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I recently had a contractor put in a new sink and disposal along with the piping. The disposal tailpipe is just barely above the drain pipe, however the p trap doesn't go straight out.

  1. Is there a minimum height requirement that the disposal tailpipe must be above the drain?
  2. Does the p trap need to be straight out to the drain or can there be an additional elbow like in the picture?
  3. Does the dishwasher drain (smaller plastic tube) need to be above the disposal, or does that get pumped in without the need for gravity?

Thank you for the help!

disposal piping configuration

  • Is the disposal tailpiece really above the drain pipe in the wall? Your pic makes it look totally below the wall-entry of the drain. – JPhi1618 Mar 19 at 16:10
  • Yes, the disposal drain is basically at the same level as the main pipe drain, maybe a millimeter or two above it. I can add another picture if necessary. – sizzick Mar 19 at 16:22
  • It is essentially a very deep Ptrap. I see a potential for water to sit in the disposal and stagnate as any un-discharged ground up food waste will never dry out. Could smell. – Alaska Man Mar 19 at 18:01
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I'll try to answer your questions as stated.

Minimum height difference

I'm not sure if there is a code for this, but it's good practice to have the bottom of the disposal drain higher than the top of the drain into the wall. So a height difference of at least one pipe-width.

But, as long as the bottom of your tailpiece is higher than the wall drain, the disposal shouldn't hold water. If the wall drain was higher, then the disposal would always have a pool of smelly water sitting in it. Your problem is that the height difference is so slight that the inside of a fitting or even a very small buildup of gunk will cause water to start pooling in the disposal. I would strive to have at least a 3/4" difference.

The depth of your new sink could be the issue. You can look for a disposal that has a shallower neck, or cut into the wall to see about lowering the drain opening there.

Deep P Trap

I worry about the depth of the trap because that is a lot of room for gunk to accumulate. The disposal acts like a water pump when it's on, so it should be able to blast water through that deep trap, but it's not on all the time. Oil, grease, and other yuck can slip down into the trap and congeal causing smells and clogs.

I don't see any reason the trap had to be that deep and I think that can be easily improved on.

Dishwasher drain

I'm not going to fully explain it all here, but that part is fine. The dishwasher drain should either have a "high loop" or an "air gap" (search terms to learn more) to help prevent siphoning of dirty water back on to your clean dishes in the event of a clog, but the height of the drain line entry doesn't matter because the dishwasher does have a pump for the drain. It's pool of water is near the floor, so it has to have a pump to get rid of the water - gravity is not an option.

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  • The trap had to be that deep because the last elbow going into the wall angles down at 45°. If it went right at 90° (so it was horizontal), then the next elbow dropped vertically, the trap could have been much more shallow. i.e. I agree with you - it didn't need to be that deep, it just needed more creative thinking and possibly one extra elbow. – FreeMan Mar 19 at 23:23
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With your clarification that the disposal drain is slightly above the drain everything is ok. The p trap snaking around is quite common and a bit of a necessity here. The dish washer pumps to the upper section providing an air space, So although it looks funny it is fine on all counts,

Could it be better Sure but I doubt you want the wall opened up and drain height and position adjusted as this won’t really help much and will cost quite a bit more.

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