We are remodeling a bathroom in a house we just purchased, that was added to the second floor in 1999. The second floor bathroom is not original to the house. We thought the main stack went all the way to the roof. During demo we found that it does not. It is a 3 inch stack from the basement, that did, and will have a toilet, shower, and sink on it. My question is with the venting. The venting is a 2 inch black plastic pipe through the roof that is individually connected to each appliance at the trap, or in the case of the toilet, a 3 x 3 x 2 T under the flange. We have a flat rubber roof, so I would prefer to use the existing 2 inch vent already there. Can I do this as it was originally installed, or do I need to extend the 3 inch stack through the roof, and eliminate the 2 inch stack. As I said, because it is a flat rubber roof, I would prefer not to add a hole, and then need to add a patch after removing the 2 inch stack. This bathroom worked just fine the way it was set up before demo. Everything flowed very well, and there were no odors. Also, it is a small bathroom, so the horizontal runs will not be very long. The longest will be the 3 inch main stack to the toilet. 4 ft. or less.
Thank you in advance for any advice.