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?