Long HORIZONTAL runs on the main stack?
I'm embarking on a remodel of three bathrooms in a terrible condition, in a 50s house in Southeastern PA.
Bathroom 1: first floor; one shower, one sink and one toilet. No venting anywhere; in fact, it would appear that the sink somehow drains into the shower drain, which in turn drains into the main stack, as there is no separate drain for the sink in evidence. I haven't demolished this yet.
Bathroom 2: second floor; one shower, one sink and one toilet. All fixtures have separate drains into the main stack. None of them are vented.
Bathroom 3: second floor; one tub, one sink and one toilet. All fixtures have separate drains into the main stack. None of them are vented.
The main stack itself discharges horizontally. In fact, the only vertical part of it is the drop from the second floor to the first floor. It is correctly sloped. It is 3" hub-and-spigot cast iron. It's laid out as follows, going downstream:
A) From the vertical roof penetration, it drops between rafters (about 45 degrees) into Bathroom 3. It then turns vertical again for four feet before turning horizontal. This is a sweeping combination of 45 degree elbows.
B) The three fixtures in Bathroom 3 enter the main with wyes: first the toilet, then the tub, then the sink. The main is still horizontal at this point.
C) The three fixtures in Bathroom 2 enter the main with wyes: first the shower, then the sink, then the toilet. The main is still horizontal at this point.
D) After the total horizontal run of about 15 feet, the main drops vertically for about 10 feet into the basement. It again turns horizontal and there is a cleanout at that point.
E) The fixtures in Bathroom 1 enter the main with wyes: first the toilet, then the shower (and presumably sink.) The main is still horizontal at this point, and it continues horizontally for a further 15 feet.
F) The main drops vertically about 18", turns horizontal again (there is a cleanout at this point) and then promptly exits the building underground.
Everything I read and see seems to suggest that the main stack ought to be vertical and only branch drains are allowed to be horizontal. Is that true, and if so, what am I supposed to do with the situation in which I find myself? The issue is not fixture vents, as I'm committed to add those, or any other fixture plumbing details, but the main itself.
Many thanks in advance for your advice,