Help with bathroom remodel plumbing

[Endo1030]

My wife and I have lived in this house for about 2 years, and when taking a shower in the master bath have noticed an objectionable smell coming from the drain on occasion.  Since we were planning to remodel the bath we figured we'd just fix the problem at that time.  Well, we have started the reno and I have now completed demo and have pulled up some of the subfloor to expose the plumbing. I now understand the source of the smell,  I need help figuring the best way to solve it.

https://drive.google.com/file/d/1IfAc-knGAPDCGKKk-GHsEVs7XvNA-kkw/view?usp=sharing
View of WC to drain stack connection - https://drive.google.com/file/d/1vgP5YQYvin_cAb4hdbDkTy9sqihoFJrm/view?usp=sharing
Vanity sink plumbing - https://drive.google.com/file/d/1v7DN2pKPdz-U8ihPmAXS5TfbqWenApnH/view?usp=sharing

I'm not a plumber, so sorry if I'm using incorrect terms here, but I hope I'll make myself clear.

Everything seems to be sloped away from the main drain stack!  the elbow where the yellow and grey lines come together is a low point.  I think you WILL believe what was sitting in the yellow pipe when I cut away the shower trap.  That was clearly making it's way into the shower trap, and causing the smell when the water was disturbed I guess.  It's now capped. 

The green line is what has me a little confused.  it's a 1 1/2" pipe that ties into the branch that is also serving the bathroom sink (the red line).  It turns down at the exterior wall and runs beside the kitchen sink, so I think it must be intended as the vent for that sink.  Can a vent from a sink on the first floor tie into a drain branch at the second floor?  because of the negative slope of the grey line, I think most of the bathroom was draining down this green line anyway.

As a solution I was thinking of removing the blue section entirely.  draining the bathroom sink down the green line with an elbow.   There is plenty of room to cut away the green line where it turns down to establish 1/4" slope .  The bathroom sink has it's own vent which can be seen in the third photo, and I guess this would become the kitchen sink vent as well under this plan.  I guess this isn't code, but what if i add an air admittance valve to the kitchen sink?

Then I could route the shower drain straight to that bay where the grey line is and raise that up so it has the correct slope.  Then tie that line into the vent marked with white. 

Anyone see a better solution?  I could replace everything and enlarge the holes that are already overblown in the joists, and I'd have to pull back a few more feet of subfloor to adjust the sink drain line.  I still wouldn't know what to do with that green line though.

All of these joists are cut all to hell, with inadequate plywood pieces scabbed on.  Not to mention the TWO adjacent wall studs that are cut completely through to accommodate drains for a double vanity which used to be there.  The new vanity is just one sink, I'll cut out that wacky pvc loop and replace with a san-T coming straight out of the wall and sister on new studs to shore up the ones that were cut.

[sonofsven]

As a carpenter I'll just say that your plumber was a savage.
Can't you just re do the low point and toilet connection?
A long sweep off the toilet and keep the shower drain line as high as possible until it meets that line coming off the toilet?
Remember, 1/4" per foot drain slope, shit goes downhill.
If I saw that for a client I'd definitely call a good plumber and most likely pull it and sister those joists.
Also, don't create a 'wet vent' on your kitchen drain.
Also also, you did have a p trap on the shower drain, correct?

[Endo1030]

Yea, they really were.

It will be a pretty big hassle to get the 3" horizontal pipe at the drain stack any lower.  I'd have to remove trim and cabinets in the kitchen below to access the vertical pipe and cut it shorter.  Without doing that the red line coming from the sink is already lower than that 3" pipe, but there might be enough room to rebuild the the whole sink branch too and bring it higher.  I'd have to measure that and check when I get home. 

If I was going to repair every joist I'd have to pull the subfloor up and cut out that section too anyway, so it might be a better option.  If I can get 4' sections of 2X8's in there to glue and screw to those joists? Do you think that's sufficient?

I will definitely do all of that if it's the only code compliant way, but if anyone sees an easier option I'm still all for it.  I know wet venting is allowed in some applications, and this is definitely not what I understand as the acceptable use, but maybe someone else knows different.

[Endo1030]

And yes, the shower drain did have a p trap.  Although it wasn't original, it was pvc and the original plumbing is All abs.  The sub floor was already cut up in that L shape when I pulled up the old shower pan.  I think there must have been a tub there at first, and was remodeled sometime in the 90's to a tile shower based on the style of tile that was in there

[lthenderson]

I'm not sure I can add much to this. If the green line is a vent line from your kitchen below, it definitely isn't code to be hooked up like it is. Like above, I would not convert it to a drain line if it was a vent and since it is meant to drain if it is actually a wet line, putting an elbow there is going to create a situation. I personally would probably get a licensed plumber to get in and check things out.

As far as the chopped up floor goes, it can be fixed several ways. The traditional way is to sister 3/4" plywood webbing the same depth as the joist on each side of the joist going back as far as you can but ideally at least a couple feet in each direction. You notch the plywood to fit around the pipe with the notch facing up on one side and the notch facing down on the other side and then nail both into the joist. I've also seen people use metal brackets made out of galvanized steel with dozens of holes for nails that come with a sized notch to fit next to your pipe. I don't know what these are called or if they are made for this application. The other method I have seen is to get a iron c-channel that gets lag bolted to the joist on either above or below the pipe notch to strengthen it from sagging. One last method if there is enough play on one side of the piping is to cut the pipe, add in a new joist with properly cut hole over the pipe and then put a splice back together the pipe.

[Endo1030]

Looks like I'll be cutting all this plumbing out and redoing it, so I can just sister on  full depth joists and drill new holes.  I'll disconnect the green line entirely and run it up into the attic to tie into the vent lines up there.  I noticed last night the wall plate is blown out directly above where it goes down to the first floor, as if it used to be a vent running straight up the wall to the attic.  It's also only 1 and 1/2" pipe, so I'd have had to change the sink drain to 1 and 1/2" anyway to use it.

Thanks for the help.

[lthenderson]

Personally, I don't think it looks all that bad. There already appears to be plywood webbing on some of the joist penetrations. I would just get some 3/4" plywood and make U-shaped notches to slide around the pipe on each side of the joist and call it good.