CPD 01 2024: The Fifth Facade: Regulations on Roofs

This latest CPD module, sponsored by Rockwool, considers the roof as a building’s fifth facade, examining its functional uses and its vital role in fire safety, and explaining the relevant regulations and what needs to be done to require to comply with them. Deadline for completion Friday 29 March 2024.

Solar energy is the fastest growing energy source in Europe

In recent years, a lot of focus has been placed on the combustibility of insulation products used within facade systems, but the importance of the roof appears to have been overlooked. Given the significant number of instances where the roof is deemed to have been mainly responsible for the development of the fire, this is something that needs to be addressed.

This CPD considers the roof as a key part of a building’s design, repositioning it as the fifth facade, and explores its functional uses, fire safety and the relevant regulations.


  • Awareness of the multifunctional uses of flat roofs
  • Knowledge of legislation and testing required to achieve compliance
  • Explore the rise of rooftop photovoltaic (PV) panels and the associated fire risk.

Fire testing the fifth facade

The legal requirement for both the roof and the walls is virtually the same: the external walls of building shall adequately resist the spread of fire over the wall and from one building to another, while the roof of the building must adequately resist the spread of fire over the roof and from one building to another. As such, no preference should be given to one over the other.

The fire resistance of an element of construction is a measure of its ability to withstand the effects of fire in one or more of the following ways:

  • Resistance to collapse – meaning the ability to maintain loadbearing capacity (which applies to loadbearing elements only)
  • Resistance of fire penetration – meaning the ability to maintain the integrity of the element
  • Resistance to the transfer of excessive heat – meaning the ability to provide insulation from high temperatures.

While resistance to fire relates to a system’s ability to resist the penetration of fire, reaction to fire testing evaluates a construction material’s contribution to fire, predominantly in the early stages of a fire starting.

Materials and products can be classified into seven different Euroclasses according to the way they react to fire. Euroclass ratings are determined in accordance with BS EN 13501-1. Through laboratory testing, this classification system assesses several key performance indicators, including flame spread, heat emission, smoke emission and character changes – such as dripping, melting or charring – to make an assessment of a product’s reaction to fire.

The ratings run from A1 to F – and for A2 and below, are suffixed with “s” and “d” ratings relating to smoke emission and burning droplets. Products rated B or below are combustible – meaning they will burn, and may char or flame.

Before 2019, the National Classification System was used to determine external fire performance for roofs. Now we refer to BS EN 13501-5. This classification system includes five ratings:

  • B Roof(t4) – the highest
  • C Roof(t4)
  • D Roof(t4)
  • E Roof(t4)
  • F Roof(t4) – the lowest

BS EN 13501-5 refers to four separate roof tests. These tests measure the performance of a roof’s resistance to external fire exposure from penetration through the roof construction and the spread of flame over the roof’s surface. The suffix “(t4)” indicates that Test 4 is to be used. Test 4 evaluates the performance of a roof under the conditions of thermal attack with burning brands, wind and radiant heat.

However, the test does not subject the roof to a fully developed fire or consider fire penetrating the underside of the roof. This means the test does not define the combustibility of a roof system or its component parts.

Balconies and terraces are subject to specific technical guidance


While there are examples of flat roof construction throughout history, the design became widespread in Europe and America during the 19th century thanks to its affordability, ease of maintenance and long life. These factors remain relevant to this day, with additional factors accelerating flat roof adoption even further.

The pandemic reinforced the importance of outdoor access to our health and wellbeing. In increasingly crowded urban areas, where space is at a premium, flat roofs provide a creative canvas that can be leveraged to make the outside more accessible.

For flat roofs, building regulation requirements are set out in Approved Document B: Fire safety, specifically requirement B4: external fire spread. Where the roof is required to perform the function of a floor, designers should also be aware of requirement B3 covering internal fire spread.

Under Approved Document B, particular attention is required: where the roof can be used as a means of escape; where it is used as a floor; to upstands, balconies and terraces; and where it crosses a compartment wall.

A compartment wall should achieve both of the following:

  • Meet the underside of the roof covering or deck, with fire stopping to maintain the continuity of fire resistance
  • Be continued across any eaves.

To reduce the risk of fire spreading over the roof from one compartment to another, a 1,500mm-wide zone of the roof, on either side of the wall, should have a covering classified as BROOF(t4), on a substrate or deck of a material rated class A2-s3, d2 or better.

