The LWF Blog

Fire Safe Design – Means of Escape Assessment – The Basics, Pt One

July 23, 2014 8:26 am

How do I know people in my building can escape safely? How many people can I have at any one time in my premises? Where should I locate my fire exits? So many questions which architects, designers and building managers cannot fully answer until a fire safety advisor or fire engineer is involved.


The main objective of the Building Regulations, Approved Document B Fire Safety (2006 edition incorporating 2010 and 2013 amendments) (ADB), is life safety.


This series of blogs will therefore look at the various parameters which must be taken into account to ensure an appropriate level of fire safety is achieved when designing or refurbishing means of escape in buildings.


While this blog focuses on ADB as the most commonly-used design guide, it is noted that other guidance documents are available and may be better suited to your building.


Prescriptive vs Fire Engineering Solutions


The assessment of means of escape within a building, whether it is an existing or a new build design, depends upon the characteristics of that building. The approach taken for such an assessment will vary depending upon the use of the building, for example an office, a shop, a cinema, a tube station, etc.


Most commonly, the first point of action will be to follow the design guidance within ADB. Taking this approach is using what is called a compliant solution, i.e. following rigidly what the design standards recommend. Such design standards are not only limited to the Building Regulations and ADB. An approximate total of 120 documents, currently in place, can be used as a design guidance for fire safety. An alternative approach to the “prescriptive way” is fire safety engineering, which will be discussed further, later in this series.


Number of occupants


The designer must determine the maximum number of occupants who will be present in the premises at any one time. In the same way, architects carry out occupancy calculations in order to determine numerous requirements, such as the number of toilets necessary in a building, the fire consultant/engineer undertakes similar calculations to design a building’s means of escape.


ADB allows for two methods to be used when undertaking such an exercise. The first method is based upon floor space factor calculations, i.e. using a value of 6m2/person to determine how many people are likely to occupy an office building (7m2/person for a library, etc. – See ADB table 1).


As an alternative to using the values in that table, the floor space factor may be determined by reference to actual data taken from comparable premises (all values taken must be referenced). Where appropriate, the data should reflect the average occupant density at a peak trading time of year. The maximum number of people likely to occupy a building can therefore be determined, as a second method, based on what the future owner is intending to use the building for.

Moving away from design objectives and when concentrating on existing premises, the maximum occupancy capacity of a building is dependent upon the size of the escape routes. While the first method determines the number of occupants in order to define the escape routes, the second method works the other way around.


There is a common conflict to consider; less occupants equals smaller escape ways, which provides more valuable (saleable/rentable) area. However, the figures used for the design will become the maximum occupancy and being restrictive can lead to a reduced future value of the building and restricted usage, therefore a balance must be struck.


Next week’s blog will continue looking at Means of Escape and will discuss escape routes and travel distance calculations.


In the meantime, if you have any queries about this blog, or about a project upon which you are currently working, please make contact.  LWF have been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.


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