How long can my dead end be?
How long is a piece of string? There is no definitive published answer to this. Approved Document B allows 7.5m in residential buildings. BS 9991 allows between 7.5m and 15m, depending upon the building and its fire precautions.
If you want to go beyond these lengths then improved smoke control is usually required to compensate. How much longer you can go is between you and your Building Control Body, but distances up to around 25m seem to be commonly accepted.
I’d expect you to really struggle to get approval for travel distances beyond 30m, not because we can’t design a suitable smoke control system, but because 30m is the maximum spacing between fire doors in a corridor and it’s difficult to find a justification for accepting longer dead end travel distances.
Justification and approval
Since extended travel distances are an area of design risk, we would always recommend talking to Building Control early to obtain agreement in principle.
At the detail design stage, CFD analysis is inevitably required to satisfy the authorities that the proposed system provides adequate performance. Originally CFD was simply used to show that conditions were demonstrably better than for a compliant solution, but nowadays it is more common to show compliance with set performance objectives. The SCA publication, “Guidance on smoke control to common escape routes in apartment blocks” provides guidance on what these objectives might be. It is available as a free download from www.feta.co.uk
Building control may also request physical smoke tests to be carried out upon completion to confirm compliance.
The smoke control systems
Extended travel distance systems: location is critical
In a compliant building the location of the smoke outlet from the lobby or corridor is not legislated for. As long as the system is compliant it can be located anywhere. For an extended travel distance system it’s very different. Location is critical.
On these systems:
- The inlet and extract should be as close to the opposite ends of the corridor as possible
- The direction of air flow should draw smoke away from the stairs whenever practicable
- Mechanical extract is usually preferred
- Remember that if natural inlet is used, it needs to be generously sized to avoid excessive depressurisation of the corridor.
The aim is to continuously flush through the corridor to ensure that any smoke entering from the fire room is quickly extracted, keeping the corridor fully tenable except when the fire room door is open and for a short period after it is closed.
Smoke shafts v pressurisation webinar
Watch out for further posts in this blog series, where we’ll look at other aspects of shaft system design. In the meantime, watch my CPD accredited webinar on smoke shafts v pressurisation, where I discuss:
Features and benefits of both systems
Common problems with both systems
How to choose which option is best for your project
Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.