Wiktionary
subflooring
n. A level of flooring below the main floor.
Usage examples of "subflooring".
After it penetrated the surface hardwood and the subflooring, little of the nail would remain to grip a joist.
Though his stomach lurched as they took the elevator to the fifth level, he focused his mind tightly on the job at hand, and managed to put down the slight wave of dizziness that broke over him when he neared the precipitous edge of the subflooring, unguarded by even the most vestigial of safety railings.
The big plywood squares that formed the subflooring were down and trimmed.