I have an older house with only one bathroom, and I would like to add one down the basement. The house was hooked up to city water and sewer a few years ago, so I can see were the sewer line runs under my concrete floor. 1. Can I just tie into that line to add a toilet and shower drain, even though the upstairs bathroom runs into it? 2. If I can, is it OK to tie the vent for the new toilet in with the old vent or do I need to run the vent all the way to the roof? 3. Also the piping is cast iron, so how do tie into it?