Drop-ceiling section — drop, plenum, and the IRC R305.1 head-height check
A drop ceiling lowers the room by its drop. Habitable space needs 84″ (7 ft) finished to the lowest projection (IRC R305.1); beams and ducts may project to 76″ (6′4″); non-habitable basements need 80″. Subtract the drop and check before ordering.
What this diagram shows
A side section from the existing joists down to the finished floor. The existing ceiling or joist line sits at 96 inches; the new suspended grid hangs 8 inches below it, and the gap between them is the plenum that holds HVAC, wiring, and plumbing. A dimension marks the drop, noting the 3-inch minimum (4 inches below an existing drywall ceiling), and another marks the finished head height of 88 inches from the floor to the grid. An amber line at 84 inches shows the IRC R305.1 minimum finished height for habitable space, and a dashed amber line at 76 inches (6 feet 4 inches) shows the exception that lets beams and ducts project lower — illustrated by a duct hanging from the joists into the plenum down to that line.
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