Shop for Hardware at The Home Depot. Crown Bolt's lag or expansion shields are used for fastening lag screws into fastening into wood and concrete. The expansion shield acts as an anchoring point into a concrete slab or foundation, expanding when the screw threads are engaged during installation. Common uses include frames, and outdoor play equipment. When used in conjunction with a lag screw it can also be used in many masonry applications. Use in pre-drilled hole. The length of a lag screw is measured from the underside of the head to the pointed tip of the screw.