CPD 01 2021: Specifying roofs to reduce fire risk

This CPD, sponsored by ROCKWOOL, explains how to apply fire-conscious design, construction and materials specification to a building’s fifth facade – its roof.


The integrity of a roof is essential in maintaining a building’s ability to resist fire

Over the years, notable fires have led to new regulations that are designed to help improve building safety. Yet serious building fires continue to occur. The rules have generally been aimed at securing the integrity of a building’s walls, but while Building Regulations and Approved Document B contain requirements for roofs, some believe they are not covered robustly enough. Regulators should pay close attention to the roof, which is in a sense the fifth facade.

This CPD will examine the impact roof fires can have, and what might be done to better protect buildings and their occupants.

Fires, investigations and bans

The Grenfell Tower in June 2017 is the latest fire that has led to investigations and subsequent amendments to the regulations governing how buildings should be protected in the event a fire breaks out.

The way in which fire spread across the tower’s facade has raised important questions that are being answered in the current public inquiry.

A simple and straightforward way of determining a product’s combustibility is by checking its Euroclass reaction-to-fire rating

While ongoing changes to legislation in England and Wales have prohibited the use of combustible materials in the external walls of relevant buildings over 18m (as defined by Regulation 7), the ban – which is also being considered in Northern Ireland – only partially extends beyond walls and associated attachments to upstands, balconies and roof terraces – leaving the majority of roofs open to a greater fire risk than the walls.

As well as prompting a raft of new measures that, it is hoped, will significantly improve building safety, these changes have made many in the construction industry think about the composition of a building’s facade. It is also a reminder that as well as a building’s four walls, it effectively has a fifth facade, namely the roof.

Fires and their consequences

Notable fires over the past 50 years which saw the roof play a role in the devastation have included the Summerland Leisure Centre fire on the Isle of Man in 1973, which saw a small blaze caused by a discarded cigarette spread across the building’s cladding and roof.

Twelve years later at Bradford City football club’s ground, a small fire in a stand at the ground during a game was fuelled with devastating consequences by the structure’s old wooden roof.

In both instances a relatively innocuous fire was able to grow into something much bigger and far more dangerous, significantly fuelled by the combustible nature of the materials used in the roof structure.

While rules may get tightened in the wake of such tragic events, it is important to look at the effectiveness of such regulations or standards, assess how they are being used, and crucially where they are perhaps being misinterpreted.

Just as the potential of a fire spreading across a building’s facade must be considered, roofs too ought to be rigorously assessed in terms of fire safety and fire performance. There needs to be recognition that similar dangers and risks are present around a roof structure, because if a building fire spreads to it there is an increased risk of the whole structure coming down.

The integrity of a roof is essential in maintaining a building’s ability to resist fire. In instances where it is to be used as a means of escape – for example, from one building to another in order to access a fire exit – it needs to provide a degree of fire resistance that will involve using non-combustible materials.

That some in the insurance industry, including Zurich, are actively campaigning for the combustible cladding ban to be extended to the whole building envelope – including flat roofs – serves to highlight the importance of non-combustible insulation being used in roofs.

What starts a roof fire?

So, how do roof fires start? The main cause is deemed to be arson. According to Zurich’s report, “The Human Impact of Fire”, arson accounts for 50.5% of all fires attended by the fire service every year. The same report estimated annual economic loss during 2017 and 2018 was between £5.4bn and £11.5bn.

Another major cause is due to hot works being undertaken on a roof. Hot works are defined as any building, maintenance or refurbishment work that requires the application of heat, such as grinding, welding or torch applied roofing. These activities are said to cause 15% of all fires in commercial and industrial properties, according to Zurich, and a fifth of all construction site fires, with annual losses of around £250m.

Solar panels and plant on roofs can be a source of fires; arson and repair works using heat are also among the main causes

A third source of roof fires is equipment, which due to space constraints or other design factors is increasingly being located on a roof, such as air-conditioning units or solar panels. Electrical arcing can occur around wiring on solar panels that has become worn or has fallen into disrepair, which can lead to a fire. This only serves to highlight the importance of designing out fire risk when solar photovoltaic panels are to be installed on a building’s roof.

