NFPA 72 code sections · Smoke & Heat Detector Technologies
Four inches from the wall — and no dead-air corners
A ceiling-mounted spot smoke detector must be at least 4 inches from any sidewall measured to the near edge. If the detector has to go on a wall instead, the top of the detector sits between 4 and 12 inches below the ceiling. The band exists because smoke stratifies and moves along the ceiling; the corner where wall meets ceiling is a dead-air pocket smoke reaches last. On a sloped or peaked ceiling the first detector goes within 36 inches of the peak, with the rest spaced on the horizontal projection.
NFPA 72 reference
Chapter 17 — Initiating Devices
17.7.3.2.1 location of spot-type smoke detectors (ceiling ≥ 4 in. from sidewall; sidewall 4-12 in. below ceiling); 17.7.3.2.3 sloped ceilings
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 finish carpenter asks the alarm installer to push a hallway detector right into the corner so it lines up with a soffit. The technician declines and shows the 4-inch rule on the drawings; the detector is mounted 6 inches off the wall instead. At acceptance, the smoke test trips it on time — a cornered detector can lag the rest of the ceiling.
Quick check
1. A ceiling-mounted smoke detector must be at least how far from a sidewall?
2. When mounted on a sidewall, the top of a smoke detector is placed:
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