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.
The original foundations could be removed and enlarged to accommodate the new weight of the roof. If the posts supporting the roofs are placed directly on the deck structure below, there are a few considerations to make. The posts should stack directly above a support post below, and not within the span of a beam. Be sure that the deck floor frame is at least as thick as the entire cross-section of the post above and below. A 1.5-inch thick rim joist cannot support a 3.5-inch thick post above. Blocking must be installed adjacent the rim joist.
To avoid any modifications to the existing deck below, or to facilitate future replacement of the deck below the roof, the roof can also be designed independently from the deck. New foundations piers would then be installed and sized only with the roof in mind.