We have a new-build house with an integrated dishwasher that was already fitted when we moved in. It's about 4 months old, and it recently stopped draining.

The engineer found that the waste pipe was clogged with fat. I was surprised, since I don't put fat-coated dishes in the dishwasher; although I do, sometimes, empty small amounts of still-liquid fat down the sink (naughty, I know - but it's not much, honest!). So the engineer pointed out that the dishwasher waste is plumbed in below the level of the exit pipe from the sink trap, so it will be sitting in the trapped sink waste-water, and any fat layer could enter and clog the dishwasher waste pipe.

I've attached a picture of the pipe arrangement below. My question is - is this a sensible arrangement for the waste pipes (and I'll just have to be more careful with fats), or is it a mistake to have fitted them like this (so I can get the builders to fix it)? enter image description here

  • 1
    For what it's worth you have a fair bit of room to raise the sanitary tee by shortening its upper leg. You could lift the tee and the trap that way.
    – isherwood
    Commented Aug 13, 2020 at 20:11
  • Is that an unused dishwasher drain connection in the fitting above?
    – BMitch
    Commented Aug 13, 2020 at 20:21
  • @isherwood - possibly, but the point is I'm not a plumber and I don't want to do it myself. But my house is less than 6 months old. If this is a mistake, I can make the builders do it.
    – Chowlett
    Commented Aug 13, 2020 at 20:30
  • @BMitch - Again, not a plumber, but the connection above (and to the right) is unused, but looks much smaller than the existing connection? Unless the white elbow inside the ringed section is an adaptor that could be left off... but again, no clue, sorry.
    – Chowlett
    Commented Aug 13, 2020 at 20:33
  • 1
    Yes, I'll defer to others with more plumbing experience, but dishwasher fittings are typically smaller than other fittings on the drain lines and I can't think of anything else that green circled fitting would be used for. I suspect they planned to use it but ran out of space with the cabinet wall and solved that with the hacked solution you have now.
    – BMitch
    Commented Aug 13, 2020 at 21:12

1 Answer 1


I'm not sure anyone has answered your question yet. It's hard to tell from the picture but I'll defer judgement to the "engineer" on site that the drain line from the dishwasher is somewhat lower than the drain feed out to the sewer line. If that's the case this would cause some of the water (weir) in the trap to back up into the lower part of the dishwasher drain.
To put this in perspective - it's probably not going to cause you major issues. However, since it's under warranty - have them fix it. I'd recommend isherwood's solution above - have them shorten the leg above the dishwasher drain and the corresponding ell going out to the mainline.
The reason I recommend that is based on the picture. The vertical rise from the trap to the horizontal drain piece looks to be border-line long. To explain, here's an illustration of a proper setup: enter image description here
The height of the weir (area between the red lines) should not be more than 4 inches. Again it's hard to tell but the weir on your trap looks higher than 4 inches which would make it less effective in flushing out debris.
If the weir height is no more than 4 inches the installers have the option of moving the dishwasher drain up to the higher unused fitting going into the tailpipe on the right or raising the trap as indicated by thye blue lines in the picture. As Harper mentioned in his comment, those pipes may be pre-fab and not adjustable so the installer may have to install new pipes above the trap to get the dishwasher drain line above the outlet pipe on the left.

  • Thanks. I'm not quite clear which bits of pipe you're suggesting they shorten. I guess you're meaning they should bring the bottom of the U up, but I'm not seeing where the space is between the dishwasher joint and the long horizontal pipe to the left (which is, btw, another drain from the sink; it's a sink-and-a-half, with the half visible in the picture and the main sink off to the left).
    – Chowlett
    Commented Aug 14, 2020 at 11:30
  • (I've measured, and the weir height looks to be exactly 4 inches)
    – Chowlett
    Commented Aug 14, 2020 at 11:34
  • Not only that, it looks like the essential pipes are pre-fab pipes, sized specifically to give that weir height. Commented Aug 14, 2020 at 15:51
  • @Chowlett - I added my response as an edit in the answer above.
    – HoneyDo
    Commented Aug 14, 2020 at 17:17
  • Thanks, that clears things up. You probably get the tick in a little while.
    – Chowlett
    Commented Aug 14, 2020 at 20:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.