Several common methods such as rivets, screws and spot welding are used to fasten materials to ductwork. Three types of screws are most often used. The first is self-tapping only and requires a hole to be drilled before the screw can be utilized. The others are both self-tapping but also create their own holes. Self-Piercing Screws have very sharp points which act much like a scratch awl in making a small opening in the steel for the threads to engage and work the sheet metal into a coned threaded hole. These work well for light gage steel.