I'm planning on adding an upstairs bathroom in my house and I've gotten two different opinions from plumbers I've had look at the job. There is an exisiting cast iron vent pipe which looks to be 4 or 5", (which is also the drain for the downstairs toilet) running through the space where I plan to add the upstairs bathroom. One plumber says I can tie my new bathroom into this pipe for drainage and the other plumber says I must install an entirely new drain pipe with a separate vent. Who's right?