Smoke ventilation shafts and facade design

Smoke ventilation shaft planning problems almost always start as a coordination failure rather than a design disagreement. The fire engineer specifies a shaft and a vent at the head. The architect has already drawn the elevation. Four weeks before submission the vent turns into a 1.5 metre square louvre on the principal frontage, and the conservation officer and the planning officer both have something to say about it.

The shaft is not optional and the vent has to go somewhere. What is negotiable is where, how big, which system, and whether anybody can see it. All four are design decisions that belong at concept stage.

Why the smoke strategy reaches the elevation

A block of flats with a single common stair needs the common corridor or lobby cleared of smoke so that firefighters can work and residents can escape. Approved Document B sets out two basic arrangements for that: a natural smoke shaft with automatic opening vents from the corridor and a vent at the head of the shaft, or a mechanical smoke ventilation system using a shaft with fans.

Both arrangements terminate somewhere visible. A natural shaft needs a vent at roof level, sized by free area, and the automatic opening vent serving the stair needs its own termination. A mechanical system needs the same shaft plus fan plant, an intake path, and electrical supplies with resilience. Neither is a small object that disappears into a parapet.

For relevant buildings, the Building Safety Act 2022 framework and Article 9A of the Development Management Procedure Order 2015 require a Fire Statement with the planning application at Gateway One, with the Health and Safety Executive consulted. That report type requires someone with the necessary fire engineering competence, and PlanScope's draft is a starting point for that professional rather than a substitute. Gateway One is precisely the mechanism that brings the smoke strategy into the planning conversation, which is a good thing even when it is inconvenient.

Natural shafts, louvres and where they terminate

The natural shaft arrangement is the simpler of the two and the one that creates the more obvious elevational problem. The shaft runs the full height of the building adjacent to the corridor, with an automatic opening vent from each corridor into the shaft, and a vent at the head sized by free area. Work the dimensions from the current edition of Approved Document B rather than from a previous project, because the figures and the design conditions have been revised.

Three terminations have to be resolved on the drawings:

• The shaft head vent, usually at roof level, which can be a horizontal louvre set behind a parapet or a vertical louvre in a plant enclosure.
• The stair vent, which serves the protected stair and often sits at high level on an elevation rather than on the roof.
• Any ground floor or basement level intake required by the design.

Behind a parapet is the answer that works most often. A horizontal grille set in a roof build-up, with the parapet raised to conceal it, removes the louvre from every view except the long one from a taller building. The cost is a slightly deeper roof zone and an extra course of brickwork, which is a modest trade for an elevation that survives planning unchanged.

Mechanical systems and the plant they need

Mechanical smoke ventilation is chosen where the shaft cannot be a straight vertical run, where corridor lengths exceed what a natural system can serve, or where the design seeks to open up the plan. It is an engineered solution, usually developed under BS 9991, and it buys design flexibility at the cost of plant, maintenance and resilience.

The planning consequence most often missed is the secondary power supply. A mechanical system needs power to be available in a fire, which on many schemes means a standby generator or a battery system, and that means an enclosure, a flue if it is an engine, a fuel store, and a noise assessment. A fire strategy that quietly introduces a generator changes the planning application rather than just the services drawings.

Acoustic and visual effects of discharge points

Fans run only in a fire and in testing, so steady state noise is not the issue. Testing is, and so is the visual effect of the enclosure.

For testing, the usual approach is a condition restricting test runs to weekday daytime hours, with the sound power level of the complete package stated in the submitted information. Attenuators on the discharge and an enclosure lined to absorb are the standard measures, and both have dimensions that need to be on the roof plan from the start.

Visually, a roof level enclosure is read against the parapet line from the street and against the skyline from further away. Two moves do most of the work: set the enclosure back from the parapet by at least its own height so it falls out of the near view, and clad it in the same material as the parapet or in a recessive dark finish rather than in galvanised mesh. Show it on a verified view if the building sits in a sensitive townscape. The Design and Access Statement is where that reasoning belongs.

Heritage elevations and concealed solutions

On a listed building or in a conservation area the question becomes where the opening can go without harming significance. The answers, in the order they are usually acceptable:

1. A new rear or flank elevation, or a modern extension, taking all terminations.
2. A roof level termination concealed behind an existing parapet or within a flat roof area not visible from the street.
3. A reinstated historic opening, where documentary evidence shows one existed.
4. A new opening on a secondary elevation, detailed as a recessed metal grille in a reveal rather than a flush louvre.
5. A new opening on the principal elevation, which is the option of last resort and needs a clear case that nothing else works.

An illustrative example. A conversion of a 1890s warehouse to 24 flats, Grade II, over six storeys with a single stair. The fire engineer's first scheme put a natural shaft head vent on the principal elevation at parapet level. The agreed scheme moved the shaft to the rear of the plan, terminated the head vent horizontally behind the existing rear parapet, and took the stair vent through a blocked loading opening on the flank wall that was reinstated as a louvred metal panel in a reveal. The fire strategy performance was unchanged. This example is illustrative.

Record the reasoning in the heritage statement as well as the fire statement, because the conservation officer is reading a different document from the one the fire engineer wrote.

Coordinating with the core layout

Four coordination points decide whether the shaft can be hidden:

The shaft has to sit adjacent to the corridor it serves, which fixes its plan position within a narrow band. A core designed without a reserved shaft zone will produce a shaft that passes through a flat, which is not acceptable and is expensive to resolve late.

The shaft must be a continuous vertical run for a natural system, so it cannot jog around a lift overrun or a transfer structure. Mark it on every floor plan including the roof, at the same grid position.

Riser access and smoke damper maintenance need a route, which usually means access from the corridor rather than from inside a flat.

And the shaft competes for space with the dry riser, the wet riser if there is one, the lift shaft, the service risers and the stair. Draw all of them together at concept stage at 1:50, not later at 1:20, because at 1:20 the only available move is to lose a unit.

What to show at planning stage

The submission does not need the full engineered design, but it needs enough that the officer and the consultee can see the strategy is deliverable:

• Floor plans showing the shaft position at every level, with the roof plan showing the termination.
• Elevations showing every louvre, grille and vent at correct size, with free area noted and the material and finish stated.
• A section through the core showing the shaft from corridor to head vent.
• A roof plan showing any fan plant, attenuators, access and enclosure, with heights.
• The fire statement, prepared by someone with the appropriate competence, describing the strategy and the design standard adopted.
• Where a standby power supply is proposed, its enclosure, fuel store and a noise assessment.

If the building sits just under the Gateway One threshold the obligations differ but the design problem does not, and our note on fire evidence for a building just below the threshold covers what councils still expect. The engineered document itself is a Fire Safety Strategy, a report type where a fire engineering professional is legally required.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. The full set of report types is listed at the PlanScope report index.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.