Smoke Detector Spacing Chart
Spot-type smoke detector spacing on smooth, flat ceilings, per NFPA 72 §17.7.3.2 (2022 edition): the nominal 30 ft on-center spacing, the 15 ft maximum to a wall, and the 0.7 rule that governs irregular rooms. These are guide spacings for ordinary conditions — not a substitute for a detection design or the AHJ.
Smooth-ceiling smoke detector spacing
| Parameter | Rule | NFPA 72 § |
|---|---|---|
| Nominal spacing on a smooth ceiling | 30 ft on center | 17.7.3.2.3 |
| Maximum distance to a wall or partition | One-half the spacing = 15 ft | 17.7.3.2.3 |
| Point (0.7) rule | Every ceiling point within 0.7 × 30 ft = 21 ft of a detector | 17.7.3.2.3 |
| Irregular areas | Spacing may exceed 30 ft if the farthest point stays within 0.7 × listed spacing | 17.7.3.2 |
| High ceilings | No prescriptive derate — use performance-based design (Annex B) | A.17.7 / Annex B |
How the 30 ft and 0.7 rules work together
The 30 ft on-center figure is the simple case: a grid of detectors no more than 30 ft apart, none more than 15 ft from a wall. But a detector's real coverage is a circle, not a square — its guaranteed reach is 0.7 × 30 ft = 21 ft in any direction. The 0.7 rule uses that circle directly: in an L-shaped or angled room, you can stretch the on-center spacing past 30 ft as long as every point on the ceiling — especially the far corners — stays within 21 ft of a detector.
These are guide spacings "in the absence of specific performance-based design criteria" and apply to ordinary locations. Beams, joists, sloped ceilings, and air movement all change the layout — see the detector placement rules for the clearances and obstruction cases.
Why smoke and heat detectors are spaced differently
Smoke detectors do not use the ceiling-height reduction table that heat detectors do. NFPA 72 deliberately leaves high-ceiling smoke coverage to engineering analysis (Annex B) rather than a prescriptive factor, because smoke stratification depends on the fire, the space, and the air handling in ways a single multiplier cannot capture. Applying a heat-style reduction to smoke detectors — or skipping smoke coverage on a tall ceiling because "the 30 ft still fits" — are both common mistakes.
Common questions
How far apart should smoke detectors be?
On a smooth, flat ceiling, spot smoke detectors are spaced a nominal 30 ft on center (NFPA 72, 2022 edition, §17.7.3.2), and no detector may be more than 15 ft — one-half the spacing — from a wall. This is a guide spacing for ordinary conditions, not a guarantee of detection.
What is the 0.7 rule for smoke detectors?
The 0.7 rule handles irregular rooms and offsets: every point on the ceiling must be within 0.7 times the nominal spacing of a detector — 0.7 × 30 ft = 21 ft. As long as the farthest corner is within 21 ft of a detector, the on-center spacing can exceed 30 ft. It reflects the fact that a detector's real coverage is a circle of radius 21 ft, not a 30 ft square.
Is there a ceiling-height reduction for smoke detectors?
No. Unlike heat detectors, spot smoke detectors have no mandated ceiling-height reduction table in NFPA 72. High-ceiling smoke coverage is addressed through performance-based design (Annex B), not a prescriptive derate — a key difference from heat detector layout.
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