Finished the demo part of the bathroom remodel to find pretty significant damage to the floor joists caused by the original builder's "design" (see photos/video). Setting aside how I'm going to fix or at least mitigate the damage - what would have been the correct way to plumb in the toilet and sink drains without weakening the structure?

Given the small floor joists, I can't imagine the right way to try to drain across them this way. So what would a pro builder have done to install this bathroom correctly?

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More photos

More context: This is the 2nd floor of a 2-story, 1920s house. This bathroom and its stack are directly above the 1st-story bathroom. The stack is perfectly vertical from the basement to the attic, where it has a slight offset to exit the roof. The existing stack is cast iron and all existing drains are steel/iron.

The toilet drain pipe in the wall drains both upper and lower commodes before Y-ing into the main stack 5' above the basement floor.

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    It's difficult to apply modern techniques to an old solid wood floor system with potentially low ceilings. Those are rare beasts anymore. The short answer is that the plumbing should've been below the joists, not through them. Is that an option given your basement ceiling height? – isherwood Nov 18 '20 at 17:47
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    @isherwood, it's not over a basement in this case, since we're looking at the second floor. I'm not sure how you'd get plumbing under the joists in that case. – Nate S. Nov 18 '20 at 18:15
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    Given the house is 100 years old, the original plumbing is intact, and you're not complaining about the floor integrity, I'd say the plan of cutting away the joists wasn't a big deal. A modern construction technique would use either smaller pipe, or larger joists. – Steve Sether Nov 18 '20 at 18:38
  • I have some ceiling height to spare. Without doing a bit more demo, it may still be a challenge to tie into structure that should be carrying load. @SteveSether I agree it has managed well enough for several decades. Basically every interior wall is load-bearing; that's an issue for other reasons but probably helped in this case. – user480402 Nov 18 '20 at 21:04

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