We are converting a peninsula into an island and need to move the kitchen sink. As a result the waste pipe in the concrete slab comes out in the front (see photos) instead of the back. We don't want to jackhammer the slab (I think HOA forbids it) but I wondered if we could come out of the slab with a 90 degree elbow, run under the cabinet floor so and come out in the back of the cabinet where it used to be, or even in the hollow wall we have built behind the cabinets. Also we need to replace the floor of the sink cabinet so we could even raise the floor a little higher.

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  • Hello, and welcome to Home Improvement. I'm no pro, but I think you're asking for guano to accumulate along that horizontal stretch of pipe. And, you should probably take our tour so you'll know the details of contributing here. Sep 24, 2020 at 13:54
  • Maybe you should talk to your HOA about approved ways to re-route in-slab drains. It sounds like you might be in a multi-floor building such as a condo complex? If so, it might be important to get your plan permitted, too. Sep 24, 2020 at 15:19

2 Answers 2


Horizontal - no.

No drain line is EVER horizontal. Unless someone was an idiot and should not have been plumbing....

Perfectly acceptable to run at an appropriate slope, however. 1/4" per foot (or 2%) is the standard minimum slope for small drain lines like this.

How will you handle the venting? That is a complex process for island sinks, and if it was a peninsula before probably not arranged for. You mention having a "wall behind" - can your vent run through that (is your island not an island in the usual sense?)

There are ways far less drastic than a jackhammer to get into concrete and rearrange plumbing.

  • Also, i'm no plumber but I'm sure 90 degree elbows are not allowed for waste. You need a sweep- a 90 degree elbow is just asking for a clog.
    – Jamie M
    Sep 24, 2020 at 14:02
  • Right - a sanitary 90 elbow, or sweep, not a sharp elbow is required.
    – Ecnerwal
    Sep 24, 2020 at 14:03
  • 88 degree elbow (but it looks like 90 degrees)
    – Jasen
    Sep 26, 2020 at 12:03

You picture leaves a lot of information out, but assuming that you're putting in a standard double-bowl sink, and that the hole in the floor of the cabinet is now located over the existing drain line:

I'd suggest that you simply run your two lines horizontally from the bowl drains from right to left, then turn up, put in your trap, then run to the drain in the floor. (Note: all directions oriented as we're looking at the picture you posted. Note 2: "horizontally" means with the appropriate 1/4" per foot of slope).

To hide the plumbing (assuming you don't want to see it right at the front of the cabinet when you open the door), make a removable wood insert out of furniture grade plywood. This will have to be removable so you can access the pipes for any future plumbing changes, leak fixes, etc.

It would look something like this top view:

 D W
 | W
 P W
 +-----B------B W

D = floor drain
P = P-trap
B = Bowl drain
W = plywood vanity panel

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