Section 3: Design for vertical escape
Introduction
3.1 The limits on horizontal travel escape distances mean most people should be able to independently reach a protected escape route or final exit. The following guidance also includes measures for people who are unable to use stairs without help.
In larger buildings, some escape stairs may need to serve as firefighting stairs, and Section 17 will also apply.
Number of escape stairs
Mixed use buildings
3.2 If a building contains storeys, or parts of storeys, in different purpose groups, it is necessary to consider providing either of the following.
a. Separate escape routes from the areas of different use.
b. Other effective means to protect common escape routes.
Single escape stairs
3.3 A single escape stair may serve a building (or part of a building) in the following situations.
a. When independent escape routes from areas in different purpose groups are not necessary (see paragraph 3.2).
b. From a basement that is allowed to have a single escape route in accordance with paragraph 2.6b and Table 2.1.
c. In small premises, provided it meets the conditions in paragraph 4.2.
d. From a building that meets both of the following conditions.
i. It has no storey with a floor level more than 11m above ground level.
ii. It is allowed to have only a single escape route in every storey in accordance with paragraph 2.6b and Table 2.1.
e. An office building with a maximum of five storeys above the ground storey where both of the following apply.
i. The travel distance from every point in each storey does not exceed the distances given in Table 2.1 for escape in one direction only.
ii. Every storey with a floor level more than 11m above ground level has an alternative means of escape.
f. A factory comprising no more than either of the following.
i. For low risk buildings, two storeys above the ground storey.
ii. For normal risk buildings, one storey above the ground storey, provided the travel distance from every point on each storey does not exceed the distances given in Table 2.1 for escape in one direction only.
g. Process plant buildings with a maximum of 10 people.
Provision of refuges
3.4 Refuges form part of the management plan and offer relatively safe areas for people to wait for a short period only. Refuges should meet the following conditions.
a. Refuges should be provided on every storey (except ones consisting only of plant rooms) of each protected stairway providing an exit from that storey.
b. Refuges do not need to be located within the stair enclosure, but should enable direct access to the stair.
c. The number of refuge spaces does not need to equal the number of wheelchair users who may be in the building. A single refuge may be occupied by more than one person during the evacuation procedure.
3.5 The following are both examples of satisfactory refuges.
a. An enclosure such as a compartment (Diagram 3.1), protected lobby, protected corridor or protected stairway (Diagram 3.2).
b. An area in the open air, such as a flat roof, balcony, podium or similar place, that meets both of the following.
i. It is protected (or remote) from any fire risk.
ii. It has its own means of escape.
3.6 Refuges should be a minimum of 900mm 1400mm in size and accessible by someone in a wheelchair. Where sited in a protected stairway, protected lobby or protected corridor, they should not reduce the width of the escape route or obstruct the flow of people escaping.
3.7 Refuges should be provided with an emergency voice communication (EVC) system complying with BS 5839-9. It should consist of Type B outstations communicating with a master station in the building control room (if one exists) or next to the fire detection and alarm panel. In some buildings, wireless technology may be more appropriate.
3.8 Refuges and evacuation lifts should be clearly identified. In protected lobbies and protected stairways there should be a blue mandatory sign worded ‘Refuge – keep clear’ in addition to fire safety signs.
3.9 Paragraph 5.32 gives guidance on using lifts, including evacuation lifts, during a fire.
Width of escape stairs
3.10 The width of escape stairs should meet all of the following conditions.
a. It should be at least as wide as any exits giving access to the stairs.
b. It should be no less than the minimum widths given in Table 3.1.
c. It should not reduce at any point on the way to a final exit.
d. It should not exceed 1400mm in stairs taller than 30m, unless a central handrail is provided. When a central handrail is provided, the stair width on each side of it should be considered separately when assessing stair capacity.
3.11 Approved Document K requires stairs more than 2000mm wide in public buildings to have a central handrail.
3.12 If an exit route from a stair is also the escape route from the ground storey and/or basement storey, the width of the exit route may need to be increased (see paragraph 2.23).
Calculation of minimum stair width
3.13 The width depends on the number of stairs provided and the escape strategy (simultaneous or phased evacuation). If the maximum number of people needing to use escape stairs is unknown, calculate it using the floor space factors in Appendix D.
Discounting of stairs
3.14 Regardless of escape strategy, where two or more stairs are provided, it should be assumed that one might not be available during a fire. Each stair should be discounted in turn to ensure the capacity of the remaining stairs is adequate. This applies to buildings with or without a sprinkler system.
3.15 Paragraph 3.14 does not apply if either of the following applies.
a. Escape stairs are protected by a smoke control system designed in accordance with BS EN 12101-6.
b. Escape stairs are approached on each storey (except the top storey) through a protected lobby.
Despite these exceptions, at least one storey exit still needs to be discounted (paragraph 2.21).
Paragraph 3.34 identifies cases where stairs need lobby protection.
Simultaneous evacuation
3.16 The width of escape stairs should take account of the number of people using them while evacuating all storeys at the same time. The following stairs should be designed to allow simultaneous evacuation.
a. All stairs serving basements.
b. All stairs serving buildings with open spatial planning.
c. All stairs serving ‘residential (other)’ (purpose group 2(b)) or ‘assembly and recreation’ (purpose group 5) buildings.
Annexes B and C of BS 9999 include designs based on simultaneous evacuation.
3.17 The capacity of stairs of widths from 1000mm to 1800mm is given in Table 3.2.
3.18 As an alternative to Table 3.2, the capacity of stairs 1100mm wide or wider can be found using either of the following formulas:
a. P = 200W + 50 (W – 0.3)(N – 1)
b. W = P + 15N – 15 / 150 + 50N
where:
P is the number of people that can be served W is the width of the stair, in metres
N is the number of storeys served.
Separate calculations should be made for stairs serving basement storeys and stairs serving upper storeys.
The population, P, should be divided by the number of available stairs.
The formula is useful to determine the width of stairs where people are not distributed evenly – either within a storey or between storeys.
In the formula, 200W represents the number of people estimated to have left the stair after 2.5 minutes of evacuation, and 50 (W – 0.3)(N – 1) represents the number of people estimated to be on the stair after 2.5 minutes of evacuation.
Worked examples
A 14 storey building contains 12 storeys of offices (ground + 11). The top two storeys contain flats that are served by separate stairs. What is the minimum width needed for the stairs that serve the office floors, for simultaneous evacuation? In the 11 above-ground-floor offices, 1200 people use the stairs. (People in the ground floor offices do not use the stairs.)
In this example, two stairs are shown to satisfy the travel distance limitations.
a. The population is distributed evenly
The top office storey is at a height greater than 18m, therefore both stairs need lobby protection (see paragraph 3.34). Because both stairs are entered at each level via a protected lobby, both stairs can be assumed to be available (see paragraph 3.15).
P = 1200/2 = 600, N = 11
From the formula:
600 = 200W + 50 (W – 0.3)(11 – 1)
600 = 200W + (50W – 15)(10)
600 = 200W + 500W – 150
750 = 700W W = 1070mm
Therefore both stairs should be at least 1070mm wide. But this needs to be increased to 1100mm, because the formula applies to stairs 1100mm wide or wider (see paragraph 3.18).
This width will also be adequate when one storey exit is discounted as described in paragraph
2.21. It also complies with paragraph 3.10a (i.e. the stair widths are not less than the minimum widths needed for 110 people in Table 2.3).
b. The population is not distributed evenly
(e.g. 1000 people occupy floors 1 to 9, and 200 occupy floors 10 and 11).
The top office storey is at a height greater than 18m, therefore both stairs need lobby protection (see paragraph 3.34). Because both stairs are entered at each level via a protected lobby, both stairs can be assumed to be available (see paragraph 3.15).
To find the width of:
• the stairs serving floors 10 and 11:
P = 200/2 = 100, N = 2
From the formula:
100 = 200W + 50 (W – 0.3)(2 – 1)
100 = 200W + (50W – 15)(1)
100 = 200W + 50W – 15
115 = 250W W = 460mm
Therefore both stairs between the 9th floor landing and the top floor should be at least 460mm wide. But this needs to be increased to 1100mm, because the formula applies to stairs 1100mm wide or wider (see paragraph 3.18).
This width will also be adequate when one storey exit is discounted as described in paragraph
2.21. It also complies with paragraph 3.10a (i.e. the stair widths are not less than the minimum widths needed for 100 people in Table 2.3).
• the stairs serving floors 1 to 9:
P = 1200/2 = 600, N = 9
From the formula:
600 = 200W + 50 (W – 0.3)(9 – 1)
600 = 200W + (50W – 15)(8)
600 = 200W + 400W – 120
720 = 600W W = 1200mm
Therefore both stairs between the ninth floor landing and the ground floor should be at least 1200mm wide.
This width will also be adequate when one storey exit is discounted as described in paragraph
2.21. It also complies with paragraph 3.10a (i.e. the stair widths are not less than the minimum widths needed for 134 people in Table 2.3).
Phased evacuation
3.19 Phased evacuation cannot be used in every type of building, but can be advantageous for escape stairs in high buildings. It requires supporting facilities, such as fire detection and alarm systems, to be provided and maintained.
In a phased evacuation, the first people to be evacuated are those with reduced mobility and those on the storey most immediately affected by the fire. If needed, subsequent evacuation is done two floors at a time, reducing disruption in large buildings.
Phased evacuation enables stairs to be narrower than with simultaneous evacuation, and may be used for any building provided it is not identified in paragraph 3.16.
3.20 Phased evacuation in buildings over 30m in height introduces the potential for escaping people to impede firefighters entering and operating within the building. This can be addressed by consulting with the fire and rescue service about special management procedures.
In very tall buildings, typically over 45m in height, physical measures may need to be incorporated, such as by discounting a stair.
3.21 A building (or part of a building) designed for phased evacuation should satisfy all of the following criteria.
a. At each storey except a top storey, stairs should be approached through a protected lobby or protected corridor.
b. At each storey, the lifts should be approached through a protected lobby (see paragraph 5.37).
c. Every floor should be a compartment floor (REI depending on height and use of the building).
d. If there is a storey with a floor over 30m above ground level, the building should be protected throughout by an automatic sprinkler system in accordance with Appendix E.
e. The building should be fitted with an appropriate fire warning system conforming to at least the L3 standard given in BS 5839-1.
f. An internal speech communication system should provide communication between a control point at fire and rescue service access level and a fire warden on every storey. The
recommendations for phased evacuation provided in BS 5839-1 should be followed. Where it is deemed appropriate to install a voice alarm, this should be in accordance with BS 5839-8.
3.22 The minimum width of stairs needed for phased evacuation is given in Table 3.3.
Worked example using Table 3.3
What is the minimum width needed for the stairs serving a 15 storey office building (ground + 14 office floors), assuming a total population of 2500 people (excluding the ground floor population, which does not use the stairs)? To satisfy the travel distance limitations, three stairs are required.
The building is over 45m in height and designed for phased evacuation. It has been decided to discount one stair to take account of fire and rescue service operations as described in paragraph 3.20. Therefore:
• Number of people per storey = 2500/14 = 179.
Each remaining stair must be able to accommodate half the population of one storey (i.e. 90 people).
Thus each stair should be 1000mm wide (maximum capacity 100 people).
This width will also be adequate when one storey exit is discounted as described in paragraph 2.21. It also complies with paragraph 3.10a (i.e. the stair widths are not less than the minimum width needed for 90 people in Table 2.3).
• At least one of those stairs needs to be a firefighting stair, therefore a minimum width of 1100mm is needed (see note 2 to Table 3.1).
Additional worked example using Table 3.3
What is the minimum width needed for the stairs serving a 9 storey office building (ground + 8 office floors), assuming a total population of 1920 people (excluding the ground floor population, which does not use the stairs)? To satisfy the travel distance limitations, two stairs are required.
As both stairs need to be entered at each level through a protected lobby (see paragraph 3.21), both stairs can be assumed to be available (see paragraph 3.15). Therefore:
• Number of people per storey = 1920/8 = 240.
• Each stair must be able to accommodate half the population of one storey (i.e. 240/2 = 120 people).
• Thus both stairs would require a width of 1100mm (maximum capacity 120 people) according to Table 3.3, but:
• Each storey exit needs to be able to serve 240 people, because of discounting as described in paragraph 2.21. The minimum exit width needed for 240 people in Table 2.3 is 1200mm. As described in paragraph 3.10a, the stair should be at least as wide as the storey exit serving it.
• The required stair width is therefore 1200mm.
Design and protection of escape stairs
Enclosure of escape stairs
3.23 Every internal escape stair should be a protected stairway (within a fire resisting enclosure). If it is also a protected shaft or firefighting shaft, additional guidance in Sections 8 and 17 applies.
There is one exception: an unprotected stair (e.g. an accommodation stair) may form part of an internal route to a storey exit or final exit, provided that the distance of travel and the number of people involved are very limited. For example, small premises (Section 4) and raised storage areas (see paragraphs 7.6 and 7.7).
Construction of escape stairs
3.24 The flights and landings of escape stairs should be constructed of materials achieving class A2-s3, d2 or better in all of the following situations.
a. If the escape stair is the only stair serving the building or part of the building, unless the building has two or three storeys and is an office building.
b. If the escape stair is within a basement storey.
c. If the escape stair serves any storey that has a floor level more than 18m above ground or access level.
d. If the escape stair is external, except where the stair connects the ground floor or ground level with a floor or flat roof a maximum of 6m above or below ground level.
e. If the escape stair is a firefighting stair.
Materials achieving class B-s3, d2 or worse may be added to the top horizontal surface, except on firefighting stairs.
3.25 Further guidance on firefighting stairs is given in Section 17. Dimensional constraints on the design of stairs are given in Approved Document K.
Single steps
3.26 Single steps on escape routes should be prominently marked. A single step on the line of a doorway is acceptable, subject to paragraph 5.22.
