Under usual circumstances a building is designed for a particular purpose, and the design and build process is well regulated with the outcome being a structure that remains more or less suitable during its lifetime.
But sometimes a building’s use may change and we may need to reconsider the suitability of the fire exits.
Examples of the times when we need to consider the suitability of exits are:
- An increase in capacity, e.g. a former office becoming used as a conference space, a storage area being converted to a shop;
- A change of use to a higher fire risk category;
- Where exits have been permanently obstructed;
- Temporary building works which might require closure of some routes;
- When temporary facilities such as marquees are erected;
- When building alterations or new buildings are proposed.
Where should you look for guidance?
There are a number of sources of advice for those in the UK. Although the Building Regulations apply to new buildings and alterations, they’re a good source of information. The Building Regulations Approved Document B covers the fire safety design of buildings in England and Wales. (In Scotland the equivalent is the Technical Handbook (Non-domestic) Section 2 – Fire and in Northern Ireland, NBS Technical Booklet E.)
Other places to look are the sector-specific guides to fire risk assessment issued by the Department of Communities and Local Government (and equivalents issued in Scotland) and BS9999, “Code of practice for fire safety in the design, management and use of buildings.”
What are the considerations?
In common with other sources of guidance on the subject, Approved Document B states:
The number of escape routes and exits to be provided depends on the number of occupants in the room, tier or storey in question and the limits on travel distance to the nearest exit.
There is a core principle that there should normally be at least two escape routes. However, exceptions are allowed where travel distances are short and numbers are low.
It wouldn’t be possible in a short article to explain every intricacy but taking an example of a single storey office, it could have a single entrance/exit provided the actual travel distance from the furthest point to the exit was less than 18 metres and there were less than 60 people using the office.
A note on “alternative exits”
As you’d expect, there are criteria as to what constitutes an alternative i.e. the routes must be more than 45 degrees apart (for a diagram, check out page 34 of Approved Document B).
The following table gives a general guide to the numbers of exits required by capacity:
| Maximum number of persons | Minimum number of escape routes/ exits |
|
60 |
1 |
|
600 |
2 |
|
More than 600 |
3 |
Recommended door width
Of course, it isn’t simply a matter of any old door does the job no matter what size, so as you’d expect there’s guidance on the design width of exits. Unfortunately, though, the available guidance differs greatly so that Approved Document B, for example, is not the same as BS9999 and is different again from the Department of Communities and Local Government (DCLG) guides. Here’s the table from the Approved Document B:
| Maximum number of persons | Minimum width in mm |
|
60 |
750 |
|
110 |
850 |
|
220 |
1050 |
|
More than 220 |
5mm per person |
In the DCLG guidance a 750mm door width in a normal fire risk premises has a capacity of 100 people and a 1050mm door can take 200. How do you decide which to use? Well, the Building Regulations apply to new builds and alterations, and the DCLG guidance applies to existing premises.
Where all the various guides are in agreement is that you should discount one exit, i.e. assume that a fire is affecting the largest of the alternative exits, and then make sure that the capacity of the remaining exits is sufficient. (If two exits are close together, you should discount both.)
Taking an example of an assembly room with a capacity of 200, you could fulfill the requirement of Approved Document B with two exits of a minimum dimension of 1050mm each or three at least 850mm wide.
Caution
Guidance should not be applied rigidly, so, for example, if there is a higher risk of fire or the occupants are at greater risk, then the acceptable travel distances reduce and you may need more exits. The most thorough way to apply these variances is to use BS9999, but it isn’t a document to cherry-pick from — you either apply the whole system to your fire strategy or leave it alone.