I have a 12'x16' Building/cabin. It is a blank slate. 4 walls, roof, door, windows with Exterior sheeting on it. We bought it like this. It has 2x10 floor joists 16 OC and frame. 2x4 exterior walls. But the guy who built it used 1/2 plywood for subfloor. It is to weak and giving in places. I need to strengthen it so it can support a shower room, Kitchenette, and mini set of stairs to a loft. I was planning on laying a second layer of 3/4 plywood but unsure of the best way to do so. Any advice will be greatly Appreciated.