Smoke Detector Spacing (and Heat Detectors): Practical Rules

Detector spacing is where most fire alarm drawings get marked up by reviewers: corners left uncovered, corridor detectors too far from the end wall, beamed ceilings treated as flat, a detector right next to a supply diffuser. The rules are simple once you see them as distance rules, not area rules. This guide gives you the EN 54-14 values, the NFPA 72 equivalents, and a full floor counted step by step.

Coverage radius and the square grid

A point detector covers every point within a set radius r. To avoid blind spots the circles must overlap, so in practice you use the square that fits inside the circle. Its side is r × √2:

Detector	Radius r	Grid side (r × √2)	Max. area per detector
Point smoke (EN 54-7)	6.2 m	8.8 m	≈ 77 m² (up to 80 m² accepted on flat ceilings ≤ 6 m)
Point heat (EN 54-5)	4.5 m	6.4 m	≈ 40 m²

Radii, distances and height limits here are the design values used in our fire alarm course, based on CEN/TS 54-14 and NFPA 72 (2022). They are typical — verify with the edition adopted locally and with the authority having jurisdiction.

The counting rule

N = ⌈L ÷ s⌉ × ⌈W ÷ s⌉

L and W are the room length and width (m), s is the grid side (8.8 m for smoke, 6.4 m for heat) and ⌈ ⌉ means round up. A 20 × 14 m office: 20 ÷ 8.8 = 2.3 → 3; 14 ÷ 8.8 = 1.6 → 2; so 6 detectors. Dividing the area (280 m²) by 77 gives 3.6 → 4 and leaves the corners uncovered. The rule is about distance, not area.

Then space the detectors evenly: actual spacing = L ÷ n along each direction, with half a spacing to each wall. Check that the farthest point of each cell – half its diagonal – is within the radius.

Small rooms: every enclosed room gets at least one detector, however small. A detector is never “shared” between two rooms across a wall. Partitions or racks reaching within 0.3 m of the ceiling count as full-height walls: treat both sides as separate rooms.

Corridors

In corridors narrower than 2 m the coverage collapses to one dimension:

• Smoke: maximum 12.4 m between detectors, end detector at most 6.2 m from the end wall.
• Heat: maximum 9 m between detectors, end detector at most 4.5 m from the end wall.
• Corridors wider than 2 m follow the normal grid (practical spacing about 10 m for smoke and 7 m for heat).
• Put a detector at every junction; never leave the corner of an L or T empty.

The quick method: n = ⌈L ÷ 12.4⌉ detectors for smoke, spaced L ÷ n apart, with L ÷ (2n) to each end. If the spacing passes, the end distance automatically passes too.

Choosing smoke or heat – and ceiling height

Use optical smoke in offices, corridors, hotel rooms and classrooms; multi-sensors where steam or cooking fumes cause false alarms; heat in kitchens, laundries, boiler rooms and enclosed car parks, with a response temperature 20–30 °C above the room’s normal maximum. Point detectors lose effectiveness as height grows, because smoke and heat dilute before they reach the ceiling. Summary of the EN 54-14 logic:

Ceiling height	Point smoke	Point heat A1	Point heat A2–G	Beam / aspirating
≤ 6 m	suitable	suitable	suitable	suitable
6–7.5 m	suitable	suitable	conditional	suitable
7.5–9 m	suitable	conditional	not suitable	suitable
9–12 m	suitable, reduced sensitivity	not suitable	not suitable	suitable
12–16 m	conditional	—	—	suitable
16–25 m	not suitable	—	—	beam (≈ 35 % sensitivity loss allowed), aspirating class A

“Conditional” means acceptable depending on use and environment, with documentation. Beam detectors cover paths up to 100 m with 6.2 m lateral coverage each side, mounted within the top 10 % of the height. In tall spaces where a warm layer can hold smoke down (stratification), add detectors at intermediate levels. Data rooms often get aspirating detection – see our guide to clean agent fire suppression for server rooms for how detection and release work together.

Walls, diffusers, beams and slopes

Distances that reviewers check

• At least 0.5 m horizontally from any wall or partition.
• A clear space of at least 0.5 m below the detector in all directions – tall racks block smoke.
• At least 0.5 m from supply diffusers and away from air velocities above 0.5 m/s. Near extract grilles is fine. A duct detector never replaces the room detector.

