Hi I am in the process of laying an engineered oak floor, the existing floor is old pine boards, I have lifted a few to see what is underneath, it appears to be concrete laid over slates. The concrete is covered in bitumen and there are wooden joists set into the concrete which the floor boards are nailed into, unfortunately these joists are starting to rot as it is damp, the whole lot is at least 100 years old. The floor boards are all good apart from a couple of places around the edge where the damp has come from the walls. Should I put them back down and put in a floating floor or pull up all the floor boards use a dpm then re lay 18mm exterior ply then a floating floor?