How can roof mounted Solar PV-related fire risks be mitigated on mission critical buildings?

The risk of BAPV, or Building Applied Photovoltaics, solar panel roof related fire on mission-critical facilities, that must operate 24/7 without interruption, can be mitigated by first understanding the root causes of outbreaks of fire and then adopting an appropriate strategy to minimise the frequency of such events occurring.

The presence of a solar panel on, for example, a data centre flat roof, introduces extra electrical equipment to the roof. It joins the other plant already highly likely to be positioned there, typically for air conditioning and ventilation, which is essential for the smooth running of a datacentre. Whenever electrical equipment is installed on a roof, it brings with it the possibility of being responsible for the outbreak of a fire if the equipment malfunctions.

For so called ‘mission critical’ buildings, such as data centres, biotech facilities and airports, the overriding objective is to ensure that operational downtime is minimised so that the vital services they provide are available 24 hours a day, 365 days a year. This requires a roof specification that can help reduce the risk when it comes to a variety of challenges, these include extra weight in the roof, additional maintenance foot traffic, protection from adhoc events that could cause mechanical damage to the roof membrane and, of course, the risk of an outbreak of fire.

Mission critical buildings’ solar roofs are increasingly specified to provide a renewable energy source for energy intensive processes. Solar roofs reduce the intensity of building’s reliance on the electricity grid, lower its carbon dioxide (CO₂) emissions and reduce energy costs. The advantages of solar panels on flat roofs are clear; however, the increase in the possibility that a fire may occur needs to be recognised and mitigated.

What is the most likely cause of an outbreak of a fire on a mission critical building solar roof?

Analysis of solar panels on roofs shows that high temperatures in excess of 6,000 °C created by electrical arcing¹ in faulty roof mounted PV systems are the most likely cause of an outbreak of fire on a mission critical building’s solar roof. Electrical arcing happens when conducting parts of the components that make up the solar panel flat roof system become separated. An electrical current flows between the separated parts by ionising the air in between them and, as the current is direct (DC), rather than alternating (AC), the arc can be sustained. The high temperature created can cause parts of the roof mounted PV system, or nearby waterproof membrane, to combust and the fire to spread across the roof.

Building Research Establishment (BRE), on behalf of the government, carried out a survey² to determine which components of the roof mounted PV system were most likely to experience arcing. DC isolators posed the greatest risk followed by DC connectors, inverters and then the roof mounted PV modules themselves. The survey went on to investigate what caused these arcing issues to develop and poor installation practices were responsible for over a third of the instances recorded, followed by system design errors and faulty products.

How can the causes of fires on mission critical building solar roofs be addressed?

A focus on careful handling of roof mounted PV systems during transportation and installation to avoid equipment damage that could lead to electrical arcing is a good practice to follow for solar panels on flat roofs. Ensuring that good installation practices, in line with manufacturer’s instructions, are employed by using trained and experienced operatives will help also help reduce instances of arcing. Selecting a proven solar panel system design with a good track record for reliability can help to mitigate against the risks of manufacturers defects.

There are also products that can be specified as part of the system to add extra layers of safety. They enclose key connections with casings that will protect materials around them should arcing occur.

It is also important to carry out regular maintenance and inspection of the roof mounted Solar PV once it is installed. This will ensure no damage or wear to the system has occurred that could allow the ingress of moisture leading to electrical faults. The maintenance should include cleaning of any electrical contacts that have a build-up of oxide contaminants as these can lead to increased temperature in those areas and may lead to arcing.

What is the probability that a fire will occur on a mission critical building solar roof?

With regards to the probability that a fire will occur on a flat roof with solar panels, according to a study of the fire dynamics of roof mounted PV installations³, analysis estimates an annual fire incident frequency of just under 29 fires per GW. In PV Magazine⁴, international data also suggests that solar rooftop fires are rare. It quotes data from Germany where only 350 out of 1.4 million solar installations reported a fire and, of Japan’s 2.4 million installations, only 127 had issues.

