How to Build Storage Shelves on a Floor Joist

Place the 3/4-by-14 1/2-by-18-inch plywood panels parallel to each other on a work surface. Space them 22 1/2 inches apart from the inside edges. These are the top and bottom panels of the shelf assembly.

Position the 3/4-by-22 1/2-by-18-inch plywood panels between the ends of the top and bottom panels to form a rectangular box assembly. These are the sides of the shelf assembly. Drive deck screws through the top and bottom panels into the ends of the shelf assembly sides every 2 inches using a screw gun.

Center the 3/4-by-12 1/2-by-18-inch panel between the sides. This is the center shelf of the shelf assembly. Drive decks screws through the sides into the ends of the center shelf every 2 inches.

Lay the 3/4-by-24-by-14 1/2-inch plywood panel over the assembly. This is the back of the shelf assembly. The edges should be flush with the edges of the assembly. Drive deck screws through the back into the edges of the sides, top and bottom and shelf to complete the assembly. Space the screws every 2 to 4 inches.

Hold the shelf assembly between two floor joists in the desired location for the shelves. The top of the shelf assembly should be flush with the bottom of the flooring above the joists. Have another person hold the assembly in place while you drive deck screws through the inside of the shelf sides into the sides of the floor joists to mount the shelf assembly to the joists. Space the screws every 2 inches.