The most important element of building an Adirondack chair is the lumber. Stick with cedar or pine for a quality look and solid support. You should never buy lumber that is less than 3/4-inch in thickness. You'll need six wood boards all together including one 1-inch by 4-inch by 8-inch board, two 1-inch by 4-inch by 10-inch boards, one 1-inch by 6-inch by 10-inch board, one 1-inch by 6-inch by 12-inch board and one 1-inch by 4-inch by 8-inch board.
Adirondack chair features a number of straight cuts, angles and curves. Building one requires multiple tools including several power saws. You will need a table saw, miter saw, jigsaw, router, drill, power sander, measuring tape, combination square and clamps. The table saw will cut the slats for the seat and back. The jigsaw helps add curves to the side rails, and the miter saw makes cross cuts for angles in the chair's legs.
To hold the chair together you'll need the appropriate hardware. For your drill, purchase countersink drills bits. For the router, you'll need 1/4-inch router bits. In addition, purchase one box of 1 1/4-inch screws and one box of 1 1/4-inch pocket hole screws with a coarse thread.
Adding lumber back slats to the chair will create additional support.With wood glue, glue the slats on before attaching with screws. Gluing holds the slats in place so that they are screwed on at the correct angle. Once the chair is structurally finished, purchase wood filler to fill any screw holes and pocket holes for a cleaner appearance. You'll also need an exterior primer and exterior paint or stain. Adirondack chairs often feature a semi-gloss paint.