Q: What code or standard are you citing when you say any ceiling that has sprinklers or smoke detectors must be maintained to resist the passage of smoke? Is this because smoke and heat will migrate above the ceiling if there are cracks, voids or missing tiles, and the detectors and sprinklers may not activate properly?

A: Section 3.3.5 of NFPA 13-2010 describes smooth ceilings for sprinkler installations. If the ceilings had gaps, holes or missing tiles, then the heat and smoke would rise above the sprinkler and smoke detector, impairing their proper use. The 1/8-inch gap dimension used by most AHJs is borrowed from NFPA 80 for clearances around fire doors.