I have a new vanity I am installing in my guest bathroom. The current drain height is the same height as a storage shelf below my sink and does not allow the vanity to sit flush against the wall.

Code says (i think) the weir at the exit of the p-trap must be continually downward sloping at 1/4" to 1/2" vertical height per 1' of horizontal run. So this means that I cannot adjust the drain height downstream of the p-trap. So my only other solution is to install a p-trap behind the wall and adjusting the drain height upstream to a new wall entrance height a couple inches higher.

Reasons why I think this might be a bad idea.

  • I've never seen it done before.
  • An inspector would have to knock down the wall to confirm there is a trap installed. (could be fixed by replacing new hole in wall with access panel)
  • A leak at the trap would be undetectable and could cause a lot of damage. (but does this have any more risk of leaking than all the other pipes behind my wall?)

So, has anyone heard of this being done before? Is it ok to install a p-trap behind a wall? Am I over looking another option that is a simpler solution? What would you do? Thanks in advance!!

I have attached a couple of pictures.

The back end of the new vanity showing the shelf. the back end of the new vanity showing the shelf.

My wall showing the drain line with the horizontal run my wall showing the drain line with the horizontal run

3 Answers 3


As far as the international plumbing code is concerned (relevant section is here), you're fine putting it pretty much anywhere that meets the distance requirements. In fact, the only mention of accessibility is in mental health facilities where traps are required to be inaccessible. This is really no different than pouring concrete over the trap in a floor drain - just make sure it gets noted during the rough-in inspection.

  • 3
    Great! Thanks! This is a guest bathroom used when my parents and in-laws visit. Therefore, it may be best to follow the mental health facility codes anyway.
    – Rob S.
    Jan 10, 2016 at 3:30
  • 1
    You also might want to note it where the next owners will find it (under vanity, bottom of sink, etc.). Otherwise they could easily think you just didn't have a trap, and the fixture could end up double trapped.
    – Tester101
    Jan 10, 2016 at 14:04
  • 2
    On the pipe to the wall I wrote "trap in wall" with an arrow pointing toward the wall.
    – Rob S.
    Jan 10, 2016 at 19:47

WARNING: I have not searched through plumbing codes, so this answer could be completely wrong.

I think it's recommend to have an accessible trap, but I don't think it's code required. Traps for tubs and showers are often not accessible, so I don't think there's a code section that specifically disallows hidden traps.

However, if you do put a trap in the wall, do not use a slip joint trap. You might also want to consider building an access panel into the vanity, so you can access the trap if need be.

  • Good call on not using a slip joint in the wall. I'm going to keep the question open a little longer to see if anyone knows more definitively what the right thing to do here is. Thanks!
    – Rob S.
    Jan 10, 2016 at 0:28
  • 3
    If it were me, I'd do my best to keep the trap accessible, even if that means installing an access panel in the back of the vanity.
    – Tester101
    Jan 10, 2016 at 1:14
  • 2
    Cleanouts absolutely have to be accessible. In fact, if they aren't, you have to extend them to where they are accessible. But a lot of traps are sealed up in walls or floors/ceilings after inspection, particularly for things like washing machines, tubs and toilets. Plumbing snakes will bend and go through the traps. If a trap is glued together (not a slip joint), it isn't as if you can pull the trap apart to look at it, anyway. You'd have to cut the pipes just like you would any other section of pipe in the wall. I figure you might as well put a nice finished wall over it in that case. Jan 10, 2016 at 5:21

I also questioned this possibility. Unless you want to go to a sink stopper on a chain, space will be required for the drain linkage. Otherwise there would only be the issues of inconvenient accessibility and late detection of leaking.

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