Hi! We are extending my daughter's room by 2ft into the attic space to create a "box" effect. The only glitch we've ran into is the 1 1/2" washer vent pipe running right in the area that we need to cut out. My question is.... can we redirect the pipe by using a 90 degree elbow to run it below the floor line, through the roof joist & over 2ft, add another 90 degree elbow and go up the wall, add another 90 degree elbow back over to the existing pipe that goes up through the roof? We'll be making sort of a squared off "C" shape to move it out of the way and keep it connected to the existing roof vent pipe at the top. I think it will add 4-5ft in pipe, not incl the elbows. If we can, anything special we need to take into consideration? Thanks so much!

Sounds like a lot of trouble and not a good venting solution. The shorter and straighter is always best for expelling the moisture & heat. I am not sure if it would work in your case but I am inclined to say abandon the original vent through roof (seal off if nothing else on it) and direct vent hose over to soffit (overhang area) and cut small hole there to point downward. You would need a small flat slotted vent cover from hardware store (cheap). Good luck.

Thanks Chilibean. I think we could do that although, it won't be shorter. It would only be straighter. The part of the roof where the vent is currently located is on a big slant and the vent would be only a foot or so above the last 90 deg elbow. The soffit is about 10 ft over. Should we abandon the project all together due to no real good solution? We are DIY weekend warriors, not professionals. Are we in over our head?

The vent you describe is a plumbing vent, not a moisture or heat vent. Re-plumbing as you described will be just fine. Use long ells or 90s or two 45s (one regular and one street), in case you have to snake it some day.