i am getting ready to begin hanging soffit on my home it is a new construction and i am not sure how to attach the house side of the soffit currently there is no lumber there to attach it to only partical sheeting do i attach the "F" channel to the sheeting and just let the soffit float in the channel, and attach it to the facia board only or do i need to attach 2X4 lumber to the house so i have something to nail the house side of the soffit to.

please help
THANKS