Spot Smoke / Heat Detector Count (Smooth Ceiling)
Spot smoke or heat detectors on a smooth ceiling per NFPA 72.
Example
You enter
- Room length (ft) 60
- Room width (ft) 40
- Device listed spacing (ft) 30
You get
- Detectors 4
- Max off each wall 15.0 ft
Checked against published worked examples from: Hochiki America Corporation.
Details, formula, and sources
Scopedetectors = the fewest grid cells that each fit the listed S x S square or keep every point within 0.7 S (NFPA 72 17.7.4.2.3.1), so a 41 x 10 ft corridor at 30 ft takes 1; the first detector sits within spacing/2 of each wall. A 60 x 40 ft room at a 30 ft listed spacing takes 4 detectors (a 2 x 2 grid, 15 ft off each wall); a 100 x 80 ft open area takes 12. Beams, high ceilings, and HVAC tighten it. An install estimate; the stamped fire-alarm plan and the AHJ govern.
Formula or tabledetectors = rows x cols, the fewest grid cells that each fit the listed S x S square or keep every point within 0.7 S (NFPA 72 17.7.4.2.3.1); wall_max_ft = listed_spacing_ft / 2.
Source and editionNFPA 72 spot-detector grid identity by name (grid count over the listed spacing; wall maximum at half the spacing); first-principles grid arithmetic.
Free accessThe grid arithmetic is public first-principles; the listed spacing comes from the device listing and the reductions from NFPA 72.
What governsEstimate. AHJ and licensed professional govern.
API field namesroom_length_ft, room_width_ft, listed_spacing_ft, detectors, wall_max_ft
- Listed spacing ~30 ft for spot smoke on a smooth ceiling; the device listing governsdevice listing / NFPA 72
- Coverage 0.7 x spacing to the farthest point; first detector within spacing/2 of each wallNFPA 72