Question:
Current shower drain (2" pipe) runs 5 feet, under slab, to vent stack (5" cast iron vent pipe) of toilet. Will need to move the drain 3ft to wall for new tub.
After moving the drain, the complete length of the pipe will still be around 5 feet.
We will have to reduce the drain line at the trap to 1 1/2 in order to connect the tub drain/overflow. Do I need to add another vent?
Or is the main stack sufficient?

Attached Chicken Scratch sketch of system

Hey Brain,

Ya think your drawing's bad? Ya otta see some of mine. Since you failed to inform me what material your pipes will be I'll assume either cast Iron or PVC.
If cast iron you may have to change traps to a threaded trap that will accept a 2 X 1 1/2" Midland Bushing so you can run up a 1 1/2" copper stub up to solder on to the tubs brass tailpiece. If PVC then all you'll need is a 2 X 1 1/2" compression fitting on the trap raiser.
If the shower vented OK then there's no difference between the old configuration and the new. Good luck, tom

Hey thanks Tom. Existing 2" pipe under the slab is cast iron (or galv, sink drain is galv). I am sure the plumber is going to connect into it with pvc (new 3 foot section + trap) my concern is about the vent. First 3 plumbers that gave estimates and said I needed a new vent 36" from trap. Then the last said, if it drains fine now it should be OK. So I decided to do some research, by a book (Black and Decker complete guide) and try and figure what I really needed. Appears that the first couple of guys were going on the premise that I had 1 1/2 drain pipe under the slab. Not true. So, now, still a little uncomfortable, but starting to realize that my 5 foot stretch of 2" pipe should be sufficient.

Would adding the 1 1/2" drain/trap to the 2" line cause any pressure problems due to the 5foot run to the vent?

"Would adding the 1 1/2" drain/trap to the 2" line cause any pressure problems due to the 5foot run to the vent?"{

Not a bit since the tub waste and overflow are 1 1/2". This will not affect the drain line. Regards, Tom

Thanks Tom! Cheers