New Research Published - Rooftop Solar Reaction to Fire

Above Roof Solar PV Panels over (BRoof) Plastic Tile Roof - 8 Minutes into Test.
  Image: Health and Safety Executive 

The UK's Health and Safety Executive (HSE) has published a new report detailing a series of fire tests performed on pitched roof installations of solar PV panels.  The results (and the pretty spectacular photos) should give the solar and construction industries much to consider, as the findings challenge long-held assumptions about how fire safety regulations apply when solar is fitted to buildings.

In this blog we'll take a look at the tests and what they may mean for future regulation of the safety of  solar installations.

You can access the full HSE report on this link: Fire Spread Over Pitched Roofs Fitted with Solar Panels 

Key Findings

1. Above-roof solar is not immune to fire.  Above-roof solar contributes to the spread of the fire by providing fuel to the fire, and by changing the fire dynamics of the roof covering for the worse.
2. Roof-integrated solar systems based on plastic trays performed far worse than those without plastic trays.  The combustible plastic tray accelerated the fire which spread twice as far and twice as fast when compared to a proprietary roof integrated solar system of panels with a push-fit aluminium flashing that avoids the use of plastic trays.
3. Glass-glass modules perform better than those with a plastic back sheet but still got involved in the fire once the glass broke and plastic encapsulant material was available to the fire.
4. Current fire regulations do not address the risk posed by fires initiated by electrical faults in the solar installation (for example faulty DC-DC connectors) which highlights the value of fire safety products like ArcBox.

This project by the HSE is an extremely helpful contribution to the ongoing debate about how to make solar installations safer, and should encourage industry participants to consider solar practice that does more than just meet the current (insufficient) regulations instead of waiting for regulations to catch up.

None of the findings should have come as a surprise to folks in the solar industry - see the featured news articles below, but the fact is that most people in the solar industry would  just prefer not to spend time thinking about this topic.  

Which another way to say that we've probably lost most of our readers already, but lets plough on and dive into the detail of the report...

The Tests

A crib of burning wood was placed behind the bottom left panel in an installation of four solar panels.  The intention is that the crib simulates an electrical fault in a DC-DC connector.  The panels are on a test roof pitched up at 45 degrees.  Fans at the base of the roof are activated after 2 minutes to simulate the effect of a wind pressure on the roof.

A range of sample combinations were tested which combined above roof systems with different panel types and roof coverings as well as testing different roof integrated systems.  The time taken for the fire to spread to the top of the sample roof was recorded.  If the fire did not reach the top of the roof within 17 minutes the fire was extinguished and the spread of the fire spread was inspected after disassembling the installation.

Above Roof Solar

The working assumption in the solar industry has been that if you install solar on racks above a traditional roof covering, that the fire performance of that roof covering is unaffected and a new fire classification is not required for the roof build up including the solar panels.

The HSE report blows a massive hole in this.  

Firstly, the tests with solar panels installed above incombustible concrete tiles showed that the backing sheet of plastic on the rear of the panels will contribute to the development and spread of a fire.  Glass backed modules were found to perform much better than those with plastic back-sheet but still provided fuel from the cell encapsulant after the glass shattered due to the heat.

Second, the test with panels installed above plastic roof tiles had the fastest spread of flame of any of the tests and needed to be stopped after only 8 minutes because of safety concerns (this is the image at the top of the page).  These plastic tiles have a BRoof (T4) fire classification - the highest possible fire classification for a pitched roof covering. 

The presence of the solar panels above the combustible roof covering changes the fire dynamics, trapping heat and reflecting it back onto the fire as well as funnelling air over the fire (a chimney effect).  

The tests make clear what has been evident from real-world fires - that above-roof solar is not immune to fire and this is especially the case when installed over combustible roof coverings.  Note also that none of the tests attempted to simulate the real world situation where combustible material such as bird nests or wind blown leaves have accumulated behind the panels.

Real World Fires

St. Martin's Hospital, Bristol, UK, 22/05/2025 - fire spread behind above roof panels on flat roof of this maternity hospital, necessitating the evacuation of the building.

Bow Wharf, Bethnal Green, London, UK, 2/7/2017 -  fire behind panels installed above slate roof during the refurbishment of this building.

Roof Integrated Solar

Roof integrated solar systems replace tiles or slates on the roof.  Two types of system were tested.  One type that covers the roof with overlapping plastic plates before fixing the panels above, and a second that does not use plastic trays and fixes the panels straight to the roof with an aluminium surround (flashing) that pushes into the panels.

