I'm meeting with my plumber to go over his plans for my drainage and venting system. It is all new plumbing. The building is two story, 3 bath. One downstairs bath, two directly above back to back. I also have a downstairs kitchen and laundry. I understand there are min. vent pipe sizes for each fixture. In a perfect world, I'd love to have one vent in the roof. I've seen diagrams where all the vents tie back into a central main vent. Is this still practiced?