I have a porch that has a 6' overhang and I would like to put soffit under it, however I would like the soffit to go up along the rafter, so that it is going from the front of the house to the end of the roof (fascia), this is my 6' length. The area is roughly 6' wide x 18' long. How do I do this? I know that I can install the soffit on one end with F channel and then J channel on the other end but how to I support the total 6' length of the soffit? Do I use furring/nailing strip and then put a nail into the soffit for support? See pic for clarification.

 

You would install continuous furring down the center of the soffit so that you could nail through the soffit down the center. You also likely would not install j-channel on both sides, since it would make it pretty difficult to fit the pieces and lock them together. Generally you put the j-channel in back (you'll likely have to screw it up since nailing back there will be difficult) and then once all the soffit is up, you use a metal break to cap the front edge somehow (custom bent piece), or use aluminum fascia cover, provided the soffit comes all the way to the front of the fascia.