Allowed subfloors are:
Concrete, Underlayment grade plywood, Exterior grade plywood (sanded face, B-C or better grade), properly prepared non-cushion backed vinyl flooring.
All subfloors must be clean, dry and free of contaminants such as oil, grease, paint, sealers, waxes, curing compounds, excessive moisture, hydrostatic pressure. Floors must be structurally sound, rigid, flat to 3/16" in 10 feet (F-32).