Hold a furring strip up against the face of the rock wall. If you are planning to install vertical siding, arrange your furring strips horizontally. For horizontal siding, apply vertical furring strips. Drill a hole through the furring strip and into the rock wall with a hammer drill. Drive a 2-inch-long concrete screw through the furring strip and into the rock wall.
Attach furring strips every 12 inches along the rock wall, securing them with concrete screws every 24 inches along each furring strip. If the rock wall has an uneven space, shim the furring strips in the low spots to create an even row of nailing surfaces for the siding.
Hold a piece of siding up against the furring strips and secure it by driving finish nails through the siding and into the furring strips.