Is sharing a single vent acceptable?

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?

As long as the total size of the vents is proper, you can probably tie them all into one...the more you add, the larger it must be, and could become impratical. It may not be possible in the space available, though. The vent must have slope so any condensation, rain, etc., won't accumulate, and there is a restriction on the horizontal component verses the vertical part, so depending on the distances, it may not be practical. But, it sounds like things are stacked, so yes, combining for one roof pentration would probably work as long as it is sized properly.

vents

There is no problem with it as long as the pipe sizes are proper. The practicality of it comes in when they are so far removed, one from the other, that the work and material for combining them becomes less economical than exiting them individually.