Plumbing – Can p-trap be installed inside wall


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

Best Answer

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.