Even though roof mounted Solar PV fires are not common, the falling cost of roof mounted PV panels and benefits of renewable energy are driving the uptake of renewable energy on mission critical buildings. As the numbers of installations increase, so will the probability that some will be affected by fire related incidents.

Is the behaviour of a fire affected by the roof mounted PV panels on a mission critical building solar roof?

The presence of roof mounted PV panels can change the fire dynamics of a mission critical building roof. Experiments have shown⁵ that roof mounted PV panels can reflect heat and deflect the flame from a fire back onto the surface of the roof and increase the fire spread over the roof to which they have been mounted.

In other testing⁶, the height of the panels above the roof was found to be a factor on the rate of fire spread over the roof. For roof mounted PV panel systems there is a ‘critical gap height’. Systems that have gaps lower than this critical height cause rapid acceleration of the flame front up to 38 times faster than the baseline measurement.

Both experiments showed that the combustible components within the roof mounted PV modules themselves, which in theory add to the fuel load on the roof, did not add to enhanced flame spread. Their presence, however, could hinder firefighters in their efforts to reach and quickly extinguish a fire, adding further complications when attempting to minimise damage from a fire affecting a mission critical solar roof.

The change in fire dynamics means that even fires on flat roofs with solar panels that are not caused by an issue with the roof mounted PV system itself, have the potential to spread more rapidly than on a standard, non-solar, flat roof.

How can fire resistant roof boards help mitigate the fire risks on mission critical building roof mounted Solar PV?

As we have highlighted, although the risks are low, even when steps have been taken to mitigate the risks of electrical arcing occurring, there remains a chance that a mission critical building may encounter a fire incident on flat roofs with solar panels.

Given the desire of mission critical buildings to ensure that the risk of downtime, disruption and damage to building contents is minimised, non-combustible fire resistant roof boards could be specified to add an extra layer of fire resistance to the roof build-up. These boards are positioned underneath the waterproofing layer and above the insulation to help slow down fire spread across the surface of the roof and help limit the damage to the insulation and roof deck from the heat of the fire.

As well as being non-combustible, fire resistant boards come with additional building performance benefits. They are rigid with a high compressive strength and appeal to mission critical buildings as they can help to add overall robustness to the roof to improve their weather resilience, wind uplift resistance and resistance to damage from foot traffic during the frequent maintenance visits required for a solar panel on a flat roof.

Georgia-Pacific manufacture a range of DensDeck® fire resistant gypsum boards. The boards have embedded non-combustible fibreglass mat facers that form the first line of defence against a fire and a gypsum core that contains crystalised water incorporated into its structure. In a fire, the energy from the heat vaporises the crystalised water creating a natural barrier to increase its fire-resistant properties.

DensDeck® Roof Boards are used in 398,487 flat roof assemblies, which is 54.57% of all FM Approved assemblies, that achieve an FM Approvals (FM) Class A fire⁷ performance with a cover board. These assemblies are typically assessed to higher standards than required by building regulations and can give specifiers of mission critical buildings the reassurance that they are selecting a proven system to help mitigate the risks posed by roof mounted Solar PV fires.

If you would like to find out how selecting DensDeck® Roof Boards as part of your mission critical building roof mounted Solar PV specification can help minimise the impact of a fire and enhance the resilience of your project please contact us today for support and guidance.

¹ Fault tree analysis of fires on rooftops with photovoltaic systems – Journal of Building Engineering 29 Nov 2021
² Fire and Solar PV Systems – Investigations and Evidence – BRE May 2018
³ Experimental Study of the Fire Dynamics in a Semi-enclosure Formed by Photovoltaic (PV) Installations on Flat Roof Constructions – Journal of Building Engineering 28 March 2022
⁴ There are – data missing – solar power fires a year – PV Magazine August 2019
⁵ Fire induced reradiation underneath photovoltaic arrays on flat roofs – Kristensen JS, Merci B, Jomaas G Fire and Materials 2018
⁶ Experimental study of flame spread underneath photovoltaic (PV) modules – Fire Safety Journal May 2020
⁷ DensDeck® Roof Board is classified as A1 in accordance with EN 13501-1 and non-combustible as described and tested in accordance with ASTM E136