Both channels are used to secure the soffit to building. The upside-down F-Channel creates a shelf for the soffit to slide into and rest upon. J-channel is a piece designed to fit over siding and soffit pieces. It is a corner or end piece that resembles the letter J. It mounts the siding to the wall and helps hold the soffit in place. The F-channel can be easily confused with the J-channel that is used to cover the ends of the vinyl at the corners of the house and around windows. The J-Channel has one shelf and the F-channel has two. Both styles of channel measure 12’ in length and are manufactured from .018 gauge aluminum.