Installing a porch roof over an existing deck is usually not recommended unless the deck was originally designed to support a future porch.  Typical decks are designed to support 55 psf (lbs per square foot).  Porch decks require 25 additional psf for roof loads for a total of 80 psf.  As a result most decks have inadequate footings to support a new porch roof.  The footings for porch decks must be larger than deck footings.  Many porches use 22"+ diameter corner and intermediate footings.  Gable end porches will also require an additional footing in the center of the side rim to transfer the truss or rafter loads to the footings because the framing of the roof is perpendicular to the floor framing.  Porch footings must be positioned on the perimeter of the porch to directly transfer the loads to the footings.  Decks with cantilevered beams are not designed to support porch roofs.