I think my plan is tragically flawed... I cannot figure out how to get the venting for the first floor bath tied back into the stack on the second floor (bath). I think this may have to do with the fact that the walls do NOT line up between 1st and 2nd floors...and the 1st flr bath is "active". This an "overlay" of the basic layout of the baths (dimensions are not included). The "pink" walls represent the first floor; the "transparent" fixtures represent the 2nd flr; the "red dot" is the stack's location: The "plan" was to build a stack that looked something like this: The "problem" is the venting on the left side of the stack (labeled) "2" to sink/shower on 1st flr". It is my belief that this vent line will collide with the waste pipe from the 2nd flr toilet...which is almost "in-line" with the stack. I know I will have to enlarge the width of the stack cavity on the second floor to accomadate those vents down to first floor and I do not have an issue with that. I also know the area about the toilet should be 30" wide (15" each way, off center line, correct?) Is there any limit to the number of bends allowed on a vent? I am thinking this is my only option....get the venting for the first floor shower and sink to the joist cavity behind/under the 2nd toilet, and then get it into stack cavity between toilet and sink on 2nd flr. I may be able to increase the useable area here by dropping the sink waste line to a lower dimension and changing it's direction to the stack (it's at 22" OC AFF as per manufac. specs) - maybe under the floor ? The only other option I can see, is to turn the 4" double fixture fitting 90Â° or 45Â°, and then run the toilet waste line over to it and the shower line also. The closet bend will connect to the stack with a 45Â° or 22.5Â°- I hope. The shower waste line may end up being a "straight shot". The concern is the floor joist which were pretty well butchered by the previous home owner's efforts. The joist are not 6x's(5 1/2") or 8x's (7 1/2")...they are 7x's ???. I bought a few 8x's that I will rip off to 7" to beef this floor up. Is anyone has any suggestions on how to get this project back on track, it would be greatly appreciated. I do NOT want to "just get it done" - and do it wrong - just to get it done and move forward with the renovation.