Research and analysis

Real fires: Open-sided car park fire resistance (introduction and conclusion)

Published 22 December 2025

Applies to England

Background

OFR Consultants were engaged by the Department for Levelling Up, Housing and Communities (DLUHC) to deliver the “Real Fires” project (CPD/004/122/039) in support of fire safety technical policy, which commenced on 22nd of October 2021. The contract was subsequently novated to the Building Safety Regulator (BSR), formed within the Health & Safety Executive (HSE). As part of this project, the contract makes allowance for ad-hoc research to be undertaken to support fire safety technical policy on matters that emerge through dialogue with industry or through observations from real fires. Through this mechanism, OFR were engaged to undertake research on the structural fire resistance for purpose-built open-sided car parks.

Introduction

The BSR stated that the research project was to be organised to generate data and evidence on the fire resistance ratings in car parks to support the BSR’s understanding of the level of safety in existing and new buildings, along with an analysis of options coupled with their respective supporting information to shape policy decisions. OFR proposed that this focus on above ground, purpose-built, open-sided multi-storey car parks. 

To deliver on the requirements, OFR considered three sub work packages (WPs) to be necessary, which are summarised below:

• WP 1: Investigation of fire spread rates and extents in open sided car parks. This work package was split into two parts. The first involved a literature review of fire spread in car parks alongside the development of empirical fire spread models (WP 1A). The second concerned the development of a cellular automata model that generates fire spread scenarios for open-sided car parks (WP 1B).
• WP 2: Monte Carlo fire severity analysis to investigate the probability of failure of structural elements subject to different fire resistance / protection solutions.
• WP 3: Lifetime cost-optimisation analysis for different fire resistance / protection solutions to establish the optimal safety level considering sensitivity to life safety, property protection and business continuity goals. This work package was also split into two parts. The first determined car park occupant loads which are essential for estimating potential casualties in the event of fire induced structural failure (WP 3A). The second undertook the lifetime cost-optimisation (LCO) analysis to review which potential policy interventions provided the best return on investment (WP 3B).

This final report summarises the work carried out and describes how the various work packages are interlinked to generate the research findings. Each work package report is then separately appended.

Conclusions and further work

Conclusions

The findings of this study highlight a broader question that falls outside the scope of this project to resolve: whether priority should be given to the option that is demonstrably optimal over the life of the building, or to the measures that deliver the greatest improvement in safety relative to their cost. As demonstrated in WP 3B, these two perspectives do not always coincide, particularly in the case of sprinklers. For the four- and eightstorey reference cases, sprinkler protection can achieve a comparable safety gain for its cost to that achieved by increasing structural fire resistance to 60 minutes when measured against current policy expectations. However, when considering the balance between upfront investment and the reduction in long-term damage costs, higher levels of passive protection may be justified alongside sprinkler protection.

A cautious alignment between the lifetime cost-optimisation analysis and the J-Value assessment in WP 3B suggests that enhanced passive fire protection represents a reasonable intervention, with an indicative range of approximately 60 to 75 minutes depending on the scale of the car park. This range strikes a balance between initial expenditure and lifetime damage reduction while also offering a favourable benefit-to-cost ratio relative to existing policy recommendations. It is additionally consistent with recent industry guidance published by the Institution of Structural Engineers.

Further work

The analysis makes certain assumptions regarding the fire load arising from the presence of the cars. WP 1A includes an assessment of the population of vehicles on UK roads in term of car body size but this has not been used to extend an analysis to the impact on fire load. Further work could be carried out to determine fire load similar to the work carried out by Spearpoint et al on New Zealand car parks. A revised study could investigate if and how the presence of electric vehicles has any measurable impact and could also consider how future changes in the car population of the UK might change the analysis. 

The LCO study includes a measure of the potential benefit on installing a sprinkler system to OH2 classification. Partly in response to the changes in car park fire load and also the growth in the use of EVs, hazard classifications such OH3 and HHP3 are also recommended for car parks in guidance Therefore, it would be informative to consider the potential costs and benefits of installing systems to these classifications. Work could include a review of the performance of sprinkler suppression in car parks. In addition OFR have previously proposed a series of full-scale sprinkler tests using a EV as the source to further understand the performance of sprinkler systems to OH3 and HHP3.

Finally, WP 3A has highlighted that current design occupancy figures for car parks in Approved Document B are highly conservative. A study could be undertaken to review the implications of WP 3A on means of escape provisions in car parks.