sentences:  Floors may be built on beams or joists or use structures like prefabricated hollow core slabs.The subfloor builds on those and attaches by various means particular to the support structure but the support and subfloor together always provides the strength of a floor one can sense underfoot.Nowadays, subfloors are generally made from at least two layers of moisture resistant ("AC" grade, one side finished and sanded flat) plywood or composite sheeting, jointly also termed "Underlayments" on floor joists of 2x8, 2x10, or 2x12's (dimensional lumber) spaced generally on centers, in the United States and Canada.