We can install a knocker in a composite door, but you need to know that this material is not as strong as steel, so the door cannot take too much force. We have two options for a solution because a knocker can damage the door if it has to work hard. The first solution is to put a metal liner where you want your knocker to be. This way, the force will be absorbed upon contact with the metal liner and not with the door itself. The other option is to use wooden bumpers on each side of the inside part of the door that holds your gear.