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NFPA 72 §17.6 & §17.7
Smoke & Heat Detector Spacing
Detector count on a smooth ceiling, with high-ceiling, beam/joist, and corridor spacing extension.
Enter the room dimensions to see the required detector count.
Nominal smooth-ceiling spacing for smoke detectors is 30 ft unless the listing says otherwise.
Heat detectors only — NFPA 72 has no mandatory height-based spacing reduction for spot-type smoke detectors.
Reduces spacing perpendicular to the member: joists by half, beams to two-thirds.
A narrow corridor doesn't need diagonal ceiling coverage the way an open room does, so detectors can sit farther apart along its length and still keep every ceiling point within 0.7 × the effective spacing. This is Annex A guidance in NFPA 72, not a mandatory design method — some AHJs won't accept it, so confirm before using it on a real design.
A necessary note. Spacing rules and the heat-detector ceiling-height table are from NFPA 72
§17.6 (heat) and §17.7 (smoke), cross-checked against a manufacturer detector application
guide citing the same sections. Corridor spacing extension uses the 0.7 rule's own point-to-point
coverage geometry (extended spacing = 2 × √((0.7 × effective spacing)² −
(corridor width ÷ 2)²)), cross-checked against an independent field reference giving 41 ft
spacing for a 10 ft wide corridor at 30 ft listed spacing — it is Annex A guidance, not a
mandatory rule, and not every AHJ accepts it. This tool assumes ordinary smooth-ceiling or beam/joist
conditions on a rectangular room using the grid method — it does not cover sloped/peaked
ceilings, high air movement, or performance-based design for high ceilings, all of which need an
engineering evaluation per NFPA 72 §17.3. This is a planning aid, not a substitute for a code
review by a licensed design professional or your authority having jurisdiction.