Plumbing for half bath in detached garage questions

[JimLS]

Building a detached garage in Illinois near St. Louis. Outlet will run to an existing septic system. My understanding is that needs to be 4" even though the use is only a half bath and an AC condensate drain. My question is where should this transition to 3". It has been suggested to make the wet wall 2 x 6 to allow the fresh water lines pass the main stack which seems reasonable since a 3" PVC pipe has about 3.5" OD and would completely fill a 2x4 cavity. Should I bring the 4" above the floor? Or should I reduce the stack to 3" at the first turn up? As I am thinking the incoming line would come in just above or below the footing (about 36" deep) and turn up with a sweep 90. Then a T for the toilet connection. Then proceed up for the sinks (one in bath and one utility sink back to back). Then on up for elevated trap for suspended furnace/AC.

I am also a bit unclear on wet venting and what I am allowed to use for combination vent and drain line.

I plan to run my plan by the local inspector but figured I need to know a bit more first so I don't do anything too obviously wrong...

 

[Reach4]

Building a detached garage in Illinois near St. Louis. Outlet will run to an existing septic system. My understanding is that needs to be 4" even though the use is only a half bath and an AC condensate drain.

How did you reach that conclusion? I am not a plumber.

My question is where should this transition to 3".

If you run 3 inch to the toilet, consider a 3 in. x 4 in. closet bend, which comes up at 4 inch. That gives good closet flange options. If you run 4 inch to the toilet, come up with 4 inch for the closet flange. https://terrylove.com/forums/index.php?threads/4x3-closet-bend-vs-3-long-sweep.68679/




You might sketch a diagram of what you propose.

 

[Sponsor]

 

[JimLS]

How did you reach that conclusion? I am not a plumber.

If you run 3 inch to the toilet, consider a 3 in. x 4 in. closet bend, which comes up at 4 inch. That gives good closet flange options. If you run 4 inch to the toilet, come up with 4 inch for the closet flange. https://terrylove.com/forums/index.php?threads/4x3-closet-bend-vs-3-long-sweep.68679/




You might sketch a diagram of what you propose.

Good idea since the closet flange will go inside the pipe. think with concrete floor the pipe is usually extended above the floor and cut off flush after the floor pour. 3" would be pretty restrictive.

On the pipe size I think I read that in the Illinois code.

 

[Reach4]

Closet flanges can go inside or outside of the pipe. Avoid a 3 inch inside flange. You can start with an outside flange if you leave space in the concrete. Look up "Styrofoam Closet Flange Spacer". That lets you switch to an inside flange if needed due to some failure in the future.