*HurriClips require at least a 2.5 inch inset to grip properly. *HurriClips work on wood, block, and stucco insets. HurriClip hurricane safety clips are made of strong carbon steel. Simply slide the clips onto your plywood, typically 4 clips per sheet (depending on the size of your window), push the plywood into the window casing and you are DONE! Once the hurricane has passed, loosen the tension on the serrated part of the clip and pull the plywood sheet out of the window. Remove the clips, place in the provided bag and store until you need them again!