"what does the expert think?"
The expert thinks he would like to have a word or two with the incompetent jackleg that mangled your basement rough in.
Now that we know that you were left with two traps and no way to vent the group let's see if we can get you out of the mess you were left.
Your idea of platforming the shower and arming out to the trap is about the only way to make it work. However you will still have to vent the shower. This MUST be done after the trap not before it as you suggest. I can't see any way around this except to go under the pour and cut a vent in downstream of the trap. The lavatory could be armed out from the stack in a half wall and the trap under the floor could be capped and covered over.
BUT, if you''re going to have to take up the floor to vent the shower why not take up the area and redo it the right way. Give me some measures. Would you have enough room to place the lavatory and the shower on the same side instead of having the toilet in the middle?
I don't see any way to use your present setup without taking up the cement.
Had the plumber not put two traps under the floor we could have worked out a wet vent situation but that went down the crapper when he installed the second trap. Sorry I couldn't be more upbeat. Tom

Thanks for the bad news... I don't think I could fit the lav and shower on the same side.here is a another crazy idea, I saw one of those saniflo grey water pumps for pretty affordable. Could I use one of those for the shower and pump it up into the main stack? Then run the lav straight over to the stack along the wall(use a p trap and vent it, run it horizontally over to the stack.cap off the floor drain)? That would just leave me the toilet, anyway to vent that with that configuration? I saw the saniflo system for toilet and everything but it is very expensive. Possible?

'i saw one of those saniflo grey water pumps for pretty affordable. Could I use one of those for the shower and pump it up into the main stack?"
The shower would have to be on a platform and still be vented. With a AAV??

"run the lav straight over to the stack along the wall(use a p trap and vent it, run it horizontally over to the stack."
You may arm out to the stack from the lavatory stack but you can't tie the vent back to the stack. I'm thinking another AAV on the lavatory arm and cap off the smaller pipes in the pour. If I read your layout correctly the toilet will be wet vented through the main stack. This isn't a good plumbing lay out but it's about the best you're going to do. Cheers, Tom

Thanks for the ideas. One last idea I want to throw at you and let me know if this would be better than the last scenario. I've attached a drawing but basically platforming the toilet and shower, running an arm that is vented from the lav through to both the toilet and the shower and connect to the main stack just above the concrete. Would that be better than what you just suggested? That way I wouldn't need the pump.

Thanks for all the advice, I really appreciate it!

That would work. However the right hand vent will have to instaill downstream from the shower trap. You have it installed up stream. You can not discharge a major fixture past a unvented minor one. Remember? Where will the two vents tie back to? Regards, Tom

I had a change of heart, I'm going to just do it the right way and dig up the concrete. I am renting a concrete saw. They told me it wouldn't be difficult to cut through the slab.
So what do I need to do? I know I need to get rid of the trap where the lav will go, move the pipe so it will be back in the wall and I can vent it up. Is that the only thing I need to do? Or do I need to also dig up over where the shower will go so I can add a vent there?

Quote:

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Originally Posted by CaseyB

i had a change of heart, i'm gonna just do it the right way and dig up the concrete. I am renting a concrete saw. They told me it wouldnt be difficult to cut through the slab.
So what do I need to do? I know I need to get rid of the trap where the lav will go, move the pipe so it will be back in the wall and I can vent it up. Is that the only thing I need to do? Or do I need to also dig up over where the shower will go so I can add a vent there?

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If you are going to go the trouble of renting a concrete saw, then go all out and cut all of the concrete out and cut the PVC back and start all over from scratch.

As Tom pointed out, the combination lav drain/wet vent was placed in the wrong order from the outset.

In addition to having a finished product that meets your local code requirements, starting from scratch gives you the flexibility to place all of the fixtures exactly where you want them to be.

Just my 2 pence.

If there were some way to place the shower and lavatory on the same side then you could connect the shower drain line to the lav drain line and be wet vented. That way one vent could service the entire group. If you retain the present configuration then a separate vent will be needed for the shower. Roll a wye up on a 45 degree angle and back a vent into the wall to a elbo. The two vents can be tied back to each other but must revent back up to a dry vent. Unless a AAV is a option. Regards. Tom

Thanks for all the advice, I appreciate it greatly! I'll let you know the outcome