What Is a Smoke Shaft?
A smoke shaft is a vertical duct or enclosure running from the lowest floor of a residential block to the roof, with inlet openings at each floor level serving the common lobbies or corridors. During a fire, smoke enters the shaft at the floor of origin and rises to the roof vent, which opens automatically to exhaust the smoke from the building.
Smoke shafts are particularly common in residential blocks where the corridor or lobby arrangement makes direct natural ventilation of each floor impractical.
When Is a Smoke Shaft Required?
Approved Document B and BS 9991 permit both direct ventilation of corridors (via louvres or windows) and shaft-based arrangements. Smoke shafts are typically used where the building layout does not allow for direct external ventilation of each floor's lobby, or where the distance from the lobby to an external wall makes direct ventilation impractical.
Shaft Sizing
BS 9991 provides a methodology for sizing smoke shafts based on the geometric free area of the inlet openings, the shaft cross-section, and the roof vent. The shaft must be designed so that the buoyancy of hot smoke is sufficient to drive adequate airflow through the system without mechanical assistance.
Key variables include the height of the shaft (taller shafts generate greater stack effect), the temperature of the smoke, and the free area of the inlet louvres at each floor.
Inlet Arrangement
Inlets to the smoke shaft are typically motorised louvres or dampers positioned in the lobby wall adjacent to the shaft. Only the inlet on the fire floor should open during a fire — inlets on other floors must remain closed to prevent smoke spread. This requires a multi-zone control panel with separate outputs for each floor's inlet damper.
Roof Vent Selection
The smoke shaft terminates at a roof vent, which must be sized to match the shaft cross-section and the designed airflow. AOV Direct's OSTRO roof vent range provides certified options with 140° opening for maximum free area. Our technical team can advise on vent sizing for specific shaft configurations.
Wind Effects and Exposed Locations
Natural smoke shaft systems rely on the temperature differential between the smoke and the external air to generate the driving pressure. In exposed locations, wind pressure on the roof vent can counteract this driving force. BS 9991 requires designers to account for wind pressure effects in the vent sizing calculation, and may require mechanical assistance or wind-resistant vent design in exposed situations.
Need Expert Advice?
Our engineers are available to advise on compliance and product selection.