My double kitchen sink drains into a 2” white plastic DWV pipe that traverses approximately 4-5’ of distance without any slope at all. I used a level to confirm my visual observation: there is indeed no slope to the pipe.

Here’s where it starts: start of horizontal drain pipe

And here’s where it traverses the next cabinet: continued run of horizontal drain pipe

The horizontal pipe seen in both photos has no slope at all. My question: does this look wrong / problematic / against code?

For context, here’s how this pipe attracted attention to itself. My sink backed up Friday night. I removed the P-trap and drained everything. Put it back together, sent some Green Gobbler and went to sleep.

Woke up Saturday morning, ran hot water and saw it back up.

Took apart the P-trap and drained everything again. Used a rubber coupling to attach my shop vac just downstream of where the trap used to be. Nothing came out, and the vac sucked air from the vent stack. Put it all together, plugged the disposal opening, ran water until it backed up and vacuumed through the sink drain. Vac took out all of the water and then sucked air through the vent stack again.

Snaked it with my 24’ x 1/4” auger. Put P-trap back in, ran the faucet for an hour: nothing backed up. Ran the dishwasher: it backed up into the sink. Drained & snaked again. Sent more Green Gobbler overnight.

Woke up Sunday morning, flushed with hot water and did not observe a back up. Currently contemplating whether to run the dishwasher.

  • Definitely not to code. I'd want to know how it got screwed up like this. Was the plumbing installed by the previous owner, vs a licensed plumber?
    – Hot Licks
    Oct 26, 2020 at 1:59

2 Answers 2


A drain pipe should have a 2% slope. This prevents water from standing in the pipes. If there is not enough flow through the pipe, you also have the risk of sediments building up in the tube and clogging it. I would guess that having no slope in the pipe can cause a problem with the dishwasher. I would recommend changing the slope of the tube.


The trap arm should have slope 1/4" per foot. No slope means, as you appear to know, opportunity for debris to build up inside the pipe and clog it. I'd re-configure this drain.

Is the non-sloping arm the reason for the dishwasher back-up? Maybe! I had a similar condition in a house I used to own; the force of the dishwasher pump was enough to cause all sorts of oddities which didn't happen when using the sink like usual. But eventually, as my drain got more clogged up by slow flowing debris, even the sink couldn't drain. My kitchen drain line was completely clogged with grease & food. Snaking it would only provide temporary relief; I was just compacting stuff that was stuck in the drain, not really augering it out or pushing it through into unclogged lines. A jetter might've been more helpful but what I ultimately did was get a pro to help reconfigure the drain (before, it was going down a length of rubber hose to a river; Eastern KY for you!)

  • 1
    How very Kentucky! LOL
    – FreeMan
    Oct 26, 2020 at 12:17

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