keshlam
    Feb 1, 2023 at 14:52
  • @keshlam If I thought ABS was the only black plastic, why would I ask if it was improper to call it so? On the other hand, learning of the existence of non-black ABS cheered me up. Not everything is a Hegelian night, in which all cows are black ;-)
    – MarcoD
    Feb 2, 2023 at 9:48

1 Answer 1

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The polypropylene trap fittings (white, or black) are mechanically joined not glued.

So you're conflating two entirely different systems. Even the sizes of the pipe are different between trap fittings and drain pipe.

Polypro trap fittings of either color join to PVC or ABS via a trap adapter mechanical joint. The PVC or ABS after that mechanical joint between the two types is glued. Since there are no polypropylene fittings suitable to connect all the way to the sewer, there will be a change to a suitable drain pipe at some point, typically where the trap enters the wall, and all the inside the wall and under the floor plumbing will be PVC or ABS (or cast iron if you like self-abuse or want a really quiet flush and are willing to abuse yourself for it.)

Your second picture appears to show a galvanized iron drain pipe at the trap adapter, which probably joins to cast iron plumbing, and no PVC or ABS at all. The brass nut is a clue. Likely that is an older house from when galvanized and cast iron were the standard drain pipe materials.

enter image description here

The only place I will use a glued trap is a 2" minimum diameter under a concrete floor. Anywhere else, a glued trap is stupid, since you have to take a saw to it and throw it away, rather than disassembling it, for any maintenance or changes to the sink. Under a concrete floor, you try to avoid ever needing maintenance, and when you do, you're chiseling up the floor anyway.

Plumbers may love the first version, as it ensures additional fees later on. Nobody else does. If somebody's diamond ring ends up in the trap, you have to disconnect that glued mass from both sinks to make any use of the union on the trap, and awkwardly try to remove it to access the trap dangling off the bottom of it. With slip joint trap fittings, you undo two nuts and pull just the trap bend off - 12 times easier. Good luck getting the first version back together, too.

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    A glance under most kitchen & bath sinks installed over the last 50+ years will reveal the PP slip joint connections. As a homeowner, the OP will greatly appreciate that!
    – FreeMan
    Feb 1, 2023 at 13:53
  • @Ecnerwal, thanks for your thoughful reply. First of all we are in full agreement on both counts: the PP fittings are mechanically joined and the two systems are totally different. I never said otherwise. I was actually wondering how come such different systems are both referred to as "ABS plumbing", especially since one is only made of PP. On the substance of the matter, I agree on the merits of installation #2. How is it code compliant though, considering the PP pipes are smaller than the required 1 1/2" for kitchen and PP is not code-listed among the allowed drainage pipe materials?
    – MarcoD
    Feb 2, 2023 at 2:42
  • My code reference is section 701.2 on page 165 at epubs.iapmo.org/2022/CPC
    – MarcoD
    Feb 2, 2023 at 2:44
  • They are 1.5 inch pipes. The wall thickness is what's different. And they are not drainage pipes. They are trap fittings, which connect to drainage pipes (the fixed things in your walls.)
    – Ecnerwal
    Feb 2, 2023 at 3:01
  • Try section 811.2
    – Ecnerwal
    Feb 2, 2023 at 3:11

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