The subfloor is the structure that provides support to the top layer of flooring, such as hardwood or laminate. Often, a subfloor is laid over the joists and is made of plywood or cement. The strips are side-matched and end-matched (tongue-and-grooved). They are for nail-down installation directly to wood or plywood subfloors, or over wood screeds on concrete slab construction. Some types can also be glued directly to a concrete subfloor.