Fire Detection and Mezzanine Floors
A mezzanine is one of the most common alterations to a warehouse, shop or workshop — and one of the easiest ways to leave a fire detection system quietly out of date. Slotting a new floor into an existing space creates areas above and below it that the original detection was never designed to see, and unless someone reviews the coverage, gaps open up. A mezzanine is not a minor change to the fire alarm; it changes the very geometry the detection depends on. This guide covers how mezzanines affect detection and what to check.
The central point is that a mezzanine changes the space the detection was designed for, so coverage above and below it must be reviewed rather than assumed.
Who this is for
This is for competent fire alarm engineers dealing with buildings that have, or are gaining, mezzanine floors. The experience level assumed is competent engineer. Use it for the principles; whether and how a mezzanine is detected comes from the fire risk assessment and BS 5839-1 applied to the altered space, not the pre-mezzanine layout. The engineer's job is coverage that matches the building as it now is, above and below.
How a mezzanine changes the space
A mezzanine adds a floor level within a space, creating an area beneath it and above it that each may need detection, and it changes the geometry the original detection was designed around. A mezzanine dropped into an existing space can leave the area under it, or on it, uncovered if the detection is not reviewed. This is why a mezzanine cannot be treated as a small internal alteration from the fire alarm's point of view — it reshapes the space. Whether and how it is detected comes from the fire risk assessment and BS 5839-1 applied to the altered space. The starting recognition is simply that adding a floor changes what the detection has to cover.
The area beneath the mezzanine
The space under a mezzanine is where gaps most often hide. Often that space is effectively a separate area that ceiling detection above the mezzanine cannot see, so it may need its own detection. Whether it does depends on the space, its use and the fire risk — a low, enclosed area used for storage is very different from an open one. This under-mezzanine space is exactly the kind of gap that appears when a mezzanine is added without reviewing the detection, because attention naturally goes to the new usable floor on top. What is needed comes from the fire risk assessment and BS 5839-1 applied to the actual arrangement, considering both above and below.
A change that triggers review
A mezzanine belongs firmly in the category of changes that should prompt a detection review. Adding one changes the building's layout and often its use and fire load, and treating it as a minor alteration is precisely how coverage gaps appear. From field experience, mezzanines installed by a fit-out contractor with no thought given to the fire alarm are a common source of under-covered areas found years later at service. Any such change should be assessed against the fire risk assessment and BS 5839-1, with detection updated to suit the new arrangement, rather than assuming the existing system still covers the space adequately. The mezzanine and the fire alarm need to be considered together.
Checking mezzanine coverage on service
On service, a mezzanine is a prompt to look above and below. Confirm both the mezzanine level and the area beneath it are detected as the risk requires, that detection was updated when the mezzanine was added, and that any change of use on or under it has been considered. From field experience, areas under mezzanines left uncovered, and mezzanines added without any detection review, are the recurring findings. Record what you find, and flag any mezzanine area that the detection does not properly cover, so it can be assessed and addressed rather than remaining a hidden gap.
Common points to check
Recurring issues include the area under a mezzanine left uncovered, a mezzanine added without a detection review, and a change of use on or under it not considered. Confirming coverage above and below the mezzanine suits the current arrangement is the essential check.
When not to rely on this alone
When not to use this article: do not use it to design detection for a specific mezzanine arrangement. That comes from the fire risk assessment and BS 5839-1 applied to the altered space, by competent professionals.
Relevant standards
Detection coverage sits within BS 5839-1, a code of practice, within the fire risk assessment for the altered space. The legal duty for fire precautions in most non-domestic premises sits under the Regulatory Reform (Fire Safety) Order 2005, with Building Regulations statutory guidance in Approved Document B applying to building work. Separate the legal duty from the recommended methods, and always work to 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, or competent judgement. Verify mezzanine detection coverage against current documentation for the altered space.
Related documentation
Use this with the current BS 5839-1 and the fire risk assessment. Record coverage above and below mezzanines, and review detection whenever a mezzanine is added or its use changes.