Trying to determine if my proposed waste pipe layout is acceptable. The stack is 4", vents through ceiling. There is only one bathroom in the place. Drawing attached.

Hi EWK:

We'll need more info. Here... OK? First, where are the vents in your drawing? I see the stack is indicated, but what about the vent(s)? Also, see if you can find out what plumbing code prevails in your area... see if you can find out if WET VENTING is allowed. If wet venting is allowed it will save you some time and some money!

Finally, draw this up so you show us the vent(s) and also the direction that the joists run. The size of the joists would be helpful as well.

Back to you...

Mark

Thanks Mark. I believe wet venting is allowable given the current configuration / lack of vents. The stack continues in both directions (i.e. north to the sky, south to the sewer) and serves as the vent with its roof penetration.

Hi EWK...

You'll still need to vent the sink. The 2" sink drain and vent can act as a wet vent for the toilet. The wye fitting for that should roll above the centerline of the toilet drain pipe to be a valid vent/wet vent. It also appears that you will need to add an 1.5" vent for the shower... depends on the joist arrangement.

Let me know what direction the joists run...

I'll be back around 6:00 PM eastern time.

Mark

Your drawing wouldn't pass in my area. However, if you vent the lavatory through the roof or revent back in the attic and then connect the tub/shower back to the lavatory drain where it will be wet vented, (see image) I think you'll be just fine. Good luck, Tom

Joists run from left to right in the original picture. Kind of a moot point as there is a drop ceiling below with ample space to run everything (with correct pitches). So for my layout to work I need to T off the sink and run one end into the 3" and the other end to above everything in the stack (which then goes out the roof)? I know nobody wants to hear this but what if I do not add the additional vent line from above sink to above fixtures? There isn't one now, am just rerouting due to a new layout...

but what if I do not add the additional vent line from above sink to above fixtures
If you do it your way you'll have a inspectors worst nightmare. To stay in code you can not vent through that stack. However, you can revent the new group back in the attic and exit the roof with the original roof vent.
Good luck in whatever you decide, Regards, Tom