Combustible

Definition and Technical Context#

In the context of the UK built environment and external wall systems, the term "combustible" refers to materials capable of catching fire and burning. This designation is primarily determined by a material's reaction to fire, specifically its contribution to the growth of a fire through heat release and flame spread. In technical assessments, combustibility is not a binary state determined by observation, but a classification derived from standardised laboratory testing.

Under the Euroclass system (BS EN 13501-1), which is the prevailing standard for assessing the fire performance of construction products in the UK, materials are graded from A1 to F. Materials classified as A1 (non-combustible) or A2 (limited combustibility) are those that provide no, or an extremely limited, contribution to fire. Conversely, materials classified from B through to F are considered combustible to varying degrees. The term encompasses a broad spectrum of materials, ranging from those that ignite with difficulty to those that facilitate rapid fire spread.

External Wall Systems and Combustibility#

The combustibility of the external wall system is a central consideration during the design, construction, and remedial phases of a building’s lifecycle. An external wall is rarely a single component; it is a complex assembly comprising the outer cladding panels, insulation, cavity barriers, breather membranes, and the structural sub-frame. Each component possesses its own fire classification.

In many historical and contemporary cladding systems, combustible materials have been utilised for various functional reasons. For example, certain types of insulation provide high thermal performance but are classified as combustible. Similarly, high-pressure laminates (HPL) and metal composite materials (MCM) with polyethylene cores are often categorised as combustible. The interaction between these components, and how they are installed, determines the overall fire performance of the building fabric. When assessing an existing building, professionals must identify the combustibility of every layer within the wall makeup to understand how the system might behave in the event of a fire.

Regulatory Context in England and Wales#

The regulatory framework regarding combustible materials has undergone significant evolution. The Building Regulations 2010, specifically Approved Document B, provides guidance on fire safety. A pivotal shift occurred with the introduction of the "combustible materials ban" via the Building (Amendment) Regulations 2018. This amendment restricted the use of combustible materials in the external walls of "relevant buildings" over 18 metres in height. Relevant buildings typically include blocks of flats, student accommodation, and hospitals.

The scope of these regulations was further refined in 2022, lowering the height threshold for certain requirements and introducing tighter controls on the types of materials permitted on buildings between 11 and 18 metres. For buildings within these height brackets, the regulations generally mandate that components of the external wall must achieve Euroclass A2-s1, d0 or A1. However, there are specific exemptions for components where non-combustible alternatives are not practically available, such as seals, gaskets, and laminated glass. The application of these rules is complex and depends heavily on the specific building use, height, and the date the building work was carried out.

Testing and Assessment Processes#

Determining whether a material is combustible and how it affects a building's safety involves several levels of professional scrutiny. The EWS1 (External Wall System) survey process is a common mechanism used by surveyors to assess the risk posed by combustible materials in residential blocks. This process often involves intrusive inspections, where small sections of the cladding are removed to identify the internal components.

Laboratory testing is the definitive method for establishing combustibility. Common tests include:

While individual components may be combustible, some systems were historically approved based on large-scale tests like BS 8414, which assess how the entire assembly performs together. However, current regulatory trends in the UK place a heavy emphasis on the combustibility of individual components for high-rise residential buildings.

Responsibilities for Building Owners and Managers#

Building owners, managing agents, and Resident Management Company (RMC) directors hold a duty of care to ensure the buildings they oversee are managed in accordance with the Regulatory Reform (Fire Safety) Order 2005 and the Fire Safety Act 2021. Understanding the presence of combustible materials is a fundamental part of the Fire Risk Assessment (FRA).

When combustible materials are identified in an external wall, it does not automatically necessitate immediate removal, but it does require a competent professional to conduct a Fire Risk Appraisal of External Walls (FRAEW) following the PAS 9980 methodology. This appraisal determines whether the presence of combustible materials poses an undue risk to life safety and what remedial actions, if any, are required. Directors and managers must ensure that these assessments are carried out by suitably qualified engineers, as the findings influence insurance premiums, property valuations, and the eligibility for government remediation funds.

Common Misunderstandings#

A frequent misunderstanding is the belief that "combustible" is synonymous with "unsafe." In technical terms, many common building materials—such as timber and certain plastics—are combustible but can be used safely in specific applications and building types according to the regulations. The risk is not determined by the presence of a combustible material alone, but by its volume, its location within the wall, and the presence of mitigating features like fire breaks.

Another common point of confusion is the distinction between "fire retardant" and "non-combustible." A fire-retardant material is often a combustible material that has been treated with chemicals to slow down the spread of flame. While this treatment improves the Euroclass rating (for example, moving a material from Class D to Class B), the material remains combustible. Only materials achieving Class A1 or A2 are considered to have reached the threshold of non-combustibility or limited combustibility required for most high-rise external wall applications under current UK standards.

Related Terminology#

To accurately interpret technical reports regarding cladding, it is necessary to distinguish between several closely related terms:

• Non-combustible: Materials that do not contribute to a fire (Euroclass A1).
• Limited Combustibility: Materials that provide a minimal contribution to fire (Euroclass A2).
• Reaction to Fire: How a material responds to heat and flame (e.g., how quickly it ignites).
• Fire Resistance: How long a structural element (like a door or a wall) can withstand a fully developed fire and prevent it from passing through to another compartment.
• Cavity Barrier: A component designed to close a gap in a wall to prevent the spread of fire and smoke within the hidden voids of a cladding system.
• Flashover: The near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area.

Professional assessment of these factors is essential for any building remediation project, as the interplay between these definitions dictates the legal and physical requirements for the building fabric.