Smoke Vent Calculator

An AOV (Automatic Opening Vent) is a ventilator that opens automatically when triggered by smoke detection or a manual call point. In residential buildings, roof-mounted AOVs are typically required at the top of common staircases to vent smoke and allow escape. They must provide a minimum 1m² free area.

Approved Document B requires smoke ventilation in common staircases of blocks of flats with a single staircase, basement car parks, firefighting shafts, and corridors/lobbies in buildings over 18m tall. BS 9991 and BS 9999 provide additional guidance for complex buildings.

For a common staircase in a block of flats, the vent at the head of the stair must provide a minimum 1m² free area when fully open (Approved Document B Volume 1, para 3.52). Free area is the declared aerodynamic free area to BS EN 12101-2, or the geometric free area at the minimum cross-section (AD B Appendix D5) — not the overall unit size. Because a real louvre or window has a free-area ratio below 1, a 1m² free area usually needs a unit around 1100mm x 1100mm or larger, depending on the product's declared ratio.

Natural smoke outlets from a basement or enclosed car park should have a combined clear cross-sectional area of at least 1/40 (2.5%) of the floor area of the storey they serve, sited at high level and evenly distributed around the perimeter (Approved Document B Volume 2, paras 18.6 and 11.4). For a 200m² basement that is 200 ÷ 40 = 5m² of free outlet area. A basement that is both no more than 200m² in floor area and no more than 3m below ground level, or whose rooms have external doors or windows, needs no smoke outlets at all (para 18.3). Mechanical smoke extract is an alternative where natural venting is impractical.

Smoke ventilation systems should be inspected and tested at least every 6 months, with a full annual service by a competent engineer. The control panel, smoke detectors, AOV actuators and manual override buttons must all be tested. Records must be kept under the Fire Safety Order.

A typical staircase AOV system including the vent unit, control panel, smoke detectors and installation costs £2,500-£4,000. Smoke shaft systems for taller buildings cost £5,000-£15,000. Mechanical smoke extract for basement car parks can cost £15,000-£50,000+ depending on size.