The LWF Blog
Fire Safety Engineering for Design – Sprinkler Protection – Part 225
March 10, 2025 11:24 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 224, LWF looked at hazard classification in relation to sprinkler systems. In part 225, we discuss NFPA 13’s approach to design densities and areas of operation.
NFPA 13 takes a different approach to minimum design densities and assumed maximum area of sprinkler operation, in that they provide a selection of curves for the design density and area of operation, as follows:
Light hazard occupancy
Design density and area of operation may be selected from 4.1 litre · min–1 · m–2 over 139 m2 to 2.8 litre · min–1 · m–2 over 279 m2
Ordinary hazard, group 1 occupancy
Design density and area of operation may be selected from 6.1 litre · min–1 · m–2 over 139 m2 to 4.1 litre · min–1 · m–2 over 372 m2
Ordinary hazard, group 2 occupancy
Design density and area of operation may be selected from 8.1 litre · min–1 · m–2 over 139 m2 to 6.1 litre · min–1 · m–2 over 372 m2
Extra hazard, group 1 occupancy
Design density and area of operation may be selected from 12.2 litre · min–1 · m–2 over 232 m2 to 8.1 litre · min–1 · m–2 over 465 m2
Extra hazard, group 2 occupancy
Design density and area of operation may be selected from 16.3 litre · min–1 · m–2 over 232 m2 to 12.2 litre · min–1 · m–2 over 465 m2
The equivalency for number of sprinklers in operation will naturally vary with the selected design area of operation.
The hazard classes given are typical occupancies and can be equated to a typical fuel loading for these types of premises, taken from PD 7974-4.
It should be noted that many of these occupancies would be classed as different hazard classes in NFPA 13.
In part 226 of LWF’s series on fire engineering we will begin to look more closely at light hazard, ordinary hazard and 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.