Does the vent stack for under slab toilet lines go before or after the toilet drain or does it just have to be within acertain distance?

Hi Dave:

I would love to know exactly why you are asking this... would help me to be more specific. Are you plumbing for an entire new bathroom or 1/2 bath.. If so, then venting can be simplified... if not and for entire house plumbing project... gets a bit more complicated.

If you are asking about the 3" minimum vent stack required by code that penentrates the roof then the toilet comes off a horizontal drain line with a wye fitting BEFORE the vent stack goes vertical. If talking vertical and just a sink with toilet then the toilet can come off the vertical stack with a sanitary tee fitting before stack continues vertical.

If you are asking about adding a bathroom then the 2" vent that comes off the toilet drain line itself then the vent comes off BEFORE the toilet and the 2" vent connects into the 3" vent in the attic just prior to it penetrating the roof.

Let me know more... MARK

I will be back in again around 6:00pm East coast time... ;) Check back then for any response I may be able to offer. Otherwise, wait to see if Tom or others chime in!