What free area does a common corridor smoke vent need in a block of flats?

Approved Document B Volume 1 gives two code-following options. A smoke vent on an external wall of the corridor or lobby needs a minimum free area of 1.5 m². Alternatively the corridor can discharge into a vertical smoke shaft, in which case the vent from the corridor into the shaft, the vent at the head of the shaft and any internal vent within the shaft each need a minimum of 1 m². A separate vent of at least 1 m² free area is provided from the top storey of the stair. BS 9991:2024 sets its own recommendations and its smoke control annex is normative, so check which document the fire strategy follows.

How long can a common corridor be before mechanical smoke ventilation is needed?

Approved Document B limits travel distance from a flat entrance door to the stair or stair lobby to 7.5 m where escape is in one direction only, and 30 m where escape is possible in more than one direction. In small single stair buildings built to Diagram 3.9 the one-direction limit drops to 4.5 m, extended to 7.5 m if smoke control is provided in the lobby. Extending a dead end beyond those limits is an engineered solution, not a code-following one. BS 9991:2024, as corrected by Corrigendum A1 in June 2025, expects extended corridors to use the two-shaft arrangement in Annex F.3.2.2 rather than a single extract shaft.

Where should a corridor AOV be positioned relative to the stair door?

Approved Document B says the vent should be as high as practicable, with its top edge at least as high as the top of the door to the stair. That stops the smoke layer banking down below the vent and spilling into the stair. On plan, the vent or shaft is normally placed remote from the stair so that smoke is drawn along the full length of the corridor and away from the stair door rather than past it. A vent sited next to the stair pulls smoke towards the very door it is meant to protect.

Do corridor AOVs and stairwell AOVs do the same job?

No. The corridor vent clears smoke that has entered the common corridor from a flat, so the escape route stays usable and the stair door can be opened without smoke following someone in. The stair vent relieves smoke and heat that has already reached the stair, and gives the fire and rescue service a way of venting it. They are sized differently, sit in different fire zones and, in Approved Document B, are triggered together on the fire floor. Removing or blocking one does not have its effect covered by the other.