The LWF Blog
Fire Safety Engineering for Design – Sprinkler Protection – Part 226
March 17, 2025 11:25 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 225, discussed NFPA 13’s approach to design densities and areas of operation. In part 226, we begin to look more closely at light hazard and ordinary hazard risks.
Light Hazard
Premises that would be classed as light hazard are non-industrial in type and where the combustibility of building contents are low. Examples of buildings that may be classed as light hazard include medical facilities (incl. hospitals), schools, hostels etc. The maximum fire loading for this type of risk would be 400 MJ.m-2.
Ordinary Hazard
An ordinary hazard risk is likely to be commercial or industrial in nature and involve the handling, processing and storage of ordinary combustible materials, in the main. This type of ordinary hazard material is less likely to develop intensely burning fires in the initial stages. Maximum fire loading for this type of risk would be 1000 MJ.m-2.
Because Ordinary Hazard covers a wide range, the classification is further subdivided, with a broad band of fuel loading from 400 MJ.m-2 through to the 1000 MJ.m-2 quoted above.
Storage risks can produce fires with a strong upward fire plume velocity and if the installed sprinkler system does not consider this, the sprinklers may underperform. While storage of goods is permitted under this classification, it is likely to be restricted in height and quantity to avoid uncontrolled fires.
Also included in the Ordinary Hazard classification are buildings with large commercial kitchens, such as hotels, restaurants, cafes and some industrial buildings. Some kitchen risks may be controlled and mitigated with standard sprinkler installations, however, the positioning of such systems over deep fat fryers can be of particular danger.
Water droplets entering hot oil at low velocity would be likely to sink below the oil and turn to steam. The volumetric expansion rate of water to steam is in the region of 1 to 1620. The resultant expansion is equivalent to a small explosion and hot oil, capable of causing significant burns, is likely to be spread far from the source of fire origin. It is possible to get special sprinklers which operate at higher pressures to avoid this issue, but it is more common for suppression in such environments to use dry powders, foam, CO2 or water mist.
In part 227 of LWF’s series on fire engineering we will begin to look at High Hazard risks. 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 since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.