Lastly, there is the danger of a fire spreading from elsewhere in the building, such as the main structure, to its roof. If such a fire reaches a roof made from combustible materials, or where combustible material is located, the fuel load provided by the roof may spread the fire.

According to the UK government, between 2010 and 2018 there were 4,290 fires in England where the roof was deemed to have been mainly responsible for the development of the fire, but often these were instances where the blaze started elsewhere and spread to the roof through poor venting or an open window.

Rules and regs
External fire spread is covered by Part B4 of the Building Regulations. While it uses virtually identical language to describe the legal requirements for both roofs and walls, the recent ban on the use of combustible materials does not currently apply to flat roofs, meaning combustible insulation can still be specified.

At present, EN 13501-5 – Fire classification of construction products and building elements, Part 5: Classification using data from external fire exposure to roofs tests – is a key method in assessing how a roof will react to the spread of fire when exposed to an external fire source.

A proposed roof system is tested by exposure to burning brands, simulated winds and radiant heat – the results of which are used to give a classification. Within the UK these ratings run from the highest-achievable BROOF(t4) down to FROOF(t4).

Approved Document B allows roof coverings achieving BROOF(t4) to be used without further consideration of the building type or relevant boundaries. But it is important to note that EN 13501-5 only assesses how a roof will behave when exposed to an external fire source. It does not provide any indication of how the roof system will perform when exposed to fire from inside the building, nor does it assess the combustibility of the roof system and its component parts, which may fuel a fire and emit toxic gases and smoke.

The roof is effectively a building’s fifth facade and should be considered as such during the design, construction and occupancy phases

It is also important to note that roof systems incorporating combustible insulation products as low as Euroclass F can achieve a rating of BROOF(t4). As such, the BROOF(t4) classification does not automatically allow such roof systems to be continued uninterrupted over compartment walls, where it is necessary for the substrate to be Euroclass A2-s3, d2 or better.

A wide range of commonly used combustible flat roof products achieve BROOF(t4) when tested in combination. So what does this actually mean? The test assesses surface spread of flame when a fire is coming from the outside. It looks for droplets and charring. And it looks at penetration, but it is important to note that this is only through the membrane.

The test does not measure combustibility, and neither does it consider fire that originates from the underside of the roof. So it cannot answer questions about whether a compartment wall detail could allow fire to break through from beneath and attack the underside of the roof. Nor does BROOF(t4) offer any kind of comment on the roof’s fire resistance. Effectively there is no integrity or insulation rating, and it does not assess penetration other than through the membrane from fire above.

Some building material manufacturers use a variety of terms, including “fire-retardant”, “fireproof”, “fire-safe”, “chars/charring”, “Class 0” and “unrestricted”. None of those mean a product does not burn; in some cases such products will actually burn a great deal.

For example, Class 0, the now obsolete national fire class, was a classification applied to the outer surface of the product, which was developed to assess the effect of adding surface finishes such as wallpaper to a building element. However, for some manufacturers this evolved into putting a thin layer of protective aluminium foil over combustible insulation to enable the product to be declared Class 0.

Need for vigilance
One simple and straightforward method of determining the combustibility of a building product is by checking its Euroclass reaction-to-fire rating. When managing fire risk in the roof and determining the correct specification for compliance, confirming the Euroclass rating of insulation is vital.

This product rating might be supplemented by a reaction to fire classification that applies to a specific combination of products tested together, such as a BROOF(t4) classification.

For all fire tests, details of the exact construction tested are required to determine the scope of such a composite classification, and any limitations on positioning the components relative to the direction of fire. For example, a product may have achieved its stated Euroclass rating by undergoing reaction-to-fire tests only when affixed to a steel substrate. This limits the product’s field of application, since its Euroclass rating is not valid when used in conjunction with poorer-performing substrates.

Euroclass combustibility ratings

There is growing understanding about how materials used in roof construction can vary vastly in their fire performance. Wherever possible, developers and designers should consider using non-combustible materials on roofs, going beyond the bare minimum requirements.

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