The LWF Blog

Fire Engineering Design and Risk Assessment – Hazard Classifications – Part 13

September 21, 2017 9:21 am

In LWF’s current blog series on Fire Engineering Design and Risk Assessment, written for architects and others in the house design and build industry, we have been looking at the use of sprinklers as part of a fire protection plan. In part 12, we discussed those areas of a build where sprinkler use might not be appropriate and how active or passive fire protection alternatives can be used instead. In Part 13, we’ll talk about hazard classification of risks.


Fire risks are assessed and split into hazard classifications in order that the capability of a sprinkler system can be sure to be suitable for the kind of risk anticipated. There are three main classes of risk, and these are based on the expected fuel load and rate of fire growth when the building is occupied.


Light Hazard – a low combustible load with a slow rate of fire growth


Ordinary Hazard – a low to moderate load with a moderate to fast rate of fire growth


High Hazard – a high combustible load with a fast to ultra-fast rate of fire growth


While these categories might seem to encompass a lot of range, both ordinary and high hazard classifications are further subdivided to more accurately reflect the type of risk.


Classifications require that both the quantity and type of combustible materials within the building are considered, as well as the speed of development of the fire and any particular processes which might affect the conditions affecting fire development.


One occupied building may also contain a variety of different risk classifications, dependent upon the activities of that part of the premises. It is important that consideration is given to risk classification by a suitably qualified person in the fire engineering field and that the opinions and influence of the building fire insurer or other relevant authorities is included.


There are situations which may be present in the work of the building occupant for which sprinkler use may not be suitable. The use of oil and flammable liquids or gas hazards will require specialist solutions, for example, through certain water spray systems or foams.


Once the risk classification is ascertained, this will inform as to the amount of water to be delivered at the site of the fire through the sprinkler heads. The amount is referred to as the ‘design density’ and is expressed in mm per minute or litres per square metre per minute.


In part 14 of this series, we will look at the area of the activated sprinkler system and typical fuel load for premises. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.


Lawrence Webster Forrest has 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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