This is a good topic. A few years ago the UPC started allowing plastic pipes in any building. What started happening is plumbing contractors started designing high rises with a typical cast iron stack and branches but would use ABS for the vent stack and trap arms/dirty arms. Problem is plumbers, like my self, are not too familiar with occupancy and fire ratings. I'm all too familiar with the rockwool and firestopping of penetrations since that's all I did my first year of apprenticeship but that was when everything was cast iron. Now with ABS coming in it was typical for some contractors to use cast iron pups through the floor but then switch to ABS for all of the stacks and all between floor piping and then go back to cast pups throught the next floor up. Now we have a nice big chimney for a fire to jump from floor to floor. It was a huge mess with some of these big high rises and big contractors. In the end we had to put out an emergency declaration about the use of "Mixed materials" in a rated building. No mixing of cast and plastic is allowed BUT ( always a big but) if you do use mixed material, say in a TI space or on a big remodel, then you need to submit fire rating plans to the city and go through a fire review. Basically if you mix cast and plastic you would need to provide plastic pups through the floor with a crush collar and provide a crush collar on every cast to plastic connection you have in the building. On a small TI would probably be no big deal but that's what was required on some pretty big new high rises down town. I think the money they saved by using ABS was wasted in the crush collars they had to purchase. Just in case you're wondering, the plans didn't show any ABS at the time of review. I guess that's another heads up call for Washington state plumbers.