Our contractor installed the Ptrap for our washer drain below the subfloor, basically in the crawl space.

We had everything nicely tucked inside the wall.

wall plumbing

Then the inspector asked him to move the Ptrap above the subfloor. All the pipes and vents would not fit so the contractor had to thicken the wall so there would be space for the Ptrap and the dryer vent.

plumbing and thicker wall

What would be the reason for this? Is this code?

  • 1
    In the UK we would not hide the trap in a wall, so that it can be cleaned out when (not if) it gets blocked.
    – Walker
    Oct 26, 2016 at 14:22
  • As an American I must admit that the UK tends to think things through and save customers time and money. Sadly here in the states, people tend to have a throw away replace it mentality. Oct 28, 2016 at 3:10
  • There is an opening to the right of the water valves. You'll see hat the black pipe runs vertically up to the white box. The drain line from the washer will be fed into it. If it gets clogged I think we can use that opening to clear the clog.
    – milesmeow
    Oct 30, 2016 at 0:40

1 Answer 1


All traps must be installed on the same floor as the fixture served with the exception of tubs and showers.

You can probably find that code bit in the DWV section, not necessarily under washing machines. In any case putting it below the floor would violate the maximum length of the stand pipe code which is 30"

The Need for a P-trap

If you create a waste pipe for any fixture that needs draining, including a washing machine, and the drain doesn't have a properly installed P-trap, you're creating a dangerous situation. The pool of water in the bottom of the trap effectively seals sewer gases inside the pipes, and if you omit the trap, the gases have a direct route into your house. Besides being laden with harmful organisms that you can inhale, some sewer gases, such as methane, are flammable. If the washer is in an enclosed area, you could actually create a fire hazard by omitting the trap or from improper installation.

  • 1
    Got it. So my contractor should have known this...
    – milesmeow
    Oct 26, 2016 at 6:30
  • I wouldn't say he/she did not know but I would like to think that as a plumbing contractor, code for P-traps should not be uncommon to say the least. I know as a general contractor, I knew most all the codes for handicap even when its not commonly used except in commercial building. Oct 26, 2016 at 6:36
  • @milesmeow Was the "contractor" a plumber? If you hire a contractor to do work in your home, and they are not having the work done by skilled trades people, you should expect the work to not be perfect. Skilled trades people know much more about their trade, than somebody who tries to be a jack of all trades. When you spend all day every day doing a thing, you learn a lot about how to do that thing. If you spend all day doing lots of things, you learn a little bit about a lot of things.
    – Tester101
    Oct 26, 2016 at 12:51
  • He hired two guys that do framing, electrical and plumbing.
    – milesmeow
    Oct 26, 2016 at 12:54
  • Yeah, any time you start dealing with Sub's, problems become more common. Anyone doing plumbing work should have known the p-trap has to be accessible.
    – ench
    Nov 1, 2016 at 20:04

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