The system using plastic trays performed far worse in the tests, with the fire spreading to cover about twice the roof area in half the time compared to the system without plastic trays.  The explanation for this is the large quantity of accessible fuel for the fire provided by the plastic trays.  

A number of test were run with the plastic tray systems that varied the type of roofing membrane below the trays using membrane with differing fire rating.  The finding that this made no difference to the outcome suggests that the plastic tray is the main contributing factor for the very rapid spread of the fire.

Real World Fires

House Fire, Roden, Netherlands, 04/09/2023 - fire spreads on plastic tray roof integrated solar system with the seat of fire near the top of the array which limited the area of damage.

North Prospect, Plymouth, 03/05/2022 - fire destroys roof of new build residential property in Plymouth.  Fire partitioning prevents spread to joined house.

Does the Rate of Fire Spread Matter?

The average total response time by the Fire Service for a house fire in England in the period from 2013 to 2023 was an amazing 7 to 8 minutes.  The length of time the tests ran for, and the growth of the fire in that time is representative of how developed a fire would be when the Fire Service arrives at the building. The differences between the reaction to fire of the different systems could make a big difference to the scale of the challenge that the fire service faces upon arrival and the amount of damage done to the building by the fire and the measures to extinguish it.

Source: UK Home Office Official Statistics

Fire Regulations

The fire regulations relating to roofs were designed for a world where roof coverings were passive materials like concrete, clay, and slate and the risk that needed to be managed was to stop a fire next door spreading to your building.  Roofs are tested and classified for their reaction to an external fire outside the outermost layer of the roof covering.  Depending on the classification achieved, there may be limitations placed on the use of the roof covering (its proximity to adjacent buildings, the maximum continuous area of roof covered).

When solar panels are added on top of an existing roof covering, the interpretation has so far been that the roof covering below the panels performs as if the panels were not there and you can use the fire classification for the roofing material when tested alone.  

This work by HSE is just the latest in a line of research that has been sounding the alarm that this approach is not a safe one (see list below).  The presence of solar panels above the roof covering clearly does change the fire performance of the roof.

• Experimental Study of the Fire Behaviour on Flat Roof Constructions with multiple Photovoltaic (PV) panels. Fire Technol. 2018 Nov;54(6):1807–28.
• Experimental study of fire propagation on sloped roof with building applied photovoltaics. J Phys: Conf Ser. 2024 Nov 1;2885(1):012047.
• Impact of flat roof–integrated solar photovoltaic installation mode on building fire safety. Fire and Materials. 2019 Dec;43(8):936–48.

The current regulations are insufficient and should be urgently reviewed, but  regulatory change is painstaking and slow.   In the meantime schemes like the Microgeneration Certification Scheme in the UK could require fire classifications for above-roof solar systems, and put this in place more quickly than traditional regulations will manage.

Mitigation Measures

While we wait for the regulatory environment to catch up, should the solar industry just carry on as it is, sheltering behind the argument that its practices are 'compliant with regulations', regulations we now know to be insufficient for the situation?

Risk = Likelihood x Consequence

The research has focused in the consequence of a fire that starts in the solar installation and how fast it spreads in different situations.  You can also reduce the risk by lowering the likelihood a fault occurring, or if it does occur stop it spreading to start the fire in the first place.  Clearly higher-quality installations with fewer errors is a good starting point, but technical mitigation measures can be specified that in the event of an electrical fault, help prevent the development of a fire:

• The ArcBox DC connector enclosure protects the solar connectors in the installation from external damage and in the event of a fault contains arcing to prevent the initiation of a fire.
• Arc Fault Circuit Interrupt (AFCI) is a technology that uses electronic monitoring of the current in the solar circuit to detect the presence of an arc and disconnect the power.
• Micro inverters reduce the DC voltage in solar systems to a level below that which can cause arcing.

Conclusion

The findings of this research, and the other studies mentioned, make it hard to avoid the conclusion that the solar industry needs to do more to reduce fire risks.

• Solar installations above combustible roofing materials (including building integrated solar installations) could be required to adopt additional mitigation measures against situations where the fire starts in the solar installation itself.
• A new test for external fire performance of roofs could be developed to allow the fire classification of a roof including above-roof solar panels and mounting system. 

Warm Homes Fund to Go All-in on Solar

 Announcement in January Expected to Include Grants for Solar PV

Zero Bills Homes by Keepmoat and Platform Housing Group (C) Viridian Solar

It is widely expected that a £13bn 'Warm Homes Fund' to be announced by Ed Miliband in January will include grants for the installation of solar PV and batteries.  If so, this will represent a big departure from previous policy in this area which have almost exclusively emphasised retrofit insulation measures.

