The most dangerous fire is often the one nobody can see. A fire starting above a suspended ceiling or under a raised floor can grow and travel through the building unnoticed, bypassing all the detection carefully placed in the rooms below. This guide explains when voids and concealed spaces need detection, why they are a hidden risk, and the practical realities of access and servicing.
The judgement here is about matching detection to genuine risk — protecting the voids that matter without burdening the system with detectors nobody can reach.
Who this is for
This is for competent fire alarm engineers who install or service void detection, or advise on it. The experience level assumed is competent engineer. Use it for the principles; whether a specific void needs detection comes from the fire risk assessment, BS 5839-1 and the design.
Why voids are a risk
Concealed spaces — voids above suspended ceilings, beneath raised floors, within risers and roof spaces — hide fire from view. A fire that starts or spreads in one can develop well before it becomes visible in the occupied space, and the detection protecting that occupied space may not see it until it breaks through. Voids also tend to concentrate cabling and services, adding both fuel and potential ignition sources. That combination of hidden growth and concentrated fire load is what makes the right voids worth protecting.
When voids need detection
A void may need detection where it presents a fire risk or offers a route for fire and smoke to spread — for instance where it holds significant cabling or combustible material, or connects areas such that an undetected fire could spread between them. Whether a particular void needs detection is a design decision, made against the fire risk assessment, the recommendations of BS 5839-1, and Building Regulations guidance in Approved Document B on concealed spaces and cavity barriers. It depends on the void's contents, size and role in fire spread — not a blanket rule.
Access and remote indication
Detectors in voids are still detectors: they must be accessible for testing and maintenance. Good design provides that access, and remote indicators are commonly fitted outside the void — on the ceiling or wall below — to show which concealed detector has operated, so a responder is not left searching an unseen space. Poor access is one of the most common real-world problems with void detection. Where a void detector exists, confirm it can actually be reached and tested, and that any remote indicator correctly identifies the device.
What changed in BS 5839-1:2025
The Fire Industry Association's guide to the 2025 edition reports three changes that bear directly on voids and the ceilings around them:
- Remote indicators. Every remote indicator's visual indication should be red, and remote indicators should be checked for correct operation at the annual service visit, so ones that have been obstructed or painted over are caught (FIA guide, section 12).
- Where to put a void detector. The 2025 edition adds a figure showing the position and depth in the void at which the detector should be mounted (FIA guide, section 18). Use that figure from the standard itself.
- Obstructions near the ceiling. Ducting or similar close to the ceiling should be treated as a wall when the gap between its top and the ceiling is less than 300 mm and it is deeper than 10% of the overall ceiling height. Detectors should not be mounted closer to beams, ducts, light fittings or other ceiling attachments (up to 250 mm deep) than twice the depth of the obstruction, and features with a gap of more than 300 mm above them are not regarded as obstacles to smoke flow. "Closely spaced" beams are now defined as about 1 m or less centre to centre (FIA guide, section 9).
These are the FIA's summary of the changes; confirm the wording and figures against BS 5839-1:2025 itself before relying on them for a design.
Not over-detecting
The opposite mistake is putting detectors in every void regardless of need. Small voids, or those with no significant fire load and no role in fire spread, may not warrant detection, and over-detecting creates a maintenance and access burden for little safety gain. The right answer is decided per void by a competent designer, balancing the genuine fire risk against practicality. From field experience, the recurring issues are void detectors that cannot be reached for service, and remote indicators that no longer correctly map to their concealed detector after changes.
Common points to check
Recurring issues include inaccessible void detectors, missing or mislabelled remote indicators, and voids altered by later services work without the detection being reviewed. Confirming access and correct indication for any void detection is worthwhile on service visits.
When not to rely on this alone
When not to use this article: do not use it to decide which voids need detection, or to design void coverage for a specific building. Those come from the fire risk assessment, the current BS 5839-1 and Approved Document B, applied by a competent designer.
Relevant standards
Detection of voids and concealed spaces is addressed within BS 5839-1, a code of practice, with concealed-space and cavity provisions in Building Regulations statutory guidance in Approved Document B. The legal duty to provide suitable detection where necessary in most non-domestic premises sits under the Regulatory Reform (Fire Safety) Order 2005. Separate the legal duty from the recommended methods when advising a client, and always work to the current editions.
Professional disclaimer
This is an educational and workflow resource for competent engineers and does not replace the current British Standards, the fire risk assessment, the fire strategy, or competent design judgement. Verify void detection decisions against current documentation.
Related documentation
Use this with the current BS 5839-1 and Approved Document B, the fire risk assessment, and the system design and as-fitted drawings. Confirm access and remote indication for void detectors and record checks in the service documentation.