DWV questions in conjunction with basement remodel

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Dgeist

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Hi, all. I'm replacing and slightly relocating (with PVC) the exposed cast iron that serves the back half of my house to allow a drain and wall clearance in a new basement bathroom(third/fourth image). The vertical section makes a 90 bend below slab, going under the basement at a 45 and eventually out (first image). The top of the exposed stack makes a 90 as well and then wyes into separate 4" branches, each serving a bathroom group above. One "powder room" group is just above the space with the toilet on one side of a wye and the sink/vent on the other (second image - camera side). The other branch has a wye for clean-out then goes vertically with 4" line up through the powder room wet wall to a second floor bathroom (second image - wall side). Each of the current bathroom groups has its own dry vent above it's fixtures going to attic/daylight (not pictured). The whole stack currently serves 11 DFUs.

The new basement bathroom group is intended to wye into the system just downstream of the pictured slab entrance with a 3" buried horizontal draining a tub, toilet, and sink; in that order from nearest to farthest (third image).

I believe my area (GA, USA) uses IPC-2012.

Here's the questions:
- Per section 704.3, I'll have to come off the vertical by 40 inches to tap the new horizontal line from the basement group . I assume that's to avoid backflow from the stack going into the new horizontal line? Is that something that's typically enforced or a good question for the AHJ?

- I understand that since the wye and vertical serving the second floor bath appears to be 4" where it comes through the floor, then all pipe and fittings I replace downstream must also maintain that diameter?

- Is the pictured clean-out (second and fourth image) necessary simply because the line changes direction from vertical to horizontal? I'm sure I could snake straight down from the 2nd floor bathroom 10-12 feet above if need-be, but if it's a code thing, I'll need to provide access for it the new finished bathroom.

- If I combine all the new basement fixture vents in one common 2" line and run it vertically into the existing wet wall above (second image), can I tie that into the existing dry vent for that group as long as it's above the spill line on all the fixtures served by THAT vent (like up the wall and behind/above the sink)? (or can I not combine vents until they're above ALL fixtures in the home?)

2017-07-11 18.25.48.jpg

2017-07-11 18.28.32.jpg


2017-06-14 18.41.40.jpg

2017-06-14 19.41.45.jpg


BTW, that separate 3" vertical that has a sump pump plumbed to it actually serves the kitchen sink, disposal, and dishwasher AND the second floor lav sink. I think the HVAC was in before the plumbers were when the house was built. I'll likely replace it with 3" PVC and wye into the new 4" vertical near the top, routing the sump via separate output to daylight and leaving room for a proper stack clean-out at the bottom of the 4" section.

Also, the red markings on the pipe are not the current diameter, but rather what I plan on replacing it with.

Thanks.
Dan
 
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