The fir T&G porch floor on a 100 year-old house that I'm working on is in rough shape with large gaps between some boards and lots of deteriorated surface. In addition, there is lead paint on the floor. Since the top of the entry door threshold is a strong inch and a half above the floor, I'm considering laying 1/2 inch exterior plywood over the existing floor and installing new T&G fir over that. There is a kneewall at the perimeter of the porch, but otherwise it's open to the Minnesota weather. Revising the stair to the porch won't be a problem as it's a 2 riser wood job. Any problem with the ply and T&G over existing T&G plan?
I don't think I'd do that. My concern is that water from a wind driven rain would get between the layers and not dry sufficiently. I believe I would install some PT'd. strips/lath in the thickness I needed and then install the new floor on top.
If you are reluctant to remove the original floor I would be very timid about offering a warranty with the job.
I am assuming the flooring is running perpendicular to the front of the house and the framing is parallel. If you install furring over the existing porch you will create a situation that is not providing the best scenario for water that seeps through the overlying floor. The backside of the new floor will also not be able to ventilate as well as the original floor.
Also, you do not mention if the new flooring will be continuous vertical grain or mixed grain finger joined material. That decision will obviously impact expansion/contraction issues (and water seepage) as well as general durability.
The other item that you have to pay attention to is the column/post situation you may have to deal with and how your decision to add a layer or layers will impact their durability and aesthetic.