Place a piece of 3/4-inch-thick pressure treated plywood on the bathroom floor flush with one of the corners of the room. Position the plywood so that the grains of the wood run perpendicular to the wooden floor joists below it.
Observe the edge of the plywood farthest away from the bathroom wall. The edge should lay in the center of one of the wooden floor joist boards below it. If necessary, cut the end of the board off with a saw until the end lies on top of the floor joist.
Position a 1 3/4-inch deck screw at the edge of the plywood above a floor joist. Insert the screw with an electric screwdriver until the top of the screw is flush with the top of the plywood. Insert additional screws along all edges of the plywood at six-inch intervals, ensuring that all the screws penetrate into the floor joists.
Insert screws into the center of the plywood board in the locations of the floor joists, which will be approximately every 12 to 48 inches. To locate the joists, look at the exposed bathroom floor and trace the wooden joists back to the piece of plywood you are attaching.
Place another piece of plywood directly beside the first piece and leave a 1/8-inch gap between them to allow for expansion. Both edges of the boards should rest on the same floor joist underneath. Insert screws into the second piece of plywood to secure it in the same manner that you secured the first piece of plywood.
Install as many full sheets of plywood as possible on the floor. When you reach the last line of the bathroom floor, there may not be enough space to install full sheets of plywood. Measure the remaining space with a tape measure and cut the plywood pieces to size with a saw. Install the smaller pieces just as you did the full sheets.