Can I run a sink drain between the joists in my basement?

I’m trying to visualize from your description, and here’s what I have so far:

Your kitchen sink drain routes into a concrete floor and from there somewhere through the concrete floor into the main drain at the same elevation as the floor. You would like to re-route that to run through the floor at the same point, but from there down to the collective drain for a bathroom on a lower elevation.

My questions are:
1. Is the downstairs bathroom above grade? Or does that plumbing route to a sump that has to be pumped back up to the main drain? Draining your kitchen sink into a sump (if so) will put a lot more load on that pump than it had faced before.

2. Where is the vent for the rerouted kitchen drain line? You’d need to have a vent somewhere between the sink drain and wherever it empties, because if you don’t have a vent then that kitchen sink drain could either (or both) suck the lower bathroom traps dry, venting sewer gases into the dwelling, or blow the sink and tub traps back into the sink and tub, which would also be messy and unsanitary.

3. Is there also a dishwasher attached to the kitchen sink drain?

4. Does anything else share the same drain line at the main / ground / kitchen elevation, such as a washing machine or other fixture? You can’t afford backups in that line that might rise above floor level, which wouldn’t have been a problem with a vented sink drain above. Now I’d wonder if that line can be permanently capped and abandoned where it is.

If any fixture shares this drain line, then I’d say that you need to abandon your plan or reroute all of the drain lines.

OK,original drain line from one end to the other: Kitchen sink (including dishwasher emptying into the disposal) to the undersink trap; trap to wall (with vent stack), ABS line runs down from kitchen, through floor, down to basement floor, where it joins an iron line under the concrete floor. Cleanout was about 18” or so above floor. I assume the line under the floor in the basement runs to the main waste line out to the city.

Other existing plumbing consists of two upstairs bathrooms, one with shower and one with tub, both with sink and toilet, as well as laundry (upstairs also). These systems all join a vented main line on the other side of the basement from the kitchen system.

There is also a floor drain in the basement; it has not backed up in the 12 years I’ve been in the house.

What I want to do is reroute the drain that (until recently) went under the basement floor with a system that will run in or just below the basement ceiling (still below the kitchen, but still above the entry point to the other line).

I would think this was doable, if you have enough length between to get a good drainage slope. My only concern would be the sizing of the existing pipe on the other side and whether it would be large enough to take care of the additional plumbing load. Make sure the pipe between the joists is well supported. Heck, you could even put a cleanout/drain plug at the bend in case it ever clogs.

I don’t know if it would meet code and that may be a problem you will need to rectify later if you ever sell but if it ever came to that I suppose you could bust out the slab around the broken fitting and redo it the way it was originally.

@rojo The original was 2” id, as is the replacement, and the line I’m using for replacement. Great minds must think alike, because I had the same ideas about the clean-out and the coding. A “guy I know” suggested hanging the pipe no less than every 24” for stability.

Yes. get ‘r done! Sounds like a solid plan.

Though I’m sure the new drain would work, given proper installation for slope and all connections, I still have reservations about the embedded line that’s going to be abandoned. That line still connects to a main drain on its lower end, so there are some things to consider with its continued presence:
1. Since it is connected to the main drain, any backup in that line (when the sewer connection at the street is blocked for maintenance or repair of the sewer line in the street, which my own town has done several times in the past few years) is going to find a lower outlet than it used to. That is, a backup that might have gone “up to the sink” (and for that reason, you may never even have noticed even if it did occur) will now be able to occur at floor level. Also, it can now carry sewer gases into the dwelling.

2. Since the elbow broke at the floor level, and the elbow is usually a thicker piece than any metal pipe it attaches to, it makes me question the condition of the embedded pipe in the concrete floor. See the objections in #1 above: if the sewer backs up into a broken pipe, your problems are magnified and spread who-knows-where, and if sewer gases are vented into a broken / corroded pipe, then that can also be widespread.

3. If the new drain cannot be a permanent code-approved drain, then you’re merely delaying the day (and adding a new drain connection), when you have to dig up the old fitting – or fitting + pipe – and replace it. And if that happens “after you’re gone” (and the executor of your estate has to take care of all this) you may not be remembered very fondly.

If the new drain meets the code requirements and can remain permanently, then I’d also suggest reworking the attachment of the existing drain to the main (house) drain, so that you have only an empty pipe, open at both ends, in the concrete floor, and it will never perform a function (wanted or unwanted) in the future.

@CWOTUS Thanks for all the thought you put into your answer. I’m going with the solution that’s most affordable and puts the kitchen back into commission soonest. We have the kitchen sink now draining into a 20-gallon trash can, which has to be emptied at least once a day, sometimes more, and we’re not using the dishwasher or disposal. Also, immediate finances are a consideration. This solution costs a lot less, especially if I can do it myself, than breaking up the basement slab.

Long term, however, I will eventually have a licensed plumber look at the system and make any changes needed code, including digging up the old line, if necessary.