Smoke Detector Spacing per NFPA 72 — The 0.7 Spacing Rule
Spot-type smoke detector spacing looks like a grid problem, but the rules that trim the grid — corners, beams, and air movement — are what a plan checker looks for.
The nominal spacing
Spot-type smoke detectors have a listed spacing of 9.1 m (30 ft) on a smooth, flat ceiling. Lay a 9.1 m grid; a detector protects a square that far on each side.
The 0.7 rule
The critical check is that no point on the ceiling is more than 0.7 × listed spacing from a detector (that is the distance to the corner of the coverage square: 9.1 × 0.7 ≈ 6.4 m). Use this to place detectors in corridors, L-shaped rooms, and irregular spaces where a plain grid leaves gaps.
Wall and boundary rules
- No detector more than 4.55 m (half spacing) from a wall.
- Not within 100 mm of a wall/ceiling junction (dead air space); 100–300 mm down on a wall if wall-mounted.
- In a corridor up to 3 m wide, detectors may be spaced up to 9.1 m along the centreline, first one within 4.55 m of the end.
Ceiling and airflow adjustments
| Condition | Effect on spacing |
|---|---|
| Beams > 10% of ceiling height, deep | treat beam pockets individually or reduce spacing |
| Ceiling height > 3 m | reduce spacing progressively (stratification) |
| Sloped / peaked ceiling | a row of detectors within 0.9 m of the peak |
| Near supply diffusers | keep detectors ≥ 0.9 m from the diffuser |
Count
EngEst Pro’s FDAS Calculator places smoke detectors per NFPA 72 spacing and the 0.7 rule from the room dimensions you enter, adjusts for ceiling type, and adds detectors, bases, and wiring to the FDAS BOM.
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