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Smoke / Heat Detector Spacing Calculator

A beam pocket with no detector, a 24-foot warehouse ceiling laid out at the 50-foot listed spacing, a corridor with one detector too few — these are the plan-review comments that stall a fire alarm permit. This tool applies NFPA 72-2022 Chapter 17: nominal 30 ft smoke spacing with the 0.7S radius rule, beam and solid-joist reductions keyed to ceiling height, heat detector spacing reduced by Table 17.6.3.5.1, and the high-air-movement table.

Educational estimator, not a sealed design. Detector layout is performed by the fire alarm designer of record and reviewed against the manufacturer's listing and installation instructions, which govern (NFPA 72-2022 §17.7.4.2.3.2). This tool applies the prescriptive spacing rules of NFPA 72-2022 Chapter 17 for scoping, takeoffs, and plan-review prep. Texas jurisdictions adopt different code editions and local amendments govern — confirm with your AHJ.

1. Spot-type smoke detectors

NFPA 72-2022 §17.7.4.2.3.1: on a smooth ceiling, spot-type smoke detectors are spaced at a nominal 30 ft with detectors within 15 ft of every wall, or all points on the ceiling are within 0.7 × 30 ft = 21 ft of a detector. Beams, joists, slopes, and high air movement change that.

Inside dimension wall to wall. For an L-shaped room, split it into rectangles and run each one.
The shorter dimension. Corridors: enter the corridor width here.
Floor to ceiling at the detector. For sloped ceilings with beams, §17.7.4.2.4.3–.5 use the average height over the slope.
§17.7.4.2.4.1: solid joists are treated as beams for smoke detector spacing. Pick "smooth" for a flat drywall or deck ceiling with no projections.
Measure from the underside of the deck to the bottom of the beam. Compared against 10 % of the ceiling height (0.1H).
Compared against 40 % of the ceiling height (0.4H). Solid joists on 16 or 24 in. centers are 1.33 or 2 ft.
Reductions apply perpendicular to the beams; parallel to the beams, smooth-ceiling spacing applies (§17.7.4.2.4.2).
§17.7.7.3.3.1: spacing is reduced where airflow exceeds 7.5 air changes per hour. Air changes per hour = supply CFM × 60 ÷ room volume (ft³). Data centers and clean rooms are the usual cases.

2. Spot-type heat detectors

Heat detectors carry a listed spacing (printed on the data sheet and the UL listing). NFPA 72-2022 §17.6.3.1.1 places detectors within half that spacing of every wall, or all points within 0.7S. Table 17.6.3.5.1 then shrinks the spacing on ceilings above 10 ft, before any further reduction for joists, beams, or slope.

§17.6.3.4.1.1: for slopes under 30°, use the height at the peak. For 30° or steeper, detectors other than the peak row may use the average slope height. Table 17.6.3.5.1 runs from 10 ft to 30 ft; above 30 ft, spot-type heat detection needs a performance-based design.
Read it from the manufacturer's data sheet or the UL listing card — it is not a code value. Fixed-temperature spot detectors are commonly listed at 50 ft; combination rate-of-rise units vary.
§17.6.3.3.1.1: beams of 4 in. or less are a smooth ceiling. §17.6.3.3.1.3: beams deeper than 18 in. on more than 8 ft centers make each bay a separate area — enter one bay as the "room".
Highest normal temperature at the ceiling (attics, boiler rooms, and kitchens run hot). Table 17.6.2.1 picks the lowest temperature classification whose maximum ceiling temperature is at or above this value.

Related

📖 Read the full guide →

The 30 ft rule and where it came from, the 0.7S radius, how 0.1H and 0.4H decide beam pockets, the heat-detector height table, slopes, and the air-change table.

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