Background: What Is BS 9991?
BS 9991 is the British Standard code of practice for fire safety in residential buildings. It covers new-build and existing buildings of all heights and provides detailed design guidance that goes beyond the minimum performance-based requirements of Approved Document B. For AOV system designers and installers, BS 9991 is an essential reference alongside ADB.
The standard was substantially revised in 2021 to reflect post-Grenfell lessons. The 2024 amendment builds on that revision with updates to several key areas.
1. Single-Stair Buildings
The 2024 amendment includes revised guidance on smoke ventilation in single-stair buildings — a major concern following investigations into evacuation failures in blocks where the single stair became untenable during a fire. The updated standard provides clearer design criteria for the interaction between the stair pressurisation or natural ventilation system and the lobby AOV arrangement.
2. Higher-Risk Buildings
Buildings over 18 m in height are now subject to more prescriptive requirements reflecting the Building Safety Act 2022 regime. The 2024 amendment aligns BS 9991 more closely with the Building Safety Regulator's requirements, covering the competence of those designing and installing AOV systems and the documentation required for the Golden Thread.
3. AOV System Design Criteria
The amendment updates the section on smoke ventilation design to include refined guidance on vent sizing, the calculation of free area relative to corridor and lobby geometry, and the treatment of wind pressure on natural AOV installations. Buildings in exposed locations must now account for wind-induced pressure effects more explicitly in the vent sizing calculation.
4. Testing and Commissioning
BS 9991:2024 strengthens requirements for system testing on completion and at regular intervals thereafter. AOV systems must be tested to confirm that all vents open fully, that the control panel triggers correctly on smoke detection, that battery backup maintains operation for the required standby period, and that manual override stations function correctly at every floor level.
Implications for AOV Specification
Designers and specifiers should review their standard AOV specifications against the 2024 amendment to confirm that vent sizing, detector placement, and system configuration remain compliant. AOV Direct's technical team can provide guidance on how the amendment affects common residential block arrangements.
Products supplied by AOV Direct — including the OSTRO roof vent, O-Range control panels, and battery backup units — meet the performance requirements of BS EN 12101 and are suitable for use in systems designed to BS 9991:2024.
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