Basic Means of Escape from Fire

The purpose of this page is to allow you to understand the basics of means of escape and not how to design a means of escape from fire. There are many considerations, not covered here, in planning means of escape, but it will give insight into emergency escape routes and final exit doors, otherwise known as fire exits.

What is a Fire Exit?

The Regulatory Reform (Fire Safety) Order (RRFSO) 2005, which came into force in October 2006, charges the responsible person(s) in control of non-domestic premises with the safety of everyone, whether employed in or visiting the building. Under Article 14 of the RRFSO, this duty of care includes ensuring that “routes to emergency exits from premises and the exits themselves are kept clear at all times” (14: 1) and that these “emergency routes and exits must lead as directly as possible to a place of safety” (14: 2: a). In other words, the entire escape route up to and including the final exit from a building must remain unobstructed at all times, while the distance people have to go to escape (the travel distance) must be as short as possible.

In terms of fire safety, the final exits on an escape route in a public building are known as fire exits. They may or may not be located on the usual route of traffic when the premises are operating under normal circumstances.  The final exit doors should open easily, immediately and, wherever practicable, “in the direction of escape”, i.e. outwards into a place of safety outside the building. Sliding or revolving doors must not be used for exits specifically intended as fire exits. The emergency routes and fire exits must be well lit and indicated by appropriate signs, e.g. ‘Fire Exit – Keep Clear’. In locations that require illumination, emergency lighting of adequate intensity must be provided in case the normal lighting fails, and illuminated signs used. This is because, as noted in the HM Government publication “Fire Safety Risk Assessment: Offices and Shops” (May 2006): “The primary purpose of emergency escape lighting is to illuminate escape routes but it also illuminates other safety equipment”.

Places of Relative Safety

It is often necessary to devise a temporary place of safety, such as when evacuating high buildings. This may be defined as a place of comparative safety and includes any place that puts an effective barrier (normally 30 minutes’ fire resistance) between the person escaping and the fire. Examples are as follows:

  1. A storey exit into a protected stairway or the lobby of a lobby approach stairway;
  2. A door in a compartment wall or separating wall leading to an alternative exit;
  3. A door that leads directly to a protected stair or a final exit via a protected corridor.

A staircase that is enclosed throughout its height by a fire resisting structure and doors can sometimes be considered a place of comparative safety. In these cases, the staircase can be known as a ‘protected route’. However, the degree of protection that enables staircases to be considered a place of comparative safety varies for differing building types, and is normally defined in the relevant codes of practice.

Place of Ultimate Safety

Ideally, this should be in the open air, where unrestricted dispersal away from the building can be achieved. Escape routes should never discharge finally into enclosed areas or yards, unless the dispersal area is large enough to permit all the occupants to proceed to a safe distance. (NB: a safe distance equates to at least the height of the building, measured along the ground.) Total dispersal in the open air therefore constitutes ultimate safety. When inspecting any building, it is important always to follow the escape route to its ultimate place of safety. Plus, the final exits on these escape routes (i.e. fire exits) must have sufficient capacity to ensure the swift and safe evacuation of people from the building in an emergency situation.

What is the Total Width of Fire Exits Required?

There are two main sources of guidance that should be consulted when considering the above question for your premises: the Building Regulations and British Standards.

1) Building Regulations: the maximum number of persons approach

Current building regulations contain guidance on the widths of escape routes and exits for new-build, non-domestic properties and the communal areas in purpose built blocks of flats in “The Building Regulations 2010, Fire Safety, Approved Document B, Volume 2 – Buildings Other Than Dwellinghouses, 2006 edition, incorporating 2007 and 2010 amendments”.

The following information is extracted from page 37 of the above document:

3.18: The width of escape routes and exits depends on the number of persons needing to use them. They should not be less than the dimensions given in the following table:

3.20: Widths of escape routes and exits

Maximum Number of Persons Minimum Width (mm)
60 750
110 850
220 1050
More than 220 5mm per person

In calculating exit capacity, the document notes the further points:

3.21: If a storey or room has two or more storey exits it has to be assumed that a fire might prevent the occupants from using one of them. The remaining exit(s) need to be wide enough to allow all the occupants to leave quickly. Therefore, when deciding on the total width of exits needed according to the above table, the largest exit should be discounted.

3.22: The total number of persons which two or more available exits (after discounting) can accommodate is found by adding the maximum number of persons that can be accommodated by each exit width. For example, 3 exits each 850mm wide will accommodate 3 x 110 = 330 persons (not the 510 persons accommodated by a single exit 2550mm wide).

 

2) British Standards: the risk profile approach

The current BSI “Code of practice for fire safety in the design, management and use of buildings” (BS 9999: 2008) takes a complementary approach to this calculation, based on two main factors: occupancy characteristic and fire growth rate. Combining these two factors creates the risk profile of a specific building. This means that, rather than the prescriptive formula evident in earlier BSi publications on the matter, there is scope for a much more interpretative approach, on a case by case basis, which takes into account the specific features of an individual building. This is especially significant when considering the issue of escape routes and fire exits in existing premises, particularly if they are of an historical or heritage nature.

The occupancy characteristic is principally determined according to whether the occupants are familiar or unfamiliar with the building (i.e. the difference between emergency and panic exits) and whether they are likely to be awake or asleep.

Page 26 of the Standard contains the following table (numbered 2 in the document):

Occupancy Characteristic Description Examples
A Occupants who are awake and familiar with the building Office and industrial premises
B Occupants who are awake and unfamiliar with the building Shops, exhibitions, museums, leisure centres, other assembly buildings, etc.
C Occupants who are likely to be asleep: This category is sub-divided as follows:
Ci Long-term individual occupancy Flats without 24-hour maintenance/management control on site
Cii Long-term managed occupancy Serviced flats, halls of residence, boarding schools, etc
Ciii Short-term occupancy Hotels

NB Two further categories of occupancy characteristic, “Occupants receiving medical care” (D) and “Occupants in transit” (E) are not covered by BS 9999: 2008.

The fire growth rate is estimated according to the nature and quantity of combustible materials in a specific building, as follows:

Category Fire Growth Rate Examples
1 Slow Limited combustible materials
2 Medium Stacked cardboard boxes
3 Fast Baled clothing, stacked plastic products
4 Ultra-fast Flammable liquids

NB Category 4 is not covered by BS 9999:20008

Examples of the risk profiles created by combining occupancy characteristic and fire growth rate include the following (BS 9999:2008, p 28):

A2: administration office, classroom

A2/A3/A4: storage and warehousing

B1: banking hall, reception area, foyer

B2: theatre/cinema, museum, restaurant

B3: department store, supermarket, furniture store

Cii2: dormitory, study bedroom (e.g. in halls of residence)

At pp 82/83, the document notes that the minimum door widths according to risk profile and when minimum fire protection measures are provided are as given in the table below. This is with the provisos that the total door width should be:

a)      not less than the aggregate of the exit widths given in the table;

b)      not less than 800mm, regardless of risk profile

Risk Profile Minimum width per person (millimetres)
A1 3.3
A2 3.6
A3 4.6
B1 3.6
B2 4.1
B3 6.0
C1 3.6
C2 4.1
C3 6.0

As with the Building Regulations, the British Standards guidance assumes that, if a storey has two or more storey exits, a fire might prevent the occupants from using one of them. Therefore, the remaining exit(s) need to be wide enough to allow all the occupants to leave quickly.

Example:

The total number of persons that two or more available exits can accommodate is found by adding the maximum number of persons for each exit width. For example, 3 exits, each 850mm wide, in a building with a B1 risk profile, would accommodate the following number of persons:

  • 850/3.6 = 236
  • discount one exit
  • 2 x 236 = 472 (not the 708 who could be accommodated through a single exit 2550mm [i.e. 3 x 850mm] wide in a building with a risk profile of B1)

It will be noted that this example indicates that a larger maximum number of persons (236) can be accommodated by an exit width of 850mm in a building with a low risk profile than as stated in the Building Regulations table above (110). This would suggest that the Building Regulations estimate is based on the worst case scenario from the point of view of fire growth rate.

 

The Process of Escape

Having considered the factors that will influence escape, and having seen how these can be related to the risk profile and / or occupancy levels of a specific building, it is important to look at the stages in the process of escape and the maximum distances people can be expected to travel.

Escape is generally considered in four distinct ‘Stages’ as follows

Stage 1 – escape from the room or area of fire origin

Stage 2 – escape from the compartment of origin via the circulation route to a protected stairway or an adjoining compartment offering refuge

Stage 3 – escape from the floor of origin to the ground level

Stage 4 – escape at ground level away from the building.

It is important that each floor plan of a building indicates the shortest route(s) to a place of comparative or ultimate safety should an emergency evacuation be triggered, e.g. by the sounding of the fire alarm. The width of final exit doors and the escape routes leading to them will dictate the maximum number of people who can safely occupy that floor or a specific area within it under normal conditions of operation.