I'm looking to install a full bath in my basement. I will need to "t" into the existing stack which is 4". My question is would a 3" line be large enough for the main waste line out of the basement bath. It will only be aprox. 9 feet to the stack.
I'm guessing you're running the drain for this bathroom under the concrete slab.
Chances are the existing main drain line is 4 inch in which case most codes require a 4 inch line tied into the main line usually a wye is used and not a 90 "T". under the floor.
Check with your local building department when you apply for the permit ---- they will tell you what's required for your local codes.
;)Yes, always use a "Y" instead of a "T". Be sure you put the "Y" WITH the flow of the waste in the drain line.