The simple device uses the weight of your body leveraged against the doorframe, and requires that you have a doorway with trim. You can also do deep push-ups with the Door Gym by placing it on the floor. NO FASTENERS REQUIRED. And when used properly it will not damage doorway and can be installed and removed in 1 second. Door Gym door size requirements: Face molding around door: minimum 1½" maximum 3½". Molding distance from wall: minimum ¼" maximum ¾". Wall thickness: minimum 4¼" maximum 6¼" (trim to trim). This DOOR GYM has been designed to fit residential (framed) doorways 24" to 32"(up to 45" with the extensions) and support up to 300 lbs. Ab loops and broad reach accessories sold separately. For use with wooden doors only.