Beamed ceilings

With beam depth h and ceiling height H (both in m):

• h ≤ 0.10 × H: treat the ceiling as flat.
• h > 0.10 × H: beams trap smoke in bays. With bay width D: D > 0.25 × (H − h) → one detector in every bay; D < 0.25 × (H − h) → every second bay; D < 0.13 × (H − h) → every third bay.
• Mount the detector on the ceiling of the bay, within the top 10 % of H.

Sloped ceilings, voids and tall spaces

• Slopes: if the high and low points differ by less than 0.6 m, treat the ceiling as flat. Otherwise the radius may grow by 1 % per degree of slope, up to 25 %, and the first row goes at the ridge.
• Ceiling voids and raised floors with a real fire load (cable bundles – a practical threshold is 25 MJ for fire-resistant and 15 MJ for other cables) get their own detectors, a separate address and a remote indicator lamp in the room.
• Coffered ceilings: one smoke detector covers cells with a total volume up to 12 m² × (H − h); heat 6 m² × (H − h).

Worked example: one office floor

NFPA 72 in one paragraph

NFPA 72 uses a nominal smoke detector spacing of 9.1 m (30 ft); the “0.7 rule” requires every point to be within 0.7 × 9.1 = 6.4 m (21 ft) of a detector – very close to the 6.2 m above. Smoke spacing is not reduced for ceilings up to 8.5 m (28 ft); above that, expect beam, aspirating or performance-based design. Heat detectors use their listed spacing (typically 15 m / 50 ft), reduced by a table for ceilings above 3 m (10 ft). Minimum distance to a wall is 10 cm (4 in) and to a supply diffuser 0.9 m (3 ft). Beams shallower than 10 cm (4 in) are ignored; deeper beams with wide bays need a detector in every bay.

Once the detectors are placed, group them into zones and loops – see our fire alarm loop design guide – and if you are still choosing the system, read conventional vs addressable fire alarm systems. Coordinate positions with sprinklers, lights and diffusers on one reflected ceiling plan; our sprinkler spacing and location rules cover the other half of that drawing.

Common mistakes

• Counting by area ÷ 77 m² and leaving the corners uncovered.
• End detectors in a corridor more than 6.2 m (smoke) from the end wall.
• One detector “shared” between two small rooms, or none in a store or cleaner’s room.
• Treating a deeply beamed ceiling as flat.
• Detectors within 0.5 m of a supply diffuser or a wall.
• Point heat detectors on a 9 m warehouse ceiling.
• Optical smoke detectors in kitchens and car parks, followed by false alarms and a switched-off system.
• No detectors in a cable-filled ceiling void, or void detectors without remote indicators.

Quick checklist

• Detector type per room suits the environment and the ceiling height.
• Counts made per dimension (8.8 m smoke / 6.4 m heat), rounded up.
• Farthest point of each cell within 6.2 m (smoke) or 4.5 m (heat).
• Corridors: spacing ≤ 12.4 / 9 m and end distance ≤ 6.2 / 4.5 m; junctions covered.
• Every enclosed room has at least one detector.
• 0.5 m to walls and diffusers; beam, slope and coffer rules applied.
• Voids with fire load protected, with remote indicators.
• Positions coordinated with sprinklers, lights and diffusers.

FAQ

What is the maximum spacing between smoke detectors?

Under EN 54-14 rules a smoke detector covers a 6.2 m radius, which gives an 8.8 m square grid and about 77 m² per detector. NFPA 72 uses a nominal 9.1 m (30 ft) spacing with every point within 6.4 m (21 ft).

How far can a smoke detector be from the end of a corridor?

In corridors narrower than 2 m, the end detector should be no more than 6.2 m from the end wall, with no more than 12.4 m between detectors. For heat detectors the values are 4.5 m and 9 m.

How much area does a heat detector cover?

A point heat detector has a 4.5 m radius, which gives a 6.4 m square grid and about 40 m² per detector – roughly half the coverage of a smoke detector.

Why not just divide the room area by 77 m²?

Because the rule is a distance rule. Dividing the area can leave corners farther than 6.2 m from any detector; divide each room dimension by 8.8 m and round up instead.