Fire resistance is measured in REI, a designation that identifies the performance of a building element in terms of its loadbearing structure (R), integrity (E) and insulation (I). This is significant for designers working around social spaces where escape routes must be considered.

Approved Document B provides guidance through minimum periods of fire resistance in Tables B3 and B4 for structural building elements including floors. For designated escape routes, 30 minutes of fire integrity is required – although it may be prudent for designers to judge independently whether 30 minutes is truly sufficient when considering how the flat roof will be used.

Using roofs as social spaces means there will be greater footfall compared with that required for maintenance or plant installations alone. While there is no legislative requirement for non‑combustible materials depending on roof footfall, specifiers should be conscious of escape requirements where the roof is occupied.

While Approved Document B and application specific guidance are the specifier’s minimum requirement, it is clear there is scope to go above and beyond legislation to provide better protection for the building and its occupants.

The approved documents set out what, in ordinary circumstances, may be accepted as one way to comply with the Building Regulations. However, it is important to note that the approved documents are provided as guidance and do not provide a guarantee of compliance.

Debunking the myths

BROOF (t4) does:BROOF (t4) doesn’t:
Test for surface spread of flame from an external fireSubject the roof to a fully developed fire
Look at fire penetration through the membraneConsider fire penetrating the underside of the roof
Look for droplets and charringOffer any level of fire resistance rating
Be achieved by virtually all commonly used roof build-upsDefine the combustibility of a roof system or its component parts
Satisfy Regulation 7


Roofs can also be used for the practical purpose of saving interior space by minimising plant room infrastructure, alongside simplifying maintenance depending on building operation and layout.

With the rising specification of heat pumps, mechanical ventilation and solar technology, alongside the struggle for interior space in some of the UK’s larger cities, roof space is a precious commodity.

Statutory guidance for flat roof fire safety, including Approved Document B, sets out key provisions for a number of applications and indicates routes to compliance. For example:

  • Plant rooms are subject to maximum travel distances in terms of escape and exit, and Approved Document B, volume 2: B26 (a. iii) indicates that a plant room on a roof may need greater fire resistance than elements of a structure that support it, and exist as an exception to the B26 (a) principle that elements of a structure giving support to other elements must match their required fire resistance period.
  • Rooflights are covered extensively in Approved Document B, with guidance detailing where rooflights can be assessed using Euroclass reaction-to-fire ratings or where they can be considered to achieve a BROOF(t4) classification, with exceptions in both cases.
  • Junctions with compartment walls are discussed with respect to achieving continuity of fire resistance and preventing the circumvention of compartment zones in the event of a fire.

Solar and risk

It is clear in the UK, as with the rest of Europe, that the market for solar energy is growing rapidly. The European Commission reports that the cost of solar power has decreased by 82% over the last decade, making it the most competitive source of electricity in many parts of the EU.

In 2023 and beyond, commercial rooftop installations in the UK are expected to grow 30% year-on-year and Solar Energy UK forecasts 40GW of solar capacity by 2030 (capacity was reported at 14.6GW in early 2023).

On top of an ongoing trend towards solar energy, external factors such as the energy crisis and subsequent move towards energy self-sufficiency have accelerated solar adoption. Net zero programmes are also driving legislation that supports – or even enforces – solar installations.

The European Commission has declared a Solar Rooftops Initiative within its EU solar energy strategy, which proposes gradually introducing an obligation to install solar energy in different types of buildings over the next seven years, starting with:

  • All new public and commercial buildings (larger than 250m2) by 2027
  • All existing public and commercial buildings (larger than 250m2) by 2028
  • All new residential buildings by 31 December 2030.

While these standards will not be automatically applicable to UK markets, it is likely that solar installations will become part of a new standard for building construction.

Neither Approved Document B nor BS 8579:2020 have any specific guidance for the use of solar (PV) panels on flat roofs. This is particularly important in light of the Building Safety Act 2022, which reinforces liabilities for all stakeholders, indicating that compensation can be claimed from anyone responsible for the defective work and that it is not a valid defence for the defendant to claim to have followed established practice at the time.

The Building Safety Act also extended the limitation periods for claims under the Defective Premises Act from six to 30 years for retrospective claims and from six to 15 years for prospective claims on buildings completed after 28 June 2022. This change gives all prospective claimants greater protection and places greater weight of accountability on all involved in the design and construction of a building.

