I need to run a vent pipe for laundry area (washer + utility sink). The easiest path would be running it about 18ft horizontally along a side wall to the back of the house and then up to the roof, rather than straight up through the 2nd floor (requiring a chase in the living room). There will be large duct that will be soffitted along that wall that will be lowered about 10" from the ceiling.
Can I run the vent behind the ducts (ie between the wall stud & duct) in the same soffit?
What is the minimum slope of the vent pipe? Will the 10" soffit be enough slope for an 18' span?
Yes, you can run along side the duct work. Grade it 1/4" per foot so it will drain back to fixture drain. You will need 4.5 inches of grade for a 18' run. To vent these fixtures will require a 1.5" vent, but you can only run 1.5" vent a maximum of 20' horizontally. You can upsize the vent to 2" and then you can run it horz. for unlimited length. Just thought I'd mention that even though you mentioned 18'.