I'm working on replumbing my utility room and I'm adding a graywater system. Local codes are remarkably permissive but require an overflow to the sewer/septic system in case of graywater blockage or backup. I've come up with two designs and would like some input.

Design 1:

enter image description here

This design conserves fall for the graywater distribution plumbing (always a good thing) but I wonder if the overflow would be able to keep up with a really large flow. I'm pretty sure that putting the valve before the trap won't be a problem since the graywater distribution plumbing won't have any sewer gasses, but please correct me if this is wrong. My idea was to bypass the trap entirely to prevent the sewer gasses from using the overflow plumbing as a path back in the house.

Design 2: enter image description here

This design squanders some fall for the graywater distribution plumbing but looks like it offers a more robust overflow control solution.

Which one of these seems more sensible, and/or what about either of them might need changing?


2 Answers 2


1 is better (so long as that valve has essentially no restriction). Otherwise you might need some height above the valve to keep the flow happy.

In version 2, turn the tee on the greywater line around so it goes where the overflow would go, and put in a trap (currently it allows sewer gas to vent into the greywater plumbing.) In fact, if the valve does what I expect, it forces septic venting into the greywater plumbing. So, not good.

  • Oh duh, I didn't even think of that.
    – iLikeDirt
    Commented Mar 11, 2014 at 0:07
  • In #1, is there any concern with the water in the trap potentially standing there stagnant for long periods of time (i.e. between the times when the valve is switched to septic, like for washing dirty diapers or something)?
    – iLikeDirt
    Commented Mar 11, 2014 at 0:09
  • If it is not used long enough to dry out, there would be a problem. I'd plan on shooting it some water every month or so, or turn the tee feeding it around, too (you'll lose some water from the greywater, but not all of it, and that loss will keep the trap wet.)
    – Ecnerwal
    Commented Mar 11, 2014 at 0:38
  • You mean the overflow tee from the graywater pipe, just below the 90 degree elbow?
    – iLikeDirt
    Commented Mar 11, 2014 at 1:55
  • Yes, that's the one.
    – Ecnerwal
    Commented Mar 11, 2014 at 14:04

Design 1 has a problem in that the sewer trap can dry out and leak gases.

Design 2 has a problem in that sewer gases enter the graywater system, and might preferentially leak out there rather than via the trap riser.

Have you tried calling your building department and asking them how people typically meet the requirement?

  • It was just over-the-phone descriptions, but they OK'd both. Thinking about it logically, there's actually no way you could avoid either the trap drying out (graywater diverted before trap, starving it of replacement water) or sewer gasses possibly entering the graywater system (overflow presents a clear path for gasses to enter the graywater system). The only way I can think of to prevent this is to put a trap on the overflow, but that too could dry out if overflow was infrequent (as it should be!).
    – iLikeDirt
    Commented Mar 12, 2014 at 0:23
  • In theory you could find a way to dribble water into a trap... This is a odd one: normally building code mandates that designs be maintenance free and idiot-proof, yet here you'd need to fill the trap every so often. Maybe they have a better plan back at the office.
    – Bryce
    Commented Mar 12, 2014 at 3:54

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