Smoke Detector Layout Calculator — BS 5839-6 Positioning

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.

These are coverage categories defined in BS 5839-6. LD3 covers escape routes only (the hallways, landings and circulation spaces that form the route out). LD2 covers escape routes plus the rooms presenting a higher fire risk — any kitchen and the principal habitable room (living room); LD2 does not by itself add bedroom detection. LD1 provides full coverage with detectors in all circulation areas and rooms (bathrooms and WCs are normally excluded). For England, Approved Document B treats a Grade D2 / Category LD3 system as adequate for new dwellinghouses, so LD3 is the regulatory minimum; many housebuilders specify LD2 as best practice, and HMOs and some conversions require LD1.

A point optical smoke detector has a 7.5 metre coverage radius, which works out to a 10.6 metre × 10.6 metre square — about 112 square metres, conventionally taken as 100 square metres per detector. A point heat detector has a 5.3 metre radius (a 7.5 metre × 7.5 metre square ≈ 56 square metres, taken as 50 square metres). These coverage figures are the same whether you are working to BS 5839-6 (dwellings) or BS 5839-1 (commercial) — the geometry does not change between the two parts. 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, light fitting or obstruction. Point detectors are only suitable up to a maximum ceiling height of roughly 10.5 metres (smoke); above that, beam or aspirating detection is needed.

Yes — wherever more than one alarm is fitted, all detectors must be interconnected so that one activation sounds them all. This applies across every grade (A, C, D1, D2, F1 and F2) and can be hard-wired or radio-linked. Grade D1 (mains, tamper-proof standby battery) is the usual standard for new-build dwellings. The F1/F2 difference is the battery type — F1 uses a tamper-proof primary battery and F2 a user-replaceable one — not whether interconnection is required; both are battery-powered and only suitable for existing dwellings where mains wiring is impractical.

Detectors should be ceiling-mounted at least 300mm from any wall, light fitting or obstruction. On a sloped or peaked ceiling, site a smoke alarm within 600mm measured vertically down from the apex along the slope; a heat alarm sits within 150mm of the apex. A pitch whose apex rise is below those figures is treated as a flat ceiling. A ceiling beam acts as a smoke barrier — and each bay between beams may need its own detector — once it projects more than 10% of the ceiling height below the ceiling (about 240mm at a 2.4 metre ceiling); for shallower beams the detector must be sited no nearer than twice the beam depth, and no nearer than 500mm where the beam is deeper than 250mm (this is a minimum clearance, not a cap). Detectors should not be mounted near air vents, fans or in locations subject to rapid temperature changes.