: I want to install a toilet and sink located next to a
: stack.I would like to break the concrete and put in
: a y fitting into the drain.Does this need to be vented? I am 2' from the stack.This stack serves
: a upstairs full bathroom with tub and shower,and also
: a main floor powder room with sink and toilet.

: thanks

You cannot use the drainage stack from an upper floor as a vent for a lower floor (wet venting between floors). The vent will either need to run through the roof by itself or tie in with the original vent stack 6" above the highest fixture that the vent serves.
And as hj said, you need to use the right fittings for what you are doing. For instance, if there were no plumbing above and you could use this as a vent for your toilet, you would need to use a sanitary tee, instead of a wye...
Deb
The Pipewench