Plumbers?

422: This is a car site but by golly there are people on here that do that kind of work to support their habits and i'm one of em so here ya go. At the top floor, or the first toilet in line, the code says that a 3" schedule 40 PVC is acceptable but when you pick up another toilet in that line you then have to bump that size to 4" at the second toilet. There is no need to place a trap under the toilet in that line as the toilet itself is traped. You will need a trap in the 1.5" line under the lavatory. Be sure and run a vent out the roof or it will never flow right. NEVER stub the vent into the attic and leave it there. If you do and your lucky all you will ever realize is the smell of turds but if your unlucky you will notice an explosion of trapped gases. If the 3" pipe is larger than your wall simply fir out one side of it and resheetrock. Place a metal guard across the plates where the pipe runs through them so as not to drive a nail where it will give you trouble later. If your lower bathroom only has a 3" line now you should replace it at that point with a 4" in the remainder of that line. You might get by ok if it only a 3" now, who knows but as i explained this is code in our area. Hope this helps.
Small Block