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I have a washing machine that I am moving 2.5m from it's original spot.

A standard p-trap is already in the original place before connecting to the drain, i am tapping into this stand pipe for the extension.

Should I add another trap to the other end of the extension next to where I am moving the washer to, or is the original enough?

  • I would think a double trap may not drain as well make sure you have a slope so water wont stand in the 2.5m section. – Ed Beal Feb 1 '16 at 20:01
  • The farther you "wet" vent, basically the length of drain before a vent, code requires upsizing the pipe. Moving a foot or so should not affect things much. But making things longer by upwards of 7-8 feet could start to be a problem if you do not upsize the pipe or vent it using an autovent/island vent or something; especially for the high flow of a washing machine when it drains. – Damon Feb 1 '16 at 21:52
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Double trapping is both a bad idea and a code violation in most jurisdictions - from the International Plumbing Code (my emphasis):

1002.1 Fixture traps. Each plumbing fixture shall be separately trapped by a liquid-seal trap, except as otherwise permitted by this code. The vertical distance from the fixture outlet to the trap weir shall not exceed 24 inches (610 mm), and the horizontal distance shall not exceed 30 inches (610 mm) measured from the centerline of the fixture outlet to the centerline of the inlet of the trap. The height of a clothes washer standpipe above a trap shall conform to Section 802.4. A fixture shall not be double trapped.

The length of pipe between the two traps will not be open to atmosphere on either side - normally, the vent side is open to atmosphere through the vent, and the fixture side is open to atmosphere through the drain. What will happen when you pour water into it is that it will drain fairly slowly because after moving through the first trap it is compressing the trapped air between the traps.

On the other end, in order to go through the second trap, it would need to pull air through the other trap to avoid having to create vacuum. This prevents the pipe between the two fixtures from completely draining.

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Not only is a single trap enough, using two would be inadvisable. It adds resistance to flow that can cause issues.

Depending on your local codes, the trap may need to be moved closer to the washing machine.

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Since the trap's purpose it to stop sewer gas from backing up into the house then one is all you need per fixture.

From the International Residential Code

P2706.2 Standpipes Standpipes shall extend not less than 18 inches but not greater than 42 inches above the trap weir. Access shall be provided to standpipe traps and drains for rodding.

There seems to be a vertical limit on the standpipe but I can't find a horizontal limit. However, 2 ½" pipe and smaller is supposed to slope ¼" per foot so that may limit you in how far you can go.

  • There's a limit for how far a fixture can be from a trap, and how far it can be from a vent. Unfortunately, I don't remember those numbers off the top of my head. I'm sure I've included them in more than one answer, so I'll try to find them. – Tester101 Feb 1 '16 at 23:45
  • I also think there's a section that forbids double trapping a drain. – Tester101 Feb 1 '16 at 23:47
  • @Tester101 Yeah I looked for that. I remember it has a limit also like 5 feet for 1 1/2" but since this is a waste receptor I don't know if it has a horizontal limit. Apparently there is a difference between that and a fixture? Like draining a water softener or an A/C condensate. Comintern just covered the double trap. – ArchonOSX Feb 2 '16 at 0:57

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