Research and analysis

Fire spread over pitched roofs fitted with solar panels (summary)

Published 22 December 2025

Applies to England

Introduction

Research aim

The aim of this work is to study how the installation of photovoltaic (PV) panels affects the fire spread over pitched roofs in residential buildings.

To achieve this, a series of 11 large scale experiments have been carried out at the Health and Safety Executive’s science and research centre. This work has been commissioned by the technical policy team within the Building Safety Regulator (BSR) and aims to assist them in determining if further guidance is required in volume 1 of Approved Document B ^{[footnote 1]}. This is the statutory guidance document published by the UK Government under the Building Regulations 2010, specifically addressing fire safety in buildings in England.

Previous incidents ^{[footnote 2]} and experiments (^{[footnote 3]} and ^{[footnote 4]}) have suggested that the installation of PV panels may increase fire spread over a roof. BSR was concerned that this could cause fire spread to adjacent buildings, over fire compartment lines and potentially into the building itself through elements such as skylights. This project was set up to determine what design elements increase fire spread over the roof and to record data on these types of fires.

Purpose

This experiment update report provides an summary of initial findings from experimental testing which has been completed. This document has been produced to share initial findings of this work ahead of the publication of the full report.

To achieve this, the information within this report is based on observations made by those conducting the tests and some limited findings from the initial interpretation of the data. The full report for this work has not yet been produced, and data interpretation and validation are not fully complete for all tests, therefore this report is issued with the caveat that findings may evolve as the final report is produced.

Discussion and summary

Building applied photovoltaic (BAPV) panels

The series of experiments showed that on plastic backed PV panels (class C, IEC 61730-2 ^{[footnote 5]}) there was significant vertical fire spread up the array reaching the top of the panels, but the fire did not spread significantly horizontally, and the fire did not pass between the panels in this direction.

Fire spread was much less significant on the glass backed panels (class A, IEC 61730-2 ^{[footnote 5]}) and it did not extend past the first PV panel. Although the panel materials did become involved the fire, the flames did not extend significantly from the area around the ignition crib.  

One experiment was carried out above plastic roof tiles which achieved a classification of BROOF(t4), when classified in BS EN 13501-5 ^{[footnote 6]} (without PV panels installed). Flame spread on this sample was much greater than all other tests in this work with the entire area under the solar panels rapidly becoming involved in the fire and large flames produced above the top of the roof.

One variable which was observed to be relevant in this series of experiments is the position of the support rails on the roof systems. Along the aluminium edge material surrounding the PV panels these mounting rails appeared to slow the spread of the fire up the rear surface of the panel. It was observed that they caused the fire to be forced down away from the rear surface of the panel and so acted as a fire break.

Building integrated photovoltaic (BIPV) panels

In these samples the significant amount of wooden support battens and the plastic trays provided a large fuel source for the fire. Significant vertical and horizontal flame spread was observed in all of the plastic tray integrated systems with flames extending above the top of the roof. The bespoke integrated system showed less vertical flame spread however, we believe this to be due to there being less fuel available in the cavity below the panels.

It was observed that these fires were significantly harder to extinguish than the above roof systems. Part of the fire was contained in the cavity behind the waterproof layer of the roof fully sealed by the solar panels and flashing, meaning it was harder to apply water to this area unless the fire had burnt an opening in these materials. This finding is highlighted as it may be relevant to the fire service when attending these incidents.

Summary

In summary, all samples which incorporated PV panels had greater flame spread than the control sample. Even where the PV panel provided limited fuel to the fire (as in sample 5), the cavity created by the solar panel allowed the flames to reach higher up the sample. 

For the BAPV samples above concrete roof tiles there was significant vertical flame spread but very limited flame spread horizontally to the adjacent PV panel. The back material of the glass backed panels contributed less to the fire than the plastic backed panels which provided a thermally thin layer of fuel for the fire. The plastic roof tiles used on sample 4 showed very rapid flame spread and quickly allowed for other roof materials to become involved in the fire that was not expected, given their high fire classification in BS EN 13501-5 ^{[footnote 6]}.

When compared to BAPV systems, with the exception of sample 4 (plastic roof tile sample), the BIPV samples showed greater fire spread both horizontally and vertically, although there were some differences between flame spread on the different types of integrated system.

All the integrated systems involved other roof elements in the fire including the roof battens, breather membrane and plastic support trays (excluding sample 11) and they all had to be extinguished early on safety grounds. In contrast, the BAPV systems burned out themselves after the ignition crib died down and the available combustible material on the back sheet of the PV panel was consumed. Additionally, it is noted that the BIPV fires were often difficult to extinguish as the fire was protected from water in the cavity created by the panels/flashing.

Following the completion of this series of tests, an additional four tests have been commissioned. These included one test of the plastic roof tiles used on sample 4, without the PV panels fitted. This will allow comparison of how much the solar panels effect the flame spread over these roof tiles.

Additionally, the significant flame spread and involvement of other roofing materials in the fire on the BIPV samples has identified that fire penetration into building may warrant further investigation.

The 3 other tests will study the penetration of a fire on BIPV systems further. These will be constructed on a more representative roof construction, that is, the system will be built above roof rafters, instead of the oriented strand board deck used in all other tests. The three different tests will study the penetration of the fire into the building and how effective fire retardant breather membranes are at preventing this, breather membranes of 3 different reaction to fire classifications (Class E, B and A2) will be used.

1. Department for Levelling Up, Housing and Communities. Approved Document B (fire safety) Volume 1: Dwellings. 2019 edition incorporating 2020, 2022 and 2025 amendments collated with 2026 and 2029 amendments. London: UK Government; 2025 [cited 2025 Oct 21]. Available from: https://www.gov.uk/government/publications/firesafety-approved-document-b ↩

2. Fire Protection Association. Surrey residents call for housing association action over solar panels. Fire Protection Association. 2025 May 21 [cited 2025 Oct 21]. Available from: thefpa.co.uk ↩

3. Stølen R, Fjærestad JS, Mikalsen RF, Jomaas G. Experimental study of fire propagation on sloped roof with building applied photovoltaics. InJournal of Physics: Conference Series 2024 Nov 1 (Vol. 2885, No. 1, p. 012047). IOP Publishing. ↩

4. Kristensen JS, Jacobs B, Jomaas G. Experimental study of the fire dynamics in a semi-enclosure formed by photovoltaic (PV) installations on flat roof constructions. Fire technology. 2022 Jul;58(4):2017-54 ↩

5. Pester S, Coonick C, Crowder D. Fire and Solar PV Systems – Investigations and Evidence. Report No. P100874-1004 Issue 2.5. Watford: BRE National Solar Centre; 2017 Jul 17. ↩ ↩²

6. British Standards Institution. BS EN 13501-5: Fire classification of construction products and building elements – Part 5: Classification using data from external fire exposure to roofs tests. London: BSI; 2016. ↩ ↩²