I have a customer who's hired me to build a screened porch. The overall size of the porch is 14 x 20 and the foundation is built with 2 x 8 framing because of low ground clearances. There is a ledger attached to house, a double beam in the middle of the framing (parallel to the house) and a double beam for the rim joist (so the longest joist span between beams is approximately 7 feet). Joists spaced 16" o.c. The subfloor is covered up with 3/4" exterior plywood and is blocked at the seams since it wasn't available T&G. The homeowner had planned to use outdoor carpet but has now asked me to lay down porcelain tile and/or slate. Just want to be sure that this scenario: exterior (but covered), freezing and thawing (Indiana) and wood foundation described above would adequately support the tiles. I would probably install a 1/4" tile backer over the plywood subfloor first. Any advice or cautions....? Thanks!!