[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

A warehouse is 40 m × 24 m with a flat concrete ceiling at 9 m. Downstand beams 0.6 m deep run across the 24 m direction at 6 m centres.

• Are point smoke detectors allowed at 9 m, or are beam detectors or aspirating systems required?
• Do the beams act like walls, so that every bay needs its own detectors?
• How do BS 5839-1 / EN 54-14 and NFPA 72 differ in the spacing result?

[MEP Center Engineering]

At 9 m, point smoke detectors are still within the usual height limits (BS 5839-1 general limit 10.5 m; NFPA 72 has no fixed limit but requires a stratification assessment). Beams 0.6 m deep are less than 10 % of the 9 m ceiling height, so both codes allow smooth-ceiling spacing. You need about 12 detectors under BS 5839-1 and about 15 under NFPA 72.

BS 5839-1 (and similar EN 54-14 national practice)

1. Height: 9 m < 10.5 m, so point smoke detectors are acceptable. For heat detectors the limits are lower and depend on the grade.
2. Beams: a downstand deeper than 10 % of the ceiling height is treated as a wall. Here 0.6 m < 0.9 m, so the ceiling counts as flat.
3. Coverage: no point may be more than 7.5 m horizontally from a smoke detector. On a rectangular grid, the half-diagonal of each cell must be ≤ 7.5 m.
4. Layout: 4 columns × 3 rows gives cells of 10 m × 8 m. Half-diagonal: √(5² + 4²) = 6.4 m ≤ 7.5 m. Corners are 5 m and 4 m from the nearest detector, again 6.4 m. Total: 12 detectors.

NFPA 72

1. Beams: depth less than 10 % of the ceiling height (0.1 H = 0.9 m), so smooth-ceiling spacing applies. Detectors may be on the ceiling or on the bottom of the beams.
2. Nominal spacing S = 9.1 m (30 ft), and every point of the ceiling must be within 0.7 S = 6.4 m (21 ft) of a detector.
3. Layout: 40 / 9.1 = 4.4, so 5 columns at 8 m; 24 / 9.1 = 2.6, so 3 rows at 8 m. Half-diagonal √(4² + 4²) = 5.7 m ≤ 6.4 m. Total: 15 detectors.

If the beams were deeper

With 1.0 m beams (more than 10 % of H): BS 5839-1 treats each 6 m bay as a separate space, each with its own detectors. NFPA 72, with a beam spacing of 6 m ≥ 0.4 H = 3.6 m, requires a detector in each beam pocket. Both routes give a detector line in every bay, often 7 bays × 3 = 21 detectors. At that point a beam detector or aspirating system is usually cheaper and easier to maintain.

High-ceiling considerations

• Stratification: a warm layer under the roof can stop smoke from reaching it. NFPA 72 asks you to consider it. In practice, add detection at a lower level (beams at 60 to 80 % of height) or use aspirating sampling. See Optical Beam Smoke Detectors: Where and How to Place Them and Aspirating Smoke Detection: When and How.
• Maintenance access at 9 m: plan for lifts or remote-test detectors.
• Racking up to the ceiling changes the airflow; recheck with the final storage layout.

Common mistakes

• Using the BS 7.5 m radius and the NFPA 9.1 m spacing interchangeably.
• Ignoring the beam depth until the ceiling is built.
• Placing detectors closer than 0.5 m to walls or beams (BS 5839-1), or less than 100 mm (4 in) from a wall (NFPA 72).

A comparison of the two codes: EN 54-14 vs NFPA 72.

For deeper study: Fire Detection and Alarm System Design course.

Check the edition of the code that applies to your project.

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