"How many square metres per detector?" is the question we hear most often on site. There is a short answer, but on its own it misleads: correct layout is derived from coverage radius, not floor area. This article sets out the basic layout rules under TS CEN/TS 54-14.
Why area alone is not enough
A detector protects a radius, not an area. Smoke rises to the ceiling and then spreads horizontally; the binding constraint is the distance from any point in the space to the nearest detector.
Area figures are derived from that radius, not the other way round. The same 80 m² covered by one detector in a square room may need two or three in a long, narrow one.
Smoke detectors
Under TS CEN/TS 54-14:2018 the maximum detection radius for a point smoke detector is 6.2 metres. From this follow:
- Maximum spacing in a square array: 8.8 m
- Maximum distance to a wall or major obstruction: 4.4 m
- Effective area per detector with overlapping coverage: approximately 77 m²
In practice this usually falls to 60-70 m², because room geometry is rarely an ideal square.
Heat detectors
Heat detectors cover less, because heat reaches the sensing element less readily than smoke:
- Maximum detection radius: 4.5 m
- Maximum spacing in a square array: 6.4 m
- Maximum distance to a wall: 3.2 m
Note that the heat detector radius was reduced from 5 m to 4.5 m in the 2018 revision. Layouts copied from older projects may no longer comply.
Corridors are treated differently
In corridors no wider than 2 m, only the centre line is considered:
- Smoke detectors may be spaced at up to 12.4 m
- Heat detectors may be spaced at up to 9.0 m
- The distance to an end wall should not exceed half that spacing
Corridors are escape routes, so they are also a protection priority in their own right.
Five factors that change the layout
1. Ceiling beams
Beams deeper than 0.25 m (250 mm) are treated as separate compartments, and each bay must be protected individually. This is the most common error in open-plan offices: the ceiling is assumed flat and beam depth is never checked.
2. Ceiling slope
For detectors at or near the apex, horizontal distances may increase by 1% per degree of slope, capped at 25%. Because smoke collects at the apex, detectors are positioned close to the ridge.
3. Ceiling height
In tall spaces smoke dilutes and cools before reaching the ceiling. In warehouses, production halls and atria, beam or aspirating detection should be considered instead of point detectors. Beam detectors work over separations of up to 100 m.
4. Ventilation
Detectors must not sit near supply or extract grilles, where airflow carries smoke away and delays detection. The HVAC layout should be reviewed before detector positions are fixed.
5. Wall and obstruction clearance
Detectors are placed at least 0.5 m from walls. Air movement is weak in the ceiling-wall corner and smoke arrives late.
Three common mistakes
- Insisting on symmetry. Detectors are spaced evenly for appearance while room boundaries and beams are ignored, leaving some points doubly covered and others not covered at all.
- Forgetting the ceiling void. Cable trays, wiring and services sit above suspended ceilings. Where that volume carries fire risk, it must be protected separately.
- Ignoring room risk. The standard figures are upper limits, not targets. In a high-risk space detectors are placed more densely, not stretched to the limit.
These figures are a starting point
The values above apply to flat, unobstructed ceilings. Slope, beam depth, room risk, ventilation and detector class all change the result. Final layout is determined through room-by-room design on the floor plans.
Let us produce your detector layout
Yansis designs, installs and commissions fire detection systems. Contact our team to have a room-by-room detector layout prepared from your floor plans.