Helical stairs and spiral stairs
3.27 Helical stairs and spiral stairs may form part of an escape route provided they are designed in accordance with BS 5395-2. If they are intended to serve members of the public, stairs should be type E (public) stairs.
Fixed ladders
3.28 Fixed ladders should not be provided as a means of escape for members of the public. They should only be provided where a conventional stair is impractical, such as for access to plant rooms which are not normally occupied.
External walls adjacent to protected stairways
3.29 With some configurations of external wall, a fire in one part of a building could subject the external wall of a protected stairway to heat (for example, where the two are adjacent at an internal angle in the façade, as shown in Diagram 3.3).
3.30 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.
External escape stairs
3.31 Where a storey or part of the building has more than one escape route available, some of the escape routes may be via an external escape stair, provided the following conditions are met.
a. There is at least one internal escape stair from every part of each storey (excluding plant areas).
b. In the case of an ‘assembly and recreation’ (purpose group 5) building, the route is not intended for use by the public.
c. In the case of a ‘residential (institutional)’ (purpose group 2(a)) building, the route serves only office or residential staff accommodation.
3.32 Any external escape stair should meet all of the following conditions (Diagram 3.4).
a. Doors to the stair should be fire resisting (minimum E 30) and be fitted with a self-closing device, except for a single exit door from the building to the top landing of a downward- leading external stair, provided it is the only door onto the landing.
b. Fire resisting construction (minimum RE 30) is required for the building envelope within the following zones, measured from the flights and landings of the external stair.
i. 1800mm above and horizontally.
ii. 9m vertically below.
iii. 1100mm above the top landing of the stair (except where the stair leads from basement to ground level).
c. Fire resisting construction (minimum RE 30) should be provided for any part of the building (including doors) within 1800mm of the escape route from the foot of the stair to a place of safety. This does not apply if there are alternative escape routes from the foot of the external escape stair.
d. Stairs more than 6m in height should be protected from adverse weather. Protection should prevent the build-up of snow or ice but does not require full enclosure.
e. Glazing in areas of fire resisting construction should be fixed shut and fire resisting, in terms of integrity but not insulation (minimum E 30).
3.33 Access to an external escape stair may be via a flat roof, provided the flat roof meets the requirements of paragraphs 2.31 and 2.32.
Access lobbies and corridors
3.34 In the following situations, protected lobbies or protected corridors should be provided at all storeys above ground, except the top storey.
a. If the stair is the only one serving a building or part of a building that has more than one storey above or below the ground storey.
b. If the stair serves any storey at a height of 18m or more above ground level.
c. If the building is designed for phased evacuation.
d. If the stair is a firefighting stair.
e. If the option in paragraph 3.15b has been used so as not to discount one stair when calculating stair widths.
As an alternative to (a) to (c), a smoke control system as described in paragraph 3.15a may be used.
3.35 A protected lobby should be provided between an escape stair and a place of special fire hazard to protect from the ingress of smoke. The lobby should have a minimum 0.4m2 of permanent ventilation, or be protected by a mechanical smoke control system.
Exits from protected stairways
3.36 Every protected stairway should lead to a final exit, either directly or via an exit passageway.
Any protected exit corridor or stair should have the same standard of fire resistance and lobby protection as the stair it serves. The exit from a protected stairway should comply with paragraphs 5.20 to 5.24.
Separation of adjoining protected stairways
3.37 The construction separating two adjacent protected stairways (or exit passageways) leading to different final exits should be imperforate.
Use of space within protected stairways
3.38 A protected stairway may only include any of the following.
a. Sanitary accommodation or washrooms, as long as the accommodation is not used as a cloakroom. A gas water heater or sanitary towel incinerator may be installed in the accommodation, but no other gas appliance.
b. If the protected stairway is not a firefighting stair: a lift well.
c. If the protected stairway is not the only stair serving the building or part of the building: a reception desk or enquiry office area at ground or access level. The reception or enquiry office area should have a maximum area of 10m2.
d. If the protected stairway is not the only stair serving the building or part of the building:
cupboards enclosed with fire resisting construction.
Gas service and installation pipes in protected stairways
3.39 Gas service and installation pipes and meters should not be within a protected stairway, unless installed in accordance with the Pipelines Safety Regulations 1996 and the Gas Safety (Installation and Use) Regulations 1998.
Basement stairs
3.40 An escape stair forming part of the only escape route from an upper storey should not continue down to serve a basement storey. The basement storey should be served by a separate escape stair.
3.41 Where multiple escape stairs serve the upper storeys, only one needs to end at ground level. Other stairs may connect with the basement storeys if there is a protected lobby or a protected corridor between the stairs and accommodation at each basement level.
