Rooftop Solar and Fire Study Recommends Non-Combustible Roof Coverings

New research into rooftop solar and fire risks is recommending that non-combustible roof coverings should be used beneath photovoltaic arrays with a minimum gap between the two.

The research also suggests that new test methods are needed to more accurately reflect the modified fire conditions created by PV systems on roofs.

The study, Thermal exposure to roofs from fires involving photovoltaic panels was commissioned by the UK government and carried out by OFR Consultants.

It explored building-applied photovoltaic systems (BAPV), which are retrofitted to existing roofs, and building-integrated photovoltaic systems (BIPV). It examined if rooftop solar alters how fires behave and whether adding solar poses additional risks to roof constructions and firefighting.

Rooftop Solar and Fire Research

One of the key findings was that adding solar panels above roofing materials can significantly affect how a fire behaves because it increases the intensity of ‘heat flux’ – increasing how much heat flows through the roof surface and contributing to a faster rate of fire spread.

Adding PV modules to a roof creates semi-enclosed spaces between the roof and the PV panel which trap heat and direct flames towards the roof surface.

The researchers point to studies showing increased heat fluxes of up to 50 kW/m² beneath PV arrays. This compares to the 12.5 kW/m² considered in standard test methods such as BS EN 13501-5 which provides the fire performance classification procedures for roofs/roof coverings exposed to external fire. These incidences of heat flux have been shown to facilitate fire spread across roofing membranes previously deemed adequate under current classifications.

Gap Between Solar and Roof

Researchers identified that most important is the height of the gap between solar modules and roof coverings.

Experimental research reviewed by the study shows that there are critical thresholds for gap height, beyond which flame spread can be worsened.

Researchers found that flame spread may accelerate by a factor of 38 when gap height falls below certain critical limits due to more heat retention and re-radiation, when compared to a roof without PV.

Tests reviewed by the study revealed that as well as the height of solar panels being critical, their pitch, spacing, mounting system, and wind conditions also affect fire behaviour.

The research adds that these findings are consistent across multiple independent studies conducted at medium- and large-scale real-life fire incidents.

Roof Fire Classifications Inadequate

The review also highlights that existing classification tests and building guidance in England such as Approved Document B and CEN/TS 1187 Test 4 do not fully capture the unique fire dynamics introduced by PV arrays.

These tests assess roof coverings in isolation and do not take into account the changes introduced by PV system geometry, installation techniques, or system-wide behaviour.

Emergency Responder Challenges

Researchers also highlighted the practical challenges for emergency responders. PV systems operating on direct current introduce persistent electrical hazards even after power disconnection.

The presence of PV arrays can also obstruct firefighting access to the fire under the PV panel, impair ventilation systems, and if combined with battery storage, pose chemical or explosion hazards.

Recommendations

Recommendations proposed to mitigate fire spread and roof penetration are:

• Use of non-combustible roof coverings beneath PV arrays to limit fire spread
• Fire-resistant construction from the exterior inward to mitigate roof penetration
• Introducing guidance on the configuration and layout of arrays with minimum gap heights between the roof covering and PV underside
  • limiting the size of arrays and ensuring that they are appropriately spaced,
  • that PV arrays do not extend over compartmentation to mitigate circumvention
  • that they don’t compromise smoke vent outlet performance
  • Development of new test methods that accurately reflect the modified fire conditions created by PV systems.

The alternative to the above recommendations is to consider the development of a roof test that better represents the fire dynamics induced by the installation of PV arrays, the study concludes.

The study is part of the UK government’s real fires investigation project, which is taking evidence from real fire incidents to understand how buildings perform in practice in order to help improve building safety.

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