Fire alarm · BS 5839-1
How many smoke detectors does a room need?
How far a detector reaches, how far apart they go, how close to a wall, and how to count the detectors a room needs, with a worked example.
How far does a detector reach?
Every point in a protected area should be within 7.5 m (measured across the floor) of a smoke or CO detector, or within 5.3 m of a heat detector. Laid out on a square grid, that means about 10.6 m between smoke detectors and about 7.5 m between heat detectors. These figures are the same in BS 5839-1:2025 as in the 2017 edition.
The 10.6 m comes from the 7.5 m reach: a detector covers a square whose corners are 7.5 m away, so each side is 7.5 × √2, about 10.6 m.
Counting the detectors for a room
For an empty rectangular room with a flat ceiling, divide each side by the 10.6 m grid and round up:
- Detectors along the length = length ÷ 10.6, rounded up
- Detectors across the width = width ÷ 10.6, rounded up
- Multiply the two numbers together
Example: a room 24 m long and 12 m wide. Along the length, 24 ÷ 10.6 rounds up to 3. Across the width, 12 ÷ 10.6 rounds up to 2. That is 3 × 2 = 6 smoke detectors, each covering a box of about 8 m by 6 m.
The free detector spacing calculator does this and draws the layout. Treat the answer as a planning count, not a design. Real rooms have walls, beams, doors and furniture that change where detectors can go.
How close to a wall?
A detector should not be within 500 mm of a wall, or of anything that counts as a wall, such as a deep beam or a partition that stops short of the ceiling.
An obstruction counts as a wall if the gap above it is under 300 mm and its depth is more than 10% of the ceiling height. Keep detectors at least twice the depth away from an isolated obstruction up to 250 mm deep.
Narrow corridors
In a corridor under 2 m wide the coverage areas do not need to overlap, so smoke detectors can be about 15 m apart and heat detectors about 10.6 m. A corridor wider than 2 m is treated as a room, and the grid above applies.
Ceiling height limits
| Detector | Normal limit | Over up to 10% of the area |
|---|---|---|
| Point smoke or CO | 10.5 m | 12.5 m |
| Heat, class A1 | 9 m | 10.5 m |
| Heat, other classes | 7.5 m | 10.5 m |
A multi-sensor that works on heat and smoke only takes the heat limit.
What changed in 2025
- Category L2 now includes rooms where people sleep. The standard now recommends smoke, multi-sensor or CO detectors there, and heat detectors only if a fire risk assessment justifies them. It is not retrospective.
- Stairway lobbies now need automatic detection.
- A firmware update to the control panel counts as a modification and needs a certificate.
Common questions
How many smoke detectors do I need in a room?
For an empty flat-ceilinged room, divide each side by about 10.6 m and round up, then multiply. A 24 m by 12 m room needs 3 × 2 = 6. A design has to allow for walls, beams and other obstructions.
How far apart should smoke detectors be?
About 10.6 m on a square grid, because no point should be more than 7.5 m from a detector. In a corridor under 2 m wide they can be about 15 m apart.
How close to a wall can a smoke detector be?
Not within 500 mm of a wall, or of anything treated as a wall, such as a deep beam or a partition that stops short of the ceiling.
What is the highest ceiling a smoke detector can be fitted under?
10.5 m, or 12.5 m over up to 10% of the area, for point smoke and CO detectors.
This guide is general information for UK fire & security companies, not legal or engineering advice. Always work to the current edition of the relevant standard and your own competent-person judgement.
See this on a real certificateKerova builds the register, readings and verdict into the certificate itself.
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