There are a variety of materials and techniques used for applying soffits and fascia to porch roofs.  Most builders choose to match the look of the house roof as close as possible. 

Soffits can be installed by attaching plywood to the underside of the rafter tails and to a 2x2 nailer that runs across the header beam.  Some soffits are made of vinyl and use vents to help cool the roof. 

Fascia is used as an exterior trim to enhance the appearance of the roof line.  Facia boards can be wood or composite material.