Q. What type of subfloor do I need for a 3/4" solid wood?
A: If you are installing solid (3/4") hardwood on grade with a concrete substrate, the thickness of the plywood needs to be ¾; we utilize 5 ply CDX. The plywood not only acts as a barrier between the concrete and the hardwood, it also is where the nails/staples adhere. When we use a pneumatic nailer to staple the hardwood to the plywood, the force that is exerted needs the ¾" thickness to adhere the staple to within the plywood. Using a lesser thickness of plywood (ex. 5/8") the force of the nailer would cause the staple to go clear through the plywood and not adhere in the plywood. Over time the board may loosen and cause a squeak.
If you are installing solid ¾" wood to an area that has existing plywood, you do not need to install another sheet of plywood. We then apply a felt underlayment over the existing plywood and nail directly into the existing plywood. The underlayment is used so there is no wood-to-wood contact that may cause squeaks. We cannot nail into MDF that is acting like plywood. The MDF is too lightweight and will crumble when trying to secure the wood with the nailer. In this instance we would glue a sheet of ¾" plywood over the MDF first, and then nail to the new sheet of installed plywood.