How Far Apart Should Fire Detectors Be?

Correct fire detector positioning is essential to an effective fire detection system. Install detectors too far apart and areas may not receive appropriate coverage; install more detectors than necessary and the system may become unnecessarily complex and costly. But determining the correct spacing isn’t simply a case of measuring an equal distance between each detector. 

The distance between fire detectors depends on the type of detector, its coverage, the layout and height of the space, ceiling features and the requirements of the relevant fire detection standard where you live. 

In the UK, recommendations for the design, installation and commissioning of fire detection and fire alarm systems in non-domestic premises are set out in BS 5839-1:2025. The standard uses the concept of maximum horizontal distance from a detector rather than prescribing one universal detector-to-detector spacing. Current guidance describes a maximum horizontal distance of 7.5 m to the nearest point smoke detector and 5.3 m to the nearest point heat detector, subject to the conditions and exceptions within the standard.  

So, what does that mean when designing a fire detection system? 

How Far Apart Should Detectors Be? 

For heat detectors, the maximum horizontal distance between any point in the protected area and the nearest detector is generally 5.3 m. On a simple square-grid arrangement, this can equate to approximately 7.5 m between heat detectors, although the actual positioning will depend on the layout, ceiling conditions and other factors set out in the relevant standard. It will also depend on the specific product you are using, and you should always adhere to manufacturer guidance. 

For smoke detectors, the maximum horizontal distance is generally 7.5 m from any point in the protected area to the nearest detector. On a simple square-grid arrangement, this can equate to approximately 10.6 m between smoke detectors. Again, this should not be treated as a universal spacing rule, as ceiling height, obstructions, room geometry and other factors can affect detector positioning.  

It is also important to distinguish between fire detectors and fire alarm devices. Smoke and heat detectors are designed to detect signs of fire, whereas alarm devices, such as sounders and visual alarm devices (VADs), are designed to alert building occupants when the fire alarm system is activated. As a result, fire alarm devices do not follow the same spacing rules as detectors. Their positioning is determined by factors such as the required sound level, the layout and use of the building, ambient noise and, where applicable, the required coverage of visual alarm devices. 

Learn More About Different Detectors Here

In simple terms: smoke and heat detector positioning is primarily about achieving appropriate detection coverage, while the positioning of fire alarm devices is about ensuring occupants can be effectively warned of a fire.

Why Aren’t Detectors Just Spaced Evenly? 

The purpose of detector positioning is to ensure appropriate detection throughout the protected area, not to create a visually uniform ceiling. A theoretical square room with a flat, unobstructed ceiling is relatively straightforward. Real buildings are rarely that simple. A fire detection system might need to account for: 

• unusual room shapes 

• high or pitched ceilings 

• structural beams 

• ceiling obstructions 

• partitions 

• corridors 

• stairways 

• ventilation and air movement 

• ceiling voids 

• different types of fire risk 

Each of these factors can influence detector selection or positioning. This is why detector spacing should be considered during the fire detection system design process, rather than detectors simply being evenly distributed around a floor plan. 

How Many Detectors Do I Need? 

There’s no reliable answer based on floor area alone. 

While the dimensions of an area are clearly important, calculating the required number of detectors also requires an understanding of: 

1. Detector type
   Smoke and heat detectors have different coverage requirements.
2. Building layout
   Walls, partitions,corridors and the shape of individual spaces influence detector positioning. 
3. Ceiling height and construction
   Height, beams,pitches and other ceiling features can change how detection needs to be arranged. 
4. Fire risk
   The expected type and development of a fire can influence which detection technology is appropriate. 
5. System category
   The extent of automatic fire detection required depends on the objectives and category of the system. 

The design should therefore begin with the building’s fire risk and system requirements, not simply with a calculation of how many detectors will fit into a given square metre area.

Why Does Detector Positioning Matter? 

Correct detector placement is about more than compliance. A detector needs to be positioned where it can effectively identify the characteristics of fire it is designed to detect. Poor positioning may affect how quickly a developing fire is recognised, while inappropriate selection or siting can also contribute to unwanted alarms. 

Effective system design therefore brings together: 

• the right detector technology 

• the right detector positioning 

• appropriate coverage 

• the characteristics of the building and environment 

• the requirements of applicable standards and risk assessments 

These factors work together to create a fire detection system appropriate to the building it is intended to protect. 

So, How Do I Get Spacing Right? 

For detectors, BS 5839-based guidance commonly starts with maximum horizontal coverage distances of 7.5 m for smoke detectors and 5.3 m for heat detectors.  

But those numbers are only part of the answer. 

Detector technology, ceiling height, room geometry, beams, obstructions, corridors and the fire risk can all influence the final design. Fire detection and alarm systems should therefore be designed with these considerations in mind, with BS 5839-1:2025 regulation emphasising the designer’s responsibility in putting safety and efficacy at the centre of how a system is organised and certified. 

Want to Learn More?

Explore our products, learn more on our training hub, or find an Apollo partner to discuss the requirements of your next fire detection system. Apollo Fire Detectors has specialised in manufacturing fire detection and alarm devices since 1980.