Last edited 14 Oct 2020

Protected stairway

1 Introduction
2 Design of protected stairway
3 Related articles on Designing Buildings Wiki
4 External references

[edit] Introduction

Escape routes provide a means of escape from any point in a building to a final exit. Escape routes can be protected or unprotected depending on whether they have a fire-resisting construction or not.

Approved Document B of the building regulations, 'Fire safety', defines a protected stairway as:

A stair that leads to a final exit to a place of safety and that is adequately enclosed with fire resisting construction. Included in the definition is any exit passageway between the foot of the stair and the final exit.

A protected stairway is designed to provide virtually ‘fire sterile’ areas which lead to places of safety outside the building. Once inside a protected stairway, a person can be considered to be safe from immediate danger from flame and smoke. They can then proceed to a place of safety at their own pace.

To enable this to be done, flames, smoke and gases must be excluded from these escape routes, as far as is reasonably possible, by fire-resisting structures or by an appropriate smoke control system, or by a combination of both these methods. This does not preclude the use of unprotected stairs for day-to-day circulation, but they can only play a very limited role in terms of means of escape due to their vulnerability in fire situations.

[edit] Design of protected stairway

Generally, every internal escape stair should be a protected stairway. They should be relatively free of potential sources of fire and so should not be used for anything else, other than providing a lift well or housing an electricity meter.

The enclosure of a protected stairway should have a fire resistance of not less than 30 min when tested in accordance with BS 476-21 or BS 476- 22, or the European equivalents BS EN 1363, BS EN 1364 or BS EN 1365.

In a building designed for phased evacuation or a stair serving any storey more than 18m high, the stair should be provided with a protected lobby or corridor or a pressure differential system. A protected corridor/lobby is, 'A corridor or lobby which is adequately protected from fire in adjoining accommodation by fire-resisting construction.'

The final exit route from a stairway should be at least as wide as the stair leading to it. Where the escape route from a ground and/or basement floors is formed from the exit route from the stairway, the width of the exit route may need to be increased accordingly and is calculated using the merging flow formula as given in BS 9999 Code of practice for fire safety in the design, management and use of buildings:

W = [(N/2.5) + (60 S)]/80

Where:

W = width of final exit, metres

N = number of people served by ground floor storey exit

S = stair width, metres

Where a protected stairway projects beyond, or is recessed from, or is in an internal angle of, the adjoining external wall of the building; then the distance between any unprotected area in the external enclosures to the building and any unprotected area in the enclosure to the stairway should be at least 1.8 m.