What type of smoke detector do I need per BS 5839-6?

BS 5839-6 recommends optical smoke detectors for most habitable rooms, hallways and landings. Heat detectors are required in kitchens, garages and plant rooms where smoke detectors would cause false alarms. Multi-sensor detectors (combined smoke and heat) are increasingly popular and can reduce false alarms whilst maintaining smoke detection capability. In open-plan kitchen/living spaces, a multi-sensor detector or a combination of optical smoke and separate heat detector in the kitchen zone is recommended.

What are LD1, LD2 and LD3 categories?

These are coverage categories defined in BS 5839-6. LD3 is the minimum requirement, covering escape routes only (hallways, landings and circulation spaces). LD2 covers escape routes plus rooms presenting a higher fire risk, such as kitchens, living rooms and the principal bedroom. LD1 provides full coverage with detectors in all rooms, hallways and landings. New-build dwellings under Building Regulations typically require LD2 minimum, while HMOs and some conversions require LD1.

How far apart should smoke detectors be spaced?

Under BS 5839-6, optical smoke detectors cover a maximum of 100 square metres with standard 2.4m ceilings, giving an approximate radius of 7.5 metres. Heat detectors cover a maximum of 50 square metres, approximately 5.3 metres radius. In corridors, detectors should be no more than 15 metres apart and no more than 7.5 metres from the end of a dead-end corridor. All detectors must be at least 300mm from any wall. Coverage reduces with higher ceilings.

Do smoke detectors need to be interconnected?

Yes, in most installations. Grade D1 (the standard for new-build dwellings) requires all detectors to be interconnected so that when one activates, all detectors sound. This can be achieved through hard-wired interconnection or radio-linked wireless detectors. Grade F1 (battery-powered) also requires interconnection via radio link. Only Grade F2 permits standalone detectors, and this grade is only acceptable in existing dwellings where mains wiring is impractical.

Where should smoke detectors be positioned on the ceiling?

Detectors should be ceiling-mounted at least 300mm from any wall. On sloped or peaked ceilings, mount the detector within 600mm horizontally of the apex, but not within 150mm of the apex on steeply pitched ceilings (over 40 degrees) due to dead air pockets. If ceiling beams are deeper than 150mm, they create separate compartments and each bay may need its own detector. Detectors should not be mounted near air vents, fans or in locations subject to rapid temperature changes.

Smoke Detector Layout Calculator — BS 5839-6 Positioning

Calculate smoke and heat detector positions, types and quantities per BS 5839-6 for domestic and small commercial buildings.

Room Length

Longest dimension of the room

Room Width

Shortest dimension of the room

Ceiling Height

Floor to ceiling height

Room Type

Determines detector type (smoke vs heat)

System Grade

Power supply and interconnection standard

Detection Category

Extent of detection coverage

Number of Similar Rooms

Multiply detector count for identical rooms

How We Calculate This

This calculator determines smoke and heat detector layout requirements based on BS 5839-6 (the code of practice for fire detection in dwellings) and Approved Document B. It considers room dimensions, ceiling height, room type and the selected detection grade and category.

How it works

Detector type: Selected based on room type — smoke detectors for habitable spaces, heat detectors for kitchens and garages.
Coverage area: Calculated per BS 5839-6 Table 2, adjusted for ceiling height. Higher ceilings reduce coverage area.
Spacing: Maximum spacing between detectors is derived from the coverage area, with corridor-specific rules applied.
Grade and category: Determines power supply requirements and extent of coverage throughout the dwelling.

Key standards

BS 5839-6: Fire detection and alarm systems for dwellings
BS EN 54-5: Heat detectors — point detectors
BS EN 54-7: Smoke detectors — point detectors using scattered light, transmitted light or ionisation
Approved Document B: Fire safety requirements for buildings in England

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.