Doing a kitchen remodel which involved removing the soffit for cabinets to the ceiling. All went well except it revealed the vent for kitchen (and also vents laundry room sink, floor drain, and washing machine below kitchen). I was hoping the vent would be covered in the top portion of a cabinet, but the cabinets stop a couple inches to the side of the pipe.
The 1.5" copper vent extends up to just below the 2x4 top plate, 90 elbow into the old soffit space, 90 straight up again into what looks like a galvanized pipe that extends out the roof. This transition to galvanized occurs below the ceiling, around the middle of the 2 top plates.
Is it possible to cut the copper below the top plate, transition to PVC, notch the top plate (and add mending plate), then cut off enough of the galvanized so the PVC can transition into it with another 90?
Currently the galvanized is connected to copper with a threaded connection + a union. If I cut off the threaded portion, can I connect PVC with a rubber coupling?
Will there be other issues with this solution? Possible problems I was thinking were: would vibrations during cutting of the pipes causing it to come loose from the roof? Does the pipe need to be secured above or below if I splice in PVC near the top like this? It's a hip roof and joists run parallel to the top plate, but the rafters (and perpendicular joists every 4ft along the wall) all line up over the studs, so I'm thinking a 2inch notch in the top plate should be ok, but perhaps not? Anything else I'm missing? If it's too difficult or dangerous I am ok building some sort of soffit again just over this section only, but that would preferably be a last resort