Coffered Ceiling question

I am putting in a coffered ceiling in my dining room as part of a major remodel. The old ceiling is already down and once I install the new recessed lighting frame-in kits, I will be ready to reinstall the new ceiling.
The question is this.
Since I need to install a new ceiling, should I just put up some 5/8" sheetrock or perhaps 1/2" plywood as the base for the coffered ceiling?
Thanks.
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Some of the answer depends a bit on what is above. If there is living space above, code may well require the fire resistance of gyp. If it is attic structure, I would put up wafer or ply to insure nailability wherever needed. I assume you are talking about a full wood coffered ceiling that will be stained and finished.
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Actually, the ceiling will be painted, and, there will be a living space above. It sounds like I should just go with the 5/8" sheetrock and "build" the coffers <?> on top of that.
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