Flat roofs typically consist of a deck, air and vapour control layer, insulation, and a waterproof covering. Despite the name, these roofs aren't perfectly flat but have a slight slope to allow rain water to run off the roof.

They are commonly built in two main configurations: cold and warm roof systems.

• In a cold roof, insulation is placed between the joists, leaving the roof deck exposed from above. This can lead to condensation risks if not properly ventilated.
• In contrast, a warm deck roof places insulation above the deck, keeping the structure warm, mitigating thermal bridging, and reducing condensation issues. This makes it a preferred solution for higher performing roofs where thermal efficiency is key.

When it comes to fire safety, the key guidance document for specifiers and contractors is Approved Document B of the Building Regulations. While this document covers both pitched and flat roofs, it's vital to interpret its requirements correctly, depending on variables such as building type and height:

• For buildings over 15 metres tall, the roof deck must be constructed using a material with a minimum classification of A2-s3, d2 (for instance, steel or concrete).
• To adequately resist external spread of fire, double-skinned roofing should incorporate a band that is at least 300 mm wide and A2-s3, d2 or better rated when used over a compartment wall.

However, for buildings under 15 metres, these restrictions are less stringent. Most flat roofs on homes or lower buildings can be built with a broader range of construction products, provided other parts of the regulation are met. That includes appropriate fire-stopping measures and avoiding thermoplastic insulation materials near compartment walls. (PIR is a thermoset material, not a thermoplastic).

The role of BROOF(t4)

The test assigns the highest classification, BROOF(t4), when:

• A roof structure withstands fire penetration for at least 60 minutes.
• The roofing material burns for less than 5 minutes after the flame source is removed.
• The flame spread across the burning area is less than 0.38 metres.

Unlike product-only fire ratings, which test the individual components of a roof, BROOF(t4) is a system-level test. That means it evaluates the complete system – from the roofing covering and membranes to the structure, roofing insulation, and ventilation. Each component must work together to perform well under fire exposure.

When is BROOF(t4) required?

According to current regulations, a roofing system must achieve BROOF(t4) in the following cases:

• Within 6 metres of a boundary
• When the roof is used as a means of escape
• Within 1.5 metres on either side of a compartment wall, if the wall does not penetrate the roof by at least 375 mm.

Outside of these specific zones, BROOF(t4) is not a legal requirement. However, many insurers now ask for BROOF(t4) across the entire roof area, especially in buildings with perceived higher risk factors like photovoltaic (PV) panel installations.

Why product classification isn't enough

A common misconception is that using construction products with better Euroclass reaction to fire ratings – for instance, A1, A2 – automatically translates to safer roof systems.

This European classification – officially known as BS EN 13501-1 – rates construction materials based on their reaction to fire, from A1 to F.

While these ratings are often used to test individual components, such as the insulation or waterproofing of a roof, they don’t tell the full story.

In reality, a significant number of systems with E or F class reaction to fire products within the build up have passed BROOF(t4), meaning they resist external fire spread effectively when used in a tested configuration.

This highlights a key principle: what matters is how the entire system performs, not just the fire rating of individual components. Waterproofing, insulation, vapour control layers (AVCLs), and roof decks must work together to meet performance standards.

According to Simon Blackham, Recticel's Senior Technical Manager, a few wrong assumptions regularly cause confusion in flat roof specifications. It's important to remember that:

1. Façade rules do not apply to roofs. Bans on combustible materials in external walls of high rise buildings described by the Building Safety Act 2022 don't apply to roofing systems. As long as a BROOF(t4) classification is achieved for all mandatory sections of the roof, combustible materials can be used regardless of the height of the building.
2. Not all combustible materials are the same. A common misconception is that only non-combustible materials can be used near compartment walls of residential buildings with a maximum height of 15 metres. That's not the case. While you can't carry thermoplastic materials over a compartment wall, since they melt with fire, PIR boards can be used if the full system is BROOF(t4) tested as they are thermoset – meaning they char and harden with fire.
3. Product ratings aren't essential. As mentioned above, individual components with high Euroclass ratings don't always lead to a safer roof if the entire system fails to perform under fire test conditions. Focus on system-level certification, not isolated classifications.
4. Intumescent fire barriers aren't necessary. Fire breaks are often specified without understanding their purpose. If a system has already passed BROOF(t4), such additions may be redundant or even counterproductive. For instance, they may alter the original roof system build-up that passed the test or complicate the installation leading to errors.

Bodies like the British Board of Agrément (BBA) offer third-party tests and certifications for construction products, proving they meet high standards of quality and safety. However, when it comes to proving external fire performance in flat roofing systems, BROOF(t4) remains the only standard legally recognised in the UK.

It's also important to note that:

• Fire performance tests are always evolving and even the Approved Document B has been amended on multiple occasions; the best way to keep up with these changes is to seek the advice of knowledgeable industry experts, like the Recticel Technical Services team.
• Scottish regulations are often more stringent, compared to the rest of the UK, already banning combustible materials on external walls above 11 metres before England.
• Northern Ireland and Wales have separate guidelines that must be referenced in local projects. When in doubt, consult your local council to ensure compliance.

At Recticel, our approach to fire safety starts with a deep understanding of the latest regulations and transparent communication with our customers. Safety is our number one priority, and we only promote our thermal insulation products for applications we have tested.

For flat roofs, our insulation boards are tested and certified in full build-ups to achieve BROOF(t4) where required.

Our commitment includes:

• CCPI Conformance
• Expert technical support, offering reliable advice on the installation of our insulation products.
• Training and guidance for contractors and specifiers on fire and structural regulations.
• System partnerships with leading waterproofing manufacturers to deliver tested solutions.

Whether you're working on a new build, a retrofit, or a high-risk site, our Technical Services team can help you identify if BROOF(t4) is required and find tested systems that meet your needs.

Get in touch with one of our experts or visit our Technical Library to learn more.