What the Building Regulations Say
Approved Document B (ADB) Volume 1 (dwellings) and Volume 2 (all other buildings) set out the smoke ventilation requirements for England and Wales. For residential blocks, ADB Volume 1 clause 2.27 requires smoke ventilation of common corridors and lobbies in buildings with a top-storey floor level above 11 metres. Below that threshold, natural ventilation of the common stair may suffice in some circumstances.
Building Types That Typically Require AOV
The following building types typically require AOV provision: residential blocks of flats above 11 m; sheltered housing and care homes; hotels and serviced accommodation above 11 m; purpose-built student accommodation; mixed-use buildings with residential upper floors; commercial buildings with enclosed stairwells or atria; and basement car parks where natural ventilation is inadequate.
The 11-Metre Rule Explained
The 11-metre threshold relates to the floor level of the highest storey, not the total building height. It is measured from the ground level at the point of fire-service access. A building with its top habitable floor at 10.5 m may not require AOV under the threshold test, but fire engineers will often recommend AOV provision for buildings approaching the threshold.
Following the Building Safety Act 2022, buildings over 18 metres (higher-risk buildings) face more prescriptive requirements and additional oversight from the Building Safety Regulator.
When Natural Ventilation Is Acceptable Instead
ADB permits natural ventilation of stairs in some lower-rise buildings. A single-stair building under 11 m with a top-hung openable vent at the head of the stair (minimum 1 m²) may meet the requirement without an AOV actuator. However, the vent must be manually openable, and this arrangement is only acceptable in limited circumstances.
Buildings over 11 m or with more than one staircase serving protected lobbies will almost always require a powered AOV system with smoke detectors, a control panel, and battery backup.
BS 9991 and Residential AOV Design
BS 9991:2021 (and its 2024 amendment) provides the design standard for fire safety in residential buildings and gives detailed guidance on smoke ventilation system design. It defines minimum vent areas, wind-effect compensation, detector placement, and the interface between the AOV system and the fire alarm system.
Frequently Asked Questions
Does a two-storey house need an AOV? No. Approved Document B does not require AOV in standard two-storey dwellings. AOV requirements apply to common areas of multi-occupancy residential buildings above 11 m and to certain commercial building types.
Can I add AOV to an existing building? Yes. Retrofit AOV installations are common, particularly in residential blocks following post-Grenfell safety reviews. The approach will depend on the existing building fabric, the size of the vent opening that can be created, and the fire strategy.
AOV Direct's technical team can help you determine whether your building requires AOV and which products are appropriate. Call 01202 725 677 or email [email protected].
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