Find the position under your patio deck where you would like to install your hanging chair. Look for a joist overhead. The joists are the heavy wooden beams running across the underside of the deck. Don't attempt to hang a chair from the wooden flooring material itself.
Make a pencil mark in the center of the overhead joist. Fit a 3/8-inch drill bit into your cordless drill. Tighten the drill's chuck around the bit.
Drill straight up into the joist, keeping the drill bit perpendicular to the joist. Drill a hole to a depth of about 4 inches.
Insert the tip of a 7/16-inch-diameter eye lag screw into the pre-drilled hole. Twist the screw clockwise until the eye is flush against the joist. The screw may become difficult to tighten near the end. If necessary, insert a long screwdriver into the screw eye. Use the screwdriver handle to turn the screw with more torque.
Hang an S-hook from the screw eye. Suspend your rope or hammock chair from the S-hook.