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)
|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):
||Occupants who are awake and familiar with the building
||Office and industrial premises
||Occupants who are awake and unfamiliar with the building
||Shops, exhibitions, museums, leisure centres, other assembly buildings, etc.
||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
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:
||Fire Growth Rate
||Limited combustible materials
||Stacked cardboard boxes
||Baled clothing, stacked plastic products
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
||Minimum width per person (millimetres)
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.
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.