Smoke shaft provision and the shape of the building envelope

A natural smoke shaft in a residential block takes roughly 1.5 m² of cross-sectional area at every floor, with a minimum internal dimension of around 0.85 m, and it has to rise above the ceiling of the highest storey it serves before terminating at roof level. A mechanical shaft can be smaller in plan and costs plant space, a secondary power supply and a maintenance regime instead. Either way, a smoke shaft residential block design decision made at building control stage will change the core, the corridor arrangement and the roofline of a scheme that already has permission.

Approved Document B has been through several amendments, including the change relating to second staircases in taller residential buildings, so work from the current edition rather than from a previous project's fire strategy. What follows is the shape of the problem rather than a substitute for the Approved Document.

Why smoke control reaches the planning drawings

Smoke control exists to keep the common escape route usable. In a block of flats that means the corridor or lobby outside the flat entrance doors and the stair itself, and the options for achieving it are constrained by geometry rather than by preference.

The chain of consequences runs one way. Corridor length limits follow from the smoke control method. The number and position of stairs follow from corridor length. The core position follows from the stairs. The unit mix follows from the core. So a change of smoke control method at stage four rewrites the plan that was granted permission at stage two.

Travel distance in a single direction along a common corridor is tightly limited, in the order of 7.5 metres from a flat entrance door to a stair or to a point where there is a choice of direction. That single figure does more to shape a residential floorplate than any policy standard.

Natural shafts against mechanical systems

Neither option is free and neither is obviously better. Natural shafts suit compact cores with short corridors and buildings where roof plant is already congested. Mechanical systems buy plan efficiency on long floorplates and on schemes with dead-end corridors, and they pay for it with plant, resilience and the running cost the management company inherits.

The decision has to be made before the core is fixed. And it should be made by a fire engineer rather than by the architect choosing the smaller box.

Space taken at every floor and at roof level

Multiply the shaft area by the number of storeys and the arithmetic becomes a commercial question. A shaft with a builder's work zone occupying around 2.4 m² of gross internal area per floor, across a fourteen storey block, removes something in the order of 33 m² of area that would otherwise be lettable or saleable. Those figures are illustrative and will change with the core layout, but the order of magnitude is what matters when the scheme is being appraised.

At roof level the shaft has to terminate, and the termination needs free area, clearance from other openings and a position where discharging smoke will not be drawn back into the building or onto a terrace. That competes with plant, with any green roof, and with the parapet detail. Where the roof is also carrying a photovoltaic array and mechanical plant, the shaft head is the element with the least flexibility about where it goes, because its position is fixed by the core beneath.

Vents from the corridor into the shaft need free area at each floor, and their position affects the corridor wall build-up and the door positions either side of them.

Terminations, louvres and the elevation

Three visible consequences reach the elevations, and all three are matters the planning officer will comment on.

• The shaft head at roof level, which on a building with a shallow parapet becomes a visible box and may need to be treated as part of the roofscape rather than hidden.
• Louvres where a smoke outlet or a make-up air inlet meets the facade, which have a different texture and colour from the surrounding cladding and cannot be moved to suit a composition.
• The stair head vent, which on a natural system is an opening at the top of the stair enclosure and appears on the elevation whether or not the design wants it there.

Draw them at the planning stage. A submission with a clean roofline and no louvres, followed by a technical design with a 2.5 metre shaft head and four facade louvres, is a submission that invites an amendment application and a fresh round of consultation.

Coordinating with the fire statement

Where Article 9A applies, the fire statement is a submission document, and where London Plan fire safety policy applies it is expected on all major development. The smoke control strategy belongs inside it, expressed in terms a planning officer can act on: the method, the shaft positions on plan, the corridor lengths achieved, the termination positions, and the elevational consequences.

A Fire Statement is a report where a professional with the necessary fire engineering competence is legally required, and PlanScope's Fire Statement draft is a starting point for that professional rather than a replacement for them. The same applies to the Fire Safety Strategy developed later to the relevant British Standards.

Keep the fire drawings and the Design and Access Statement consistent on storey heights, core positions and roof levels. A reviewer who finds two versions of the core will ask which one is being applied for, and the answer costs weeks.

Fixing the core before submission

Four decisions should be locked before the application goes in, because each of them is hard to change afterwards.

1. The smoke control method, natural or mechanical, chosen by a fire engineer against the achievable corridor lengths.
2. The number of stairs, taking account of the current Approved Document B position on second staircases in taller residential buildings and its transitional arrangements.
3. The shaft positions on plan and their termination positions on the roof plan, drawn rather than noted.
4. The louvre positions on the elevations, with a material and colour strategy that treats them as part of the design.

Existing buildings make this harder rather than easier. In a conversion or an upward extension, the shaft has to find a route through a structure that was not designed for it, often against the grain of the existing frame, and the roof termination has to negotiate an existing parapet. Heritage fabric adds the further tension between escape provision and what can be cut, which is the subject of the note on fire safety in listed buildings.

The honest uncertainty here is the transitional position on second staircases and how individual authorities are handling schemes designed under the previous guidance. Check the current Approved Document and the authority's expectations before committing to a single stair on a building near the threshold, because the core is the one thing a section 73 application cannot easily fix.

This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.