Hi there. I'm in the process of moving and attic bathroom several feet for some better head room, and so I don't hit my head when I use the toilet. Anyways, the one dilemma I'm having is the toilet drain. I have to move the closet flange approximately 8 feet, and the joists are 2x8. Also, I'm dealing with 3" pvc. Would this be able to be done with the correct pitch, and still be able to fit the connection under the subfloor? The drain pipe would have to go through one joist, but for the most part the drain will run parallel with the joist. I am hiring a plumber, but just wanted to get some other thoughts.
You're only supposed to drill the middle one third of a joist, so you shouldn't be drilling a 3 5/8" hole (O.D. of a 3" PVC pipe) through a 2" x 8" joist. The highest up into a joist bay you can get the bottom of a 3" pipe where it picks up a toilet is about 5 7/8" (from the bottom of the subfloor). It can be higher if you're laying durock and tiling the floor of the bathroom, but if it's linoleum, the 5 7/8" measurement will be about right. By the time you run 8 feet at 1/4" per foot, the bottom of the pipe will hang 7 7/8" from the bottom of the subfloor.
The problem with that is the hub of the 3" 90 (where the pipe goes from horizontal to vertical) will have to be drilled through the top plate of the wall; you may even have to notch the plate to get that 90 in (another structural consideration - especially if it's an outside wall).
So given the first consideration (drilling a large hole through a 2x8, you should probably consult an engineer (if there's no builder involved) to see what you could do to remediate the structural integrity.
I had a similar situation a couple of years ago where they ended up fabricating a steel plate with a pre-drilled 4" hole (I told them where to cut the hole). The builder sistered it to the joist, and then I was able to drill the joist and complete the piping.