The Regs: External escape stairs call for fire-resistant façades

The basic principle of means of escape design is that there should always be an alternative route, or, where there isn’t, the distance occupants have to travel to safety should be limited. One exception is the design of flats, where there is effectively no limit to the height of building where a single stair is acceptable.

However, all that is set to change as the government reviews Approved Document B (ADB) once again in the latest consultation, which is due to close in March.

The consultation considers an initial preference towards 30m as the building height threshold at which a second staircase must be installed. This is at odds with the current expectations for BS9991, as this standard will likely recommend second stairs in residential buildings between 18m and 30m, unless other mitigating measures are installed, such as pressurisation.

Meanwhile, other jurisdictions (such as Scotland) require two escape stairs where buildings have floors over 11m, or even down to 7.5m in some cases. The consultation does reference cost options for 11m+, 18m+, 30m+ and 50m+ buildings – so, until it is complete, any of these heights could end up being the trigger for alternative escape routes.

As we enter a new era of liability and competence requirements under the Building Safety Act, many schemes have stalled and designers are hastily revisiting designs to see how they might incorporate a second stair. In many cases the design lends itself to incorporation of a second stair within the central core.

The guidance states that scissor-type stairs (where two different flights are located within one stairwell enclosure) are not acceptable, as smoke ingress would affect both stairs at the same time. Equally, having two stairs in close proximity will offer little benefit, as a shared lobby could equally prevent access to both if it became smoke-logged.

As a result, many designs consider a New York-style open external stair as the alternative. However, these are generally only permitted up to a maximum of 6m, due to weather protection and acrophobia (fear of heights). If the external part of the stair is under 6m in height (eg serving a podium) then there is still a requirement to ensure that the external façade provides fire protection to the stair. This not only limits the choice of external wall material but also the provision of doors or windows close to the staircase. Any external escape stair should meet all the following conditions:

a Doors to the stair should be fire-resisting (minimum E30) and be fitted with a self-closing device

b Fire-resisting construction (minimum RE30) is required for the building envelope within the following zones (see diagram below), measured:

i) 1800mm above and horizontally from the flights and landings of the external stair

ii) 9m vertically below

iii) 1100mm above the top landing of the stair

c Fire-resisting construction (minimum RE30) should also be provided for any part of the building within 1800mm of the escape route from the foot of the stair to a place of safety, unless there are alternative escape routes from the foot of the external escape stair

d Glazing in areas of fire-resisting construction should be fixed shut and itself be fire-resisting (in terms of integrity but not insulation – minimum E30).

However, even internal stairs can impose limitations on the external fabric, as some configurations of external wall could subject the external wall of a protected stairway to heat in event of a fire (for example, where the two are adjacent at an internal angle in the façade). Therefore, if a protected stairway projects beyond, is recessed from, or is in an internal angle of the adjoining external wall of the building, then the minimum distance between an unprotected area of the building enclosure and an unprotected area of the stair enclosure should be 1800mm.

In short, there is no simple solution to providing a second stair and the changing requirements in ADB will add to uncertainty and design challenges for some time.

Geoff Wilkinson is managing director of approved inspectors Wilkinson Construction Consultants