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Discussion Starter #1 (Edited)
The previous owners of my house dropped the ceiling in my kitchen to accommodate plumbing for an upstairs bathroom. I'm in the process of remodeling my kitchen and am planning on running the waste and supply lines through the joists in order to restore the ceiling to its original height.

I'm trying to make sense of the way the waste lines are currently set up -- see the attached pic. The line from the sink runs diagonally for 5 ft before joining up with the main waste arm. Why do this, rather than run a much shorter arm perpendicular and into the waste arm (see blue line)?

For context, the picture is taken from below looking up at the kitchen ceiling.

Thanks
 

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Could be because the entire run is vented by one vent and the rest is wet vented, therefore the toilet has to be the last to drain to the line before the stack.

You might want to re-think drilling the joists with the drains running like that. Any mistakes and you could affect their integrity.
 

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Could be because the entire run is vented by one vent and the rest is wet vented, therefore the toilet has to be the last to drain to the line before the stack.

You might want to re-think drilling the joists with the drains running like that. Any mistakes and you could affect their integrity.

I second ALL of this. What size are the joists? I'm assuming they are dimensional lumber?

There are only certain places you can drill dimensional lumber. There is a reason they ran it below the joists. Your other option is to completely replumb the underside of your house as well and get it to where your drains run parallel with the joists. How low is the ceiling?
 

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Discussion Starter #4
The joists are dimensional lumber, 2x10 (9.25"). My reading tells me that I can put holes no larger than 25% of the depth through the centerline of the joist. That gives me 2.3" max diameter holes I can put through the joists, which ought to accommodate 2" waste line for bath and sink. The toilet line is 3" but will only need to run through two joists, one of which is sistered, as will the other be.

As for venting, so far as I can tell the main waste line shown in the pic isn't vented itself, but rather goes straight into the house's main vertical sewer line. The main sewer line is vented. This seems contrary to my understanding of when/where venting is needed, but we've never had any issues with the plumbing setup as is.
 

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The joists are dimensional lumber, 2x10 (9.25"). My reading tells me that I can put holes no larger than 25% of the depth through the centerline of the joist. That gives me 2.3" max diameter holes I can put through the joists, which ought to accommodate 2" waste line for bath and sink. The toilet line is 3" but will only need to run through two joists, one of which is sistered, as will the other be.

As for venting, so far as I can tell the main waste line shown in the pic isn't vented itself, but rather goes straight into the house's main vertical sewer line. The main sewer line is vented. This seems contrary to my understanding of when/where venting is needed, but we've never had any issues with the plumbing setup as is.
You usually will need a hole about 2-9/16 for 2" which is greater than 2.3" .

There's also the issue of maintaining grade if you have to drill on the centerline of the joist. :eek:
 
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