Smoke control in blocks of flats explained: vents, shafts and maintenance

Approved Document B Volume 1, paragraph 3.53, accepts that despite the fire doors, some smoke will probably get into the common corridor or lobby from a fire in a flat. So the corridors and lobbies should be ventilated, to control smoke and protect the common stair. That adds to the protection the fire doors give.

Since the 2026 amendments, where evacuation lifts are provided, the same protection extends to the evacuation lift and its lobby. The ventilation can be natural or mechanical.

Layout comes first. Someone escaping through the common parts should be able to turn away from a fire in another flat. A single escape direction from a flat door is allowed in two cases.

With a single common stair, every flat must be separated from the stair by a protected lobby or corridor, and the travel distance in one direction is no more than 7.5 metres. With two or more stairs, a flat can sit in a dead end within the same 7.5 metres. Where people can escape in more than one direction, the limit is 30 metres.

With natural ventilation, the corridor or lobby in front of each stair has a smoke vent. It goes as high as practicable, with its top edge at least as high as the top of the door to the stair. The simplest option is a vent on an external wall, with a free area of at least 1.5 square metres.

Free area means the opening the air can actually flow through, measured either as a tested aerodynamic area or as the smallest clear geometric area.

Where there's no external wall to vent through, the vent can open into a smoke shaft. The shaft is closed at the bottom, at least 1.5 square metres in cross section and at least 0.85 metres across, and built from class A1 materials. It rises at least 2.5 metres above the ceiling of the highest storey it serves, and opens at roof level, at least half a metre above anything within 2 metres of it.

The vents into the shaft, the opening at its head, and any grilles inside it, each give at least 1 square metre of free area.

The order matters. When smoke is detected in a common corridor or lobby, three vents open together: the one on the fire floor, the one at the top of the shaft, and the one to the stair. The vents on every other floor stay shut, even if smoke reaches them later, so the shaft only ever draws from the fire floor.

The vents into the shaft are either smoke-sealed fire doorsets or smoke control dampers, with the same fire resistance.

At the top storey of the stair, there should be a vent to the outside with a free area of at least 1 square metre. In a building with a single stair, smoke detectors in the common parts open the vent on the fire floor and the vent at the head of the stair. In a building with more than one stair, the vents may be opened manually.

But the vent at the head of the stair must open before, or at the same time as, the vent on the fire floor.

Small single-stair buildings get simpler rules, under paragraph 3.32. That's a top storey no more than 11 metres up, and no more than three storeys above the ground storey. Instead of corridor vents, the fire and rescue service needs a high-level openable vent of at least 1 square metre on each storey.

Or one at the head of the stair that they can open remotely from their access level. Where there's no lobby between the stair and the flats, an automatic vent of at least 1 square metre at the top of the stair should open when smoke is detected anywhere in the stair.

Mechanical systems are the other route. For pressure differential systems, which keep the stair at a higher pressure so smoke can't get in, Approved Document B points to BS EN 12101-6. Fan-driven extract systems for corridors are usually designed as engineered solutions and agreed with building control.

And in buildings with a storey more than 18 metres up, which need firefighting shafts, the smoke control for a shaft serving only flats can follow the same corridor provisions.

Many designers also follow BS 9991, the British Standard code of practice for fire safety in residential buildings, published by BSI. Its 2024 edition goes further than Approved Document B in two ways. First, it ties the system to height.

Up to 18 metres, a corridor vent straight to outside is one option. With a storey at 18 metres or more, it recommends a smoke shaft or a mechanical system, and at 30 metres or more, a mechanical system. Second, it sizes vents by aerodynamic free area, which is what a tested vent actually lets through: at least 0.9 square metres for a corridor vent to outside, and 0.7 for the stair vent that goes with it.

Smoke control has to work on the day. The Fire Safety Order requires it to be kept in efficient working order and good repair. In high-rise blocks, the Fire Safety (England) Regulations require monthly checks of smoke control systems, and faults that can't be fixed within 24 hours must be reported to the fire and rescue service.

The building plan for firefighters shows where the smoke control controls are. A check should make sure vents open freely, nothing blocks them, and the control panel shows no faults.