Until recently the accepted wisdom has been coined 'fabric first’.  This catchy phrase summarised a prevailing energy-efficiency orthodoxy which held that until you fix the insulation and airtightness of a building (its fabric) there is no point using ‘expensive bolt-ons’ like solar.  

Fabric first became sacrosanct for some in the energy efficiency industry (and not only those who manufacture insulation), with every consultation on building regulations met with howls of criticism from some quarters for not going far enough on required insulation levels and any inclusion of solar or other technology criticised as 'green bling'.  

Numerous government policies have been influenced by fabric first thought – with ECO, Low Carbon Buildings Programme, and most recently the ‘Scottish Passivhaus’ rabbit hole that building regulations north of the border appear to be about to disappear down all prioritising insulation over renewable energy.

This dismissal of solar bling as unserious and insulation as the only ideologically pure approach to decarbonisation misses three important points: 

First, that thermal efficiency is a game of diminishing returns.  

Second, (and linked to the previous point), having run out of easy targets such as loft and cavity wall insulation, more ambitious retrofit approaches aiming for big improvements in thermal efficiency can be highly invasive, complex and risk unwanted side effects like damp and mould.

Third, that the orthodoxy arose at a time when our energy system was dominated by fossil fuel and renewable energy was expensive.  This is now out of date.  What matters more today and in future is when you use energy not how much you use.

Diminishing Returns

It's physics.  The more you insulate a building the more difficult becomes the next improvement in performance, until you are adding large amounts of insulation for only marginal gains.  Building regulations for new homes appear to have now reached this point since the Future Homes Standard consultation proposes no change to the fabric performance of new homes over those of current (2021) regulations.

In retrofit scenarios, the payback for simple low cost measures like loft insulation chimney balloons, lagging hot water tanks and pipes and draft excluders is measured in months while more expensive improvements like external wall insulation for solid walls can take many years to pay back their costs.

Unwanted Side Effects

Once you've lagged every hot water cylinder, topped up loft insulation where you can and blown insulation into walls with cavities, you're left with a large number of hard-to-treat properties with solid brick or block walls.  These require a layer of insulation to be fixed to the external walls either on the inside face which makes the rooms smaller or on the outside face which needs to be carefully protected against the weather. 

The challenges with solid wall insulation really became apparent once we moved from theory and pilot studies to pushing into volume in the real world.  The work is complex, expensive and intrusive and has sadly proven to be easy to get badly wrong at scale, with unwanted side effects such as damp and mould widely reported. 

A National Audit Office review of works done under the ECO4 and the Great British Insulation Scheme found that an amazing 98% of homes fitted with external wall insulation and 29% of those with internal wall insulation had major issues that need fixing.

The Economics Has Shifted

Fabric first approaches to energy conservation in buildings emerged at a time when renewable energy was ruinously expensive and the energy supply system was dominated by coal, oil and gas.  At this time, the careful conservation of energy was the only logical way to reduce emissions and lower energy bills.

Renewable energy is now the cheapest form of energy.  It getting more and more plentiful as investments in new solar and wind capacity expands.  The nature of renewables is that the timing of generation cannot be controlled in the same way as it can for fossil fuel based energy, but the falling cost of battery energy storage and advent of smart controls that react to time-of-use pricing signals are combining to overcome the weakness of intermittency in renewable generation.

Those who can adjust their energy demand to use power when energy is in over-supply can now take advantage of these tariffs to pay very low (sometimes zero, sometimes negative) prices for their power.  Space heating, domestic hot water and electric vehicle charging are all amenable to time-shifting or rate shifting.

New housing developments such as Hollymead Square in Essex and Beeston Canalside in Nottingham offer so-called Zero Bills Homes where the combination of solar PV, battery energy storage and electric heating with time of use tariffs and smart energy controls allow the energy supplier, Octopus Energy, to guarantee that the householders will pay nothing for energy for ten years after moving in. 

In this approach to low-carbon living it's not how much energy you use, its when you use it and how you combine that with maximising the use of low cost renewable energy you generate for yourself.

Learn to Love the Bling 

The case for using energy sparingly has not gone away, and simple, low-cost insulation improvements will always be high up the to-do list.  

However renewable energy is combining with smart energy management, electrification of transport and heating and battery energy storage to offer an alternative vision in which the when of your energy use is as important as the how much.  If the Warm Homes Fund recognises this fundamental shift, then that is to be welcomed.