Hi, All:

I added a new bathtub in the basement, the new bathtub 2 inch horizontal drain pipe (less than 5 ft long) is connected to the vertical portion of 3 inch toilet drain pipe using y fitting. The question is can the new bathtub drain pipe share the existing venting of toilet drain pipe ?

Thanks

even a rough single line pencil sketch would help us answer that question.
basically if the tub is tied into the toilet vent you should be OK.

Thanks for the reply.
So let me clearly rewrite what you said, it is not necessary to install additional vent for the tub drain pipe, since it is connected to the toilet drain pipe, and hence the toilet vent will be shared by tub and toilet drain pipes. is that right?

no.. the toilet drain is either 3 or 4 inch pipe. the toilet vent is a 2 inch pipe. you connect to the 2 inch not the larger.

The way you state it, the answer is probably that you DO need an independent vent for the tub, because that pipe below the toilet is a WASTE LINE, not a vent, and cannot be used as a vent.

That's the way I read it, they are just putting in a wye downstream of the toilet for the tub drain. That is not the correct way to do it, the trap needs to be vented before tying into the toilet drain.