If it were my project, I would make sure the plywood is securely fastened to the shingles by using plenty of screws, and them you will have the solid foundation we are looking for. As long as that is solid, I would not expect any problems. Just make sure the floor is level. In another home, I had to use a 1/4 ply, taped off all my seams, used a liquid leveler, and then added a 3/8 inch to that. It worked great and for the next 9 years I lived there, not a gap or squeak to be found! THe floors sold the house for me.