NFPA 72 code sections · Smoke & Heat Detector Technologies
Smooth ceilings: 30 feet between smoke detectors, and nothing farther than 21
On a smooth, level ceiling the code lets you space spot-type smoke detectors 30 feet apart. The real test is coverage: every point on the ceiling has to be within 0.7 times that spacing — 21 feet — of a detector, and no detector covers more than a 30-by-30-foot square. Spacing shrinks when beams, joists, slopes or high airflow get in the way, so the 30-foot number is the ceiling (pun intended), not a target. Manufacturer instructions and the listing govern where they are tighter.
NFPA 72 reference
Chapter 17 — Initiating Devices
17.7.3.2 Spot-Type Smoke Detectors (17.7.3.2.3.1 nominal 30 ft spacing on smooth ceilings; 0.7 × spacing coverage rule, 17.7.3.2.4 beams and joists)
References use NFPA 72 2019/2022 numbering — section numbers can shift between editions, so confirm in the edition your AHJ has adopted.
On the job
A warehouse office is 40 ft long and 20 ft wide. The installer puts one detector in the middle — 20 ft from each end wall. Every point on the ceiling is within 21 ft of it? No: the corners sit roughly 22.4 ft away (20 ft lengthwise plus 10 ft to the side wall, diagonal). Two detectors at the one-quarter and three-quarter points bring every corner within reach and the room passes plan review.
Quick check
1. Nominal spacing for spot smoke detectors on a smooth ceiling is:
2. Every point on the ceiling must be within what distance of a detector (30 ft spacing)?
3. Beams, joists and slopes generally:
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