Smoke control in common corridors at planning stage

Corridor smoke control design decides the plan of a block of flats more firmly than almost any other technical requirement, and it is usually resolved after the layout has been drawn. Approved Document B expects means of controlling smoke in the common escape routes of a block of flats, and the three ways of providing it have very different spatial consequences. Openable vents on the facade need an external wall at the right place on every storey. A natural smoke shaft needs a vertical void running the full height of the building next to the corridor. A mechanical system needs a shaft plus fans, a control panel and a secondary power supply.

Choose the wrong one at concept stage and the core either grows or the number of flats per landing falls. Both cost money that was not in the appraisal.

Why smoke control affects layout

The purpose is to keep the escape route usable while people leave and while the fire service enters. That objective sets three constraints at once: how far a flat entrance door can be from the stair, where the smoke can go, and how the corridor is subdivided.

Travel distance is the first. Approved Document B limits travel in a common corridor to a short distance where escape is possible in one direction only, and allows a much longer distance where escape is possible in two directions. The single direction figure is what produces the familiar pattern of two flats either side of a stair in a small block, and the two direction figure is what makes a through corridor with stairs at both ends efficient.

Smoke control is the second. A corridor that fills with smoke stops being an escape route, so the design has to give the smoke a route out at every storey served.

Subdivision is the third. Cross-corridor doors, lobbies and the position of the stair enclosure all follow from the smoke strategy rather than from the arrangement of flats.

Natural shafts against mechanical systems

Natural ventilation through openable vents on the facade is the simplest option and works where the corridor has an external wall on every relevant storey. Automatic opening vents are triggered by detection and the free area required is set out in Approved Document B.

A natural smoke shaft replaces the facade opening with a vertical void. Approved Document B sets the minimum cross-sectional area of the shaft at 1.5 square metres with a minimum dimension of 0.85 metres in any direction, and specifies the free areas required at the corridor opening and at the head of the shaft. Take the exact figures from the current edition, since the guidance has been amended and the free areas are easy to misremember.

A mechanical smoke extract system uses a shaft with fans, sized by a fire engineering calculation rather than by a table. It becomes necessary where the corridor is longer than the natural options allow, where the shaft cannot be positioned as the guidance requires, or where a single stair arrangement needs performance the prescriptive route cannot demonstrate.

Shaft area and its position in the core

A shaft has to sit adjacent to the corridor at every storey it serves, and it has to run vertically. Offsetting it around a structural frame or a lift overrun undermines the performance the guidance assumes, and a fire engineer will either refuse the arrangement or move to a mechanical solution with modelling to justify it.

That is a coordination problem rather than a design one. The core already contains the stair, the lift, the lobby, the riser, the electrical cupboard and the water main, and the smoke shaft arrives last. Reserving 1.5 square metres with a clear vertical path, from the lowest served storey to above roof level, is the single most useful thing to do at concept stage.

Two details that get missed: the shaft has to terminate above the roof with the vent clear of the roof surface, which produces a structure on the roof plan, and the shaft needs fire resisting construction with access for inspection.

Corridor length and door positions

Once the strategy is fixed, corridor geometry follows. In a dead-end arrangement the flats have to sit within the single direction travel distance of the stair, which limits the number of doors. In a two direction arrangement the corridor can be long, but it usually needs a cross-corridor door subdividing it, and the smoke control provision has to serve both halves.

Flat entrance doors are part of the strategy too. They have a fire resistance and self-closing requirement, and their position relative to the vent or shaft opening affects how smoke moves when one is opened. A door immediately adjacent to the shaft opening is a problem worth avoiding on the plan rather than discovering in the fire strategy review.

The practical test at concept stage is simple. Measure from each flat entrance door to the stair enclosure along the actual route, and check it against the figures in the current Approved Document. If a door fails, the plan changes, not the standard.

Plant space and maintenance access

Mechanical systems bring plant. Expect fans at roof level or in a dedicated plant room, attenuation to control noise, a control panel in an accessible location, and an electrical supply arranged so that a fire does not disable it, which usually means a secondary supply or a generator.

Maintenance access is a planning matter as much as a technical one, because a roof-level plant enclosure with a safe access route and a fall arrest arrangement appears on the elevation and in the roofscape. Show it on the submitted drawings at its real size. A plant enclosure added between permission and construction is a change that can need a formal amendment.

Noise from test running is worth designing out early. Periodic testing next to a bedroom window generates complaints even though the system runs for minutes a year.

Facade openings and elevation impact

Smoke control is visible. Automatic opening vents on a corridor facade read as louvres or as glazed panels with an unusual opening pattern, repeated on every floor at the same position. A smoke shaft head reads as a louvred box or a raised structure on the roof.

Three consequences for the elevation. The louvre position is fixed by the corridor, so it cannot be moved for composition. Louvre colour and profile matter more than usual because the element repeats vertically. And in a retained historic facade the insertion of vents is a heritage question in its own right, which sits alongside the issues discussed in facade retention and structural heritage evidence.

Explain the louvres in the Design and Access Statement rather than leaving an officer to wonder what they are. A short paragraph saying the openings are required for smoke control and showing how they have been integrated usually removes the comment before it is made.

What to fix before submission

1. The smoke control strategy: facade vents, natural shaft or mechanical extract, decided with the fire engineer rather than assumed.
2. The shaft reservation, at the area and minimum dimension the strategy needs, running vertically from the lowest served storey to above the roof.
3. Travel distances from every flat entrance door, measured along the route.
4. The position and appearance of every facade opening, drawn on the elevations.
5. Roof plant extent, height and access arrangement, drawn on the roof plan and shown in any views.
6. Whether a second staircase is required, since changes to Approved Document B have introduced that requirement for taller residential buildings with transitional arrangements attached. Check the current edition, because it changes the whole basis of the core.

Where the building is a relevant building under Article 9A of the Development Management Procedure Order, a Fire Statement accompanies the application and the Building Safety Regulator is consulted at Planning Gateway One. A Fire Safety Strategy to BS 9991 records the smoke control decisions in full. Both are fire engineering documents requiring a competent author, and an AI-assisted draft is a starting point for that professional rather than a substitute for them.

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This article is general guidance, not legal advice. Fire safety design is a matter for a competent fire engineer and for the Building Regulations, so check the current standards and the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.