Thanks, guys. I decided to leave well enough alone.
See if you can picture this. The room behind the closet is about 10‚Äô wide, flanked on each side by dormers, which form the sidewalls. Its depth is about 14‚Äô, but is not all usable because of the 7/12 sloped roof, which is only 6‚Äô high when you enter, and slopes down from there. The entrance is actually in the best location.
However, in the middle of this area are these vent pipes, two sets joining into one about 2‚Äô up. The main stack is the 4‚Äù variety, while there is an offshoot comprised of length‚Äôs of 1 ¬Ω‚Äù (or so) galvanized pipe and one 45 degree threaded elbow. The whole thing looks like a big, inverted ‚ÄúY‚Äù.
My intentions were to remove the pipes where it comes out of the floor and where it comes down along the ceiling. Then, I would add enough plastic vent pipe so as to route it behind the hip-wall I‚Äôm building. Believe me, this would be a very straightforward procedure, and if not for the pipes being in iron, the project would already be a done deal. But the iron pipes sound somewhat difficult to move, and the extra usable floor space is probably not worth it.
This past weekend I laid down the floor. I simply worked around the pipes. When complete, the little room will be somewhat ‚ÄùL‚Äù shaped, with only a small section of the 2‚Äù pipe actually in the room. The rest will be accessible from the remaining attic space, through a new trap door.
Again, thanks for all the input.