What is a construction void in fire detection?
A construction void is a space created by the building structure but not occupied during normal operation: the gap between a false ceiling and the slab, the hollow of a technical false floor, or a vertical technical duct connecting several levels. These spaces pose a specific problem: smoke can circulate there without being perceived by occupants, and a fire developing there remains invisible until it crosses a wall or ceiling.
These voids often house electrical cabling, ventilation ducts, sometimes combustible insulation materials. They can also constitute a propagation path between compartments that were supposed to be fire-tight. This is why their treatment is subject to specific criteria in the design of a fire detection installation, independently of the detection installed in occupied rooms.
In which cases is detection in a false ceiling mandatory?
Detection in a construction void is not automatic: it results from an analysis conducted by the design office based on criteria set by NBN S21-100-1, the standard for design and installation of fire detection systems in Belgium. This analysis combines several cumulative factors rather than a single universal threshold.
The main criteria are the height of the void, the presence of combustible materials or electrical cabling in significant quantities, the function of the void (simple duct passage or actual exploited technical volume), and the role of the compartmentation crossed. A false ceiling of a few centimetres without combustible load will not call for the same treatment as a high void housing dense cable trays.
This assessment must be documented in the design file and validated with the BOSEC certified installer. It also takes into account the building classification (low, medium or high) from the Royal Decree of 7 July 1994, which influences the overall requirement level of the installation.
Do technical false floors follow the same rules?
False floors, common in computer rooms, technical rooms and certain offices, are subject to the same risk analysis logic as false ceilings, with reinforced attention related to the nature of the equipment they house.
The void under technical flooring often concentrates a high density of electrical and IT cabling, continuously powered, which constitutes a risk of localized heating difficult to detect from the occupied room. In server rooms in particular, this void is generally subject to dedicated detection, separate from that of the inhabited volume, because a fire starting under the floor can develop for several minutes before being perceptible on the surface.
As with false ceilings, the decision to equip or not the void under false flooring falls to the design office, which applies the criteria of NBN S21-100-1 taking into account the height of the void and the actual combustible load, and not a standard rule applied without analysis.
What does the standard say for vertical technical ducts?
Vertical technical ducts (risers, cable ducts, lift shafts) present a particular risk: they cross several compartments and can transform a localized fire into rapid vertical propagation between floors if fire tightness is compromised.
Detection in these ducts is examined according to the compartmentation role that the duct is supposed to ensure and the presence of electrical equipment or cabling inside. When the duct crosses several levels with a significant combustible load, detection at the top of the duct or at each floor crossing is generally retained by the design office.
This point relates to the compartmentation requirements of the Code on Well-being at Work (Book III, Title 3) on fire prevention in workplaces, which requires preserving the integrity of separations between compartments, including at technical penetrations.
What design errors occur most often on site?
Errors observed on site concerning detection in construction voids are recurrent and often linked to a lack of coordination between trades rather than ignorance of the standard.
- Insufficient or poorly positioned access hatches: the detector is installed but becomes inaccessible for periodic inspection once the false ceiling is permanently closed.
- Modification of the void after design: addition of ventilation ducts or additional cabling during construction, without reassessment of the initial risk analysis.
- Absence of remote action indicator: a detector in a hidden void without a visible indicator light on the facade makes location impossible in case of alarm.
- Plans not updated: the file handed over to the operator no longer reflects the actual layout after site modifications, which complicates subsequent inspections.
- Unsuitable detector choice: a standard detector placed in a dusty void or subject to vibrations from ventilation ducts, without appropriate protection rating.
These errors are easily corrected upstream, during coordination between the design office, the installer and other trades, but become costly to fix once the false ceiling is closed.
What type of detector is suitable for these confined spaces?
In a construction void, the choice of detector must take into account the relative inaccessibility of the space and particular environmental conditions (dust, vibrations, absence of light, dense cabling).
Point smoke detectors are commonly used there, associated with a remote action indicator visible from the occupied room or corridor, to allow rapid location without having to open each hatch. In large volume voids or heavily loaded with cabling, aspirating detection may be selected, as it facilitates air sampling in hard-to-reach areas.
Whatever type is selected, the installation must remain accessible for periodic inspection and maintenance organized by NBN S21-100-2, which sets the verification procedures for proper functioning of the entire installation, including elements installed in construction voids. An invisible and inaccessible detector is a detector that cannot be maintained in compliance.
Who validates the location of detection in construction voids?
Validation of detection location in construction voids falls within a precise chain of responsibilities, which begins with the design office and ends with competent local authorities.
The design office establishes the risk analysis and design file compliant with NBN S21-100-1. The BOSEC certified installer implements this design and documents actual locations. ANPI (Association Nationale pour la Protection Incendie) can be consulted for technical advice on complex configurations. Finally, the territorially competent emergency zone (hulpverleningszone) examines the file during opinions related to environmental permits or operating authorizations, and carries out periodic on-site inspections.
This chain ensures that detection in construction voids is not decided randomly on site, but results from a traceable technical file, enforceable in case of subsequent inspection.