Hand framing a porch roof involves first setting the ridge beam and the side walls to the proper height that the roof will hang from.  Most porch roofs use 2x6 or 2x8 rafters that form an upside down V across the ridge beam.  The rafters can be attached 16” on center to the ridge beam using rafter ties.  On the header side the rafters are typically notched with a bird’s mouth over the top of the header or top plate of the porch wall.  The angles you cut the rafter ends determine the pitch of the roof and the length of the rafters. 

Collar ties are horizontal braces that can be installed between opposite rafters to add strength and lateral stability.  The gable end rafter should be filled in with studs or webbing to support the overhang.