NAICS 238330 is for Flooring Contractors while PSC Z2FB is for Repair Or Alteration Of Recreational Buildings. Replacement of gym flooring is not a repair or an alteration. It seems that PSC Z1FB Maintenance of Recreational Buildings (rather than Z2FB) more accurately describes the replacement (i.e., not a significant upgrade) of the gym floor, and your specific job requirement is not considered construction.