I would like to add a diverter valve to my washer's drain line to facilitate a graywater system. I'm thinking of shortening the washer drain so I can relocate the P-trap higher up and add the diverter valve beneath it on the main stack. If I were to do this, what's the shortest that I could make the washer drain? What does the code say, and what does sanity say? If necessary, I can just move the washer box higher up; it's a little low as it is. Here's a picture to show my thoughts:

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The dryer ducting is all going to be removed, and the 220v outlet is going to be relocated, so ignore those. And needless to say, that cast iron is going to be replaced with ABS.


Practically speaking, what you want to do is fine. The minimum horizontal slope is 1/4 inch per foot and the max is 3 inches per foot unless vertical. the inlet of the washer drain must be above the height of the washer. This is usually 36 to 38 inches minimum for floor seated washers. Obviously, if your washer is set on a platform, you will need to adjust the height of the drain inlet to be above the washer. I like to see at least a foot to 18 inches higher than the washer so that the drain can keep up with the washer pump and not overflow. I caution you however, if the diverter valve slows the drain flow at the current drain diameter, you may need to increase the pipe/valve diameter size to avoid back-up and overflow.

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  • It already overflows right now. The inlet is 35 inches above the ground and can't keep up with the washer's pump, explaining the water damage you see in the bottom of the picture. I can't tell yet whether this is due to a blockage or simply too small of a pipe too low off the ground. – iLikeDirt Feb 15 '14 at 14:25
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    looking at you pic, best I can, I bet that old cast trap has a ton of rust and possible blockage restricting flow. Personally, i'd cut everything out to the main drain, clean the main real well and replace everything with the same size PVC as the main is. Usually we see a washer drain into the main 4 inch stack, but not sure of your situation below the floor. – shirlock homes Feb 15 '14 at 15:38
  • The main drain is cast iron too, and it's also a 2-incher, at least above the slab. It's not the only drain though; other drains all meet up at some invisible point below the slab and head toward the septic tank. I was planning to use ABS instead of PVC. Any reason not to? – iLikeDirt Feb 15 '14 at 16:11
  • Some area codes require one not the other. For what reason is a mystery to me. Abs is a bit easier to work with, doesn't need primer before glue, cuts a little easier. they both work the same and last the same amount of time. No real advantage, one over the other. – shirlock homes Feb 15 '14 at 17:04
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    To prevent water from leaking out of the drain due to the water seeking its level in the drain pipe. If the pipe outlet is below the level it would seek (i.e. the level of water in the tub), the drain will never stop gurgling out. Basically, the washer-and-drain act like a water level: en.wikipedia.org/wiki/Water_level – iLikeDirt Feb 24 '14 at 15:04

International residential code says that a standpipe must be no less than 18" above the trap weir.

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International Residential Code 2012

Chapter 27 - Plumbing Fixtures

Section P2706 Waste Receptors

P2706.2 Standpipes.
Standpipes shall extend not less than of 18 inches (457 mm) but not greater than 42 inches (1067 mm) above the trap weir. Access shall be provided to standpipe traps and drains for rodding.

Check local codes, to determine what's acceptable in your area.

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You probably ought to ask your local building code enforcement.

The numbers I find are wildly variable state-by-state. Wisconsin seems to have a minimum drop of 12 inches and Michigan is 18 inches. Most states seems to mention the drain opening must be at least 34 (or 36) inches off the finished floor without specifying any minimum drop.

If you can't get a straight answer from the inspectors, I would play it safe and move the box up 36+ inches off the floor so that it is a) out of the way of those pipes, b) replaced by a nice, non-crufty connection box, and c) handier to reach when a washer is in position.

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