Track down joists with an electronic stud finder, or simply probe for joists by driving a nail throughout the drywall. All of the joists needs to be centered both 16 or 24 in. aside, but double-Examine with a nail. Mark each joist location with a chalk line on the ceiling http://landeniomic.jaiblogs.com/15456032/the-5-second-trick-for-removing-painted-popcorn-ceiling