With solar solutions becoming a new norm, it is essential that all buildings stakeholders are aware of potential risks and engaged in mitigating them.

Flat roofs now commonly house social spaces as well as plant equipment

Rooftop PV fires

Research and real-world evidence point to solar solutions introducing additional fire risk to flat roofs. There are known incidences of solar panel arcing – in which electrical energy passes through air gaps and can cause ignition of nearby materials or the solar panel itself, due to the high temperatures involved.

After a fault-tree analysis of fires related to PV systems was made – a combination of data obtained from reports, research studies and fire incident statistics of four countries – the failure rate of different components of these systems was calculated. The analysis highlighted the significant causes of fire on the component level and various failure patterns. Results identified seven major events that led to incidents caused by a PV-related ignition source, with electrical arcing being the main cause of fires.

Rooftop PV fire studies have shown the following:

  • PVs are at risk of electrical arcing and fire ignition (Allianz, AXA, IF)
  • Mechanical components exposed to the elements degrade over time, increasing fire risk (IF)
  • Panels can trap, collect and radiate heat towards materials, increasing rooftop temperatures (IF, AXA, RSA, Zurich, FM Global)
  • Installation on inadequate or combustible roof materials increases fire spread risk (IF)
  • Improper positioning of rooftop PV makes roof access and system inspections difficult (AXA, Allianz, IF)

A roof fire featuring solar panels at We The Curious, a science museum in Bristol, is widely reported to have been caused by birds having damaged the solar panels, which resulted in electrical faults. This event also highlights the wider impact of fire. Although the fire service was able to manage the fire quickly, the volume of water needed to extinguish the fire led to water damage in areas of the building where the fire itself did not reach.

Solar installations must not provide a route for a fire to bypass zones of compartmentation, or otherwise support or accelerate fire spread. A potential source of fire risk not always considered by designers and installers is the cables and penetrations associated with PV.

Specification and installation guidance, such as that covering waterproofing membrane warranties, discusses the waterproofing of penetrations without always also giving due consideration to the fire performance of the complete roof system. Many PV systems rely on cables that route through the roof system and it is vital that such cables and other penetrations in the roof are appropriately fire-resistant.

This is not to suggest that solar installations are inherently unsafe, nor that the risk outweighs the benefits. It merely highlights the need for stakeholders to ask themselves whether it is necessary to go above and beyond legislative requirements to specify non-combustible building materials when installing solar technology.

The role of the roof in modern building design has expanded


The insurance industry is increasingly aware of the risks associated with solar installations, and several major providers have published guidance on the topic. AXA’s Property Risk Consulting Guidelines state not to install PV systems on combustible roofs and Aviva’s Loss Prevention Standards state that PV panels should not be installed directly on top of combustible roofs.

While it is not the case that providers will or should refuse to cover buildings where solar PV systems are present, it is a recognition that the specification of non-combustible materials where possible is considered best practice.

Significantly, it is also aligned with new guidance published by the Fire Protection Association (FPA) under its RISCAuthority research scheme. RISCAuthority is a membership group comprising a series of UK insurers that actively support expert working groups developing and advising best practice for the protection of people, property, business and the environment from loss due to fire and other risks.

In a 2023 Joint Code of Practice document entitled RC62: Recommendations for fire Ssafety with PV panel installations, RISCAuthority recommended that PV installations should be installed on non-combustible roofs meeting Class A1/A2 s1, d0 to BS EN 13501-1. Fires involving combustible roofs will spread quickly, without the benefit of any protection installed within the building. Adjoining or nearby buildings can potentially also be at risk.

In the absence of solar PV-related guidance in the approved documents, there is nonetheless a groundswell of opinion that non-combustible specifications are the most sensible approach.

Mitigating risk

From the outset, designers should be aware of the project-specific influences with regard to fire safety and compliance. This will help the technical team to ensure they are designing a roof that will meet the requirements of all key stakeholders as well as adhere to building regulations. Fire compliance can significantly impact the design details of the roof, so early engagement is key.

It is vital to consider the use of non‑combustible materials early in the design process, particularly where the materials are substantive or when specifying solar technology. Some manufacturers recommend specifying non‑combustible insulation across the entire flat roof, as opposed to mixing non-combustible and combustible insulation, depending on zoning requirements. This not only provides an added layer of fire safety but offers the additional benefit of simplifying the specification process by removing the need to specify a number of different products. It can also help to ensure a more effortless process in terms of on-site handling and the installation of products.

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