I had to open up a wall to do some electrical work downstairs in my workshop, and had one of those "a ha!" moments as I discovered that the sewer line for the upstairs bathroom passed right through it, and there was a cold water line there as well, so I said to myself, gee, I could add a toilet right here without going to a lot of expense, and since it is the old workshop, I could use that old toilet I have out in the shed and all. I could build a little plywood box, install the commode on top of it, and have just enough room without having to cut a hole in the slab,and then run the vent up the outside wall to the roof. So my first question is, do I need to install a P-trap in the sewer line or does the internal plumbing in the commode handle all that? If I can just use a Y pipe from the commode to the sewer line, everything will fit into my little white trash plumbing box I want to sit the commode on.