Combustible Materials

Definition and Physical Properties#

The term 'combustible materials' refers to substances that possess the chemical and physical capacity to ignite and burn, thereby contributing to the spread of fire and the release of heat energy. In the context of the built environment, combustibility is not a binary state but a spectrum defined by how a material reacts when subjected to thermal stress. While all organic materials are inherently combustible, the rate at which they contribute to a fire—measured through parameters such as ignitability, flame spread, and heat release—determines their specific classification under standardised testing regimes.

Within an external wall assembly, combustibility is assessed through the European classification system (Euroclasses), as set out in BS EN 13501-1. Materials are categorised from A1 (non-combustible) and A2 (limited combustibility) through to E and F (highly combustible). The distinction between these categories is fundamental to how materials are specified, as it dictates the potential 'fuel load' an external wall system adds to a building’s facade.

Combustible Materials in External Wall Systems#

An external wall is rarely a single product; it is a complex assembly comprising multiple layers, each with varying degrees of combustibility. The term applies not only to the visible outer cladding panels but also to the internal components that facilitate thermal performance and weatherproofing. Common combustible elements found within these systems include:

• Insulation: Many traditional high-performance thermal insulators, such as PIR (polyisocyanurate), PUR (polyurethane), and phenolic foams, are combustible.
• Core Materials: Composite panels (ACM, MCM, or HPL) often feature a core material—such as polyethylene—sandwiched between non-combustible skins.
• Ancillary Components: Vapour barriers, breather membranes, tapes, and thermal break gaskets are frequently manufactured from polymers that are combustible.
• Timber Components: Structural frames or decorative cladding slats made from treated or untreated wood.

The interaction between these materials is a critical focus of cladding testing. A material that appears inert in isolation may behave differently when combined with others in a system that includes ventilated cavities, which can potentially create a chimney effect if fire enters the wall void.

UK Regulatory Framework and Context#

The regulatory treatment of combustible materials in England and Wales has undergone significant transformation, particularly regarding multi-storey residential buildings. The Building Regulations 2010 (as amended) and the accompanying Approved Document B provide the framework for how these materials may be used. A pivotal shift occurred with the introduction of the 'combustibility ban' (Regulation 7(2)), which restricted the use of combustible materials in the external walls of 'relevant buildings' over a specific height threshold, currently 18 metres, with further nuances applied to buildings between 11 and 18 metres.

In the context of existing buildings, the assessment of combustible materials is often driven by the EWS1 (External Wall System) process and the requirements of the Regulatory Reform (Fire Safety) Order 2005. The presence of combustible materials does not automatically necessitate remediation; rather, it triggers a requirement for a comprehensive Fire Risk Appraisal of External Walls (FRAEW) conducted in accordance with PAS 9980. This professional appraisal evaluates whether the specific arrangement of combustible materials poses an undue risk to life safety.

Testing and Classification Methodologies#

To determine whether a material is combustible, it must undergo specific standardised tests. The Euroclass system relies on several different testing methods to categorise performance.

For large-scale system testing, BS 8414 is frequently utilised. Unlike component-level tests, BS 8414 evaluates the entire wall assembly, including rails, fixings, and fire barriers. This determines how combustible materials behave in a holistic environment, providing data on whether the system meets the performance criteria set out in BR 135.

Roles and Responsibilities for Building Fiduciaries#

For building owners, managