Last reviewed 28 May 2021
How long should it take to evacuate your premises in the event of a fire? Mike Sopp explains the different factors that affect evacuation times.
Fire safety practitioners are often asked, particularly after a fire drill, how long it should take to evacuate the premises.
What, on the face of it, appears to be a simple question has quite a complex answer, particularly in view of the UK’s risk-based fire safety regime.
Those responsible for fire safety need to know what would be deemed to be a safe evacuation time and the factors that influence this.
Legislation, such as the Regulatory Reform (Fire Safety) Order 2005 (RRFSO), does not prescribe any evacuation times. Rather, the RRFSO states that “in the event of danger, it must be possible for persons to evacuate the premises as quickly and as safely as possible”.
The reason for this is that, unlike fire certification arrangements under the old regime, where each certificate was building specific, the RRFSO applies to a wide range of buildings, therefore making it impossible to prescribe evacuation times.
Guidance to the RRFSO does contain further information on evacuation times. For example, guidance for office environments notes that “escape routes in a building should be designed so that people can escape quickly enough to ensure they are not placed in any danger from fire”.
The key phrase here is “should be designed”. This clearly indicates that designers of buildings need to consider the design of the means of escape as part of the overall design of the property so that occupants can move to a place of reasonable and/or total safety before the conditions in the property become untenable.
Certainly, designers should be following the requirements of the respective Building Regulations and associated guidance such as that contained in Approved Document B: Fire safety, BS9999:2017 Fire Safety in the Design, Management and Use of Buildings and BS ISO 20414:2020 Fire Safety Engineering. Verification and Validation Protocol for Building Fire Evacuation Models.
It is key that the fire safety practitioner responsible for fire evacuation in a property as occupied has access to and can share necessary information in relation to the design of the means of escape are provided.
Although this information will not provide an exact evacuation time, it will enable:
fire risk assessments/analysis
the fire evacuation strategy
escape route design.
This information is particularly important given the current emphasis on the “golden thread” of information approach for buildings that is recommended following a review into the Grenfell fire.
Influences on evacuation times
Designers will take into account the building’s complexity, size and occupation when designing means of escape and ensure that the following basic principles are met:
there are sufficient routes with enough capacity and minimum travel distances that lead to a place of reasonable or total safety
escape routes are protected from the effects of fire and ingress of smoke (tenability)
there is a means of giving early warning of fire to occupants of the building.
In achieving the above, designers should also be building in factors based upon human behaviour. It is widely recognised that up to two-thirds of actual evacuation time consists of occupants delaying when the evacuation signal is given.
These factors are built into what are known as the:
available safe escape time (ASET): calculated time available between the moment of ignition and the estimated time at which conditions become untenable
required safe escape time (RSET): calculated time available between ignition and the time occupants in a specified space in a building are able to reach a place of safety.
For safe evacuation to occur, the ASET must be significantly longer than the RSET. There are a number of factors that will affect the RSET and ASET. These include the time:
from ignition to detection of fire
from detection to alarm
taken by occupants to recognise the alarm
for response once the alarm is recognised
to move to safety.
In theory the fire safety practitioner should be able to rely on the RSET and ASET and be confident that the evacuation time will be within this scope.
However, there are many factors that may affect the RSET and ASET in “real world” conditions, not least:
if the fire detection and alarm system is not maintained in a state of good repair and efficient working order
where unauthorised changes affect the means of escape and/or damage the integrity of the protection against the ingress of fire and smoke
changes in use of the property that have altered the fire risk profile or occupant profile
general wear and tear and/or lack of maintenance of elements of the means of escape
older escape design not meeting current standards of design
The fire risk assessment
If occupants are unable to evacuate in a timely manner this could result in a breach of relevant fire safety legislation and increase the risk of injury or death.
PAS 79-1:2020 Fire Risk Assessment. Premises Other Than Housing. Code of Practice, notes that “an assessment should be made of the likely consequences of fire”.
The assessment should “understand that all persons within the premises should be able to reach a place of ultimate safety before life threatening conditions arise; either unaided or with the assistance of staff — without FRS assistance (RSET versus ASET)”.
In assessing this aspect, the fire risk assessor may consider a number of inputs into the process including:
previous fire evacuation outcomes (how long it took to evacuate)
whether the fire evacuation strategy was followed
the needs of those who may require assistance
the condition of the means of escape (eg damage/unauthorised changes)
the type of fire detection and warning system in place
fire safety culture influencing behaviours.
These could then be used to determine (usually in a qualitative manner) whether the ASET/RSET requirements are being met or whether real-life evacuation takes longer, creating a situation where occupants may be at risk of harm.
The key elements of fire evacuation
All occupants must be able to reach a place of reasonable and/or total safety before the means of escape become untenable.
Information regarding the fire evacuation design and means of escape should be provided to those responsible for fire safety in the occupation stage of the building’s lifecycle. For older premise, however, such information may not be available.
As such, the duty holder/person responsible for fire safety should, through the fire risk assessment, seek assurance that the occupants can reach a place of reasonable or total safety before the buildings means of escape become untenable.
The risk assessment should take into account any factors that may influence the ASET/RSET and confirm that the time needed to evacuate the premises is within the limits that the means of escape have been designed for.
Where the fire risk assessment identifies problems with evacuation, the responsible person will need to determine the actions required to address these issues, eg influencing work culture to improve response times or material changes to the means of escape or alarm systems.
The following are available from the British Standards Institution at shop.bsigroup.com.
BS9999:2017 Fire Safety in the Design, Management and Use of Buildings
PAS 79-1:2020 Fire Risk Assessment. Premises Other Than Housing. Code of Practice
PD7974-6:2019 Application of Fire Safety Engineering Principles to the Design of Buildings. Human Factors: Life Safety Strategies. Occupant Evacuation, Behaviour and Condition
BS ISO 20414:2020 Fire Safety Engineering. Verification and Validation Protocol for Building Fire Evacuation Models.
The following is available from the UK Government website .
The Building Regulations 2010: Approved Document B. Fire Safety