Why Must Fire Detection Be Enhanced in Solid Wood or CLT Buildings?
Solid wood or CLT (cross-laminated timber) structures do not behave like concrete or steel when exposed to fire: wood itself constitutes a calorific load and can continue to smoulder latently, sometimes invisibly, after apparent extinction. Detection must therefore intervene as early as possible, before the fire reaches load-bearing elements or spreads into concealed spaces.
This challenge relates to a general observation about fire in Belgium: 70% of victims are killed by smoke, and half of the deaths recorded in 2025 occurred during night-time fires, a time when early detection is the only safeguard before occupants are alerted by their senses. In a timber structure, this detection delay must be even shorter given the propagation kinetics specific to the material.
What Is the Specific Risk of Concealed Spaces in Timber Construction?
Concealed spaces (false ceiling plenums, cavities between CLT panels, assembly joints, service ducts) form pathways where smoke and hot gases circulate without being detected by conventional point detection installed only in occupied volumes. In a timber structure, these voids are often in direct contact with combustible elements, which accelerates internal propagation that is difficult to detect from inside the room.
The main risk is not visible flame but temperature rise in a confined space, which can weaken a load-bearing panel before any external sign. This is why the risk analysis preceding the detection system design, according to NBN S21-100-1, must specifically identify these hidden volumes as areas to monitor, not treat them as standard technical voids.
What Does NBN S21-100-1 Provide for Detection in Timber Structures?
NBN S21-100-1 governs the design and installation of fire detection systems in Belgium, without providing a dedicated chapter on wood as a material, but it requires a risk analysis that must consider the nature of walls, actual compartmentation and concealed volumes of the building. For a solid wood or CLT structure, this analysis must lead to detection coverage extended to concealed spaces identified as critical.
The standard also requires the choice of detector type (optical, thermal, aspirating) based on the fire risk identified in each zone, not uniformly across the entire building. In timber construction, this translates concretely into denser coverage in panel junction areas and integration of detectors in service ducts crossing load-bearing walls, where standard detection would have sufficed in an equivalent concrete structure.
How Does the Royal Decree of 7 July 1994 Govern Compartmentation of a Timber Building?
The Royal Decree of 7 July 1994 establishing basic standards for fire and explosion prevention classifies buildings into low, medium and high categories, and imposes compartmentation and fire resistance requirements for walls according to this classification, regardless of the structural material used. A solid wood or CLT building must therefore meet the same compartmentation requirements as an equivalent concrete or steel building.
The practical difficulty arises from the fact that theoretical compartmentation (fire-resistant walls) can be compromised by concealed spaces internal to the timber if these are not themselves fire-stopped. Fire detection never replaces deficient compartmentation: it compensates for the actual propagation delay inside these voids, which justifies enhanced detection coverage precisely at compartment penetration points (ducts, floor penetrations, CLT panel joints).
Which Detection Technologies Should Be Prioritised in Timber Concealed Spaces?
The technological choice depends on volume, accessibility and expected propagation speed in each concealed space identified by the risk analysis. There is no single solution: the following table summarises the most relevant uses in solid wood or CLT structures.
| Technology | Recommended use in timber structure | Main limitation |
|---|---|---|
| Optical smoke point detector | Occupied volumes, accessible plenums of limited height | Poorly suited to narrow or poorly ventilated voids |
| Aspirating smoke detection (ASD) | Confined concealed spaces, service ducts, extensive false ceilings | Higher cost and maintenance, capillary tube cabling |
| Heat detector | Areas where cold smoke or wood dust disrupts optical detection (timber processing workshops) | Later detection than optical |
| Linear smoke detector | Large open volumes, glulam halls | Not suited to narrow concealed spaces |
Aspirating detection is often selected for solid wood concealed spaces because it allows continuous sampling in spaces difficult to access for maintenance, while detecting very low smoke concentrations before any visible emission.
What Role Does the Emergency Zone Play in Validating a Timber Project?
The emergency zone (hulpverleningszone) with jurisdiction over the building issues an opinion on the fire safety file before permit issuance, and this role is all the more decisive for a solid wood or CLT structure as the compartmentation rules of the Royal Decree of 7 July 1994 leave room for interpretation regarding the treatment of concealed spaces specific to this construction method.
In practice, it is recommended to involve the emergency zone from the project sketch phase, not after structural design, because certain detection choices (location of aspirating detectors, fire-stopping of voids) depend directly on architectural decisions difficult to correct at the end of construction. The operator's prevention advisor also has an interest in being involved in this consultation, as they will need to maintain the file in the building's safety register once it is in operation.
How to Organise Maintenance of a Detection System in a Timber Building?
Maintenance and periodic inspection of a fire detection system follow NBN S21-100-2, which sets verification operations and their frequency, regardless of building structure. In solid wood or CLT construction, the additional challenge is physical accessibility of detectors installed in concealed spaces, often more difficult to reach than a standard ceiling detector.
It is useful to provide, from the design stage, access hatches dedicated to maintenance of these hidden detectors, failing which periodic inspection becomes complex to carry out within the timeframes provided by the standard. Using a BOSEC-certified company for installation and a qualified technician for maintenance remains the guarantee that these access points will actually be inspected, not just the detectors visible in false ceilings or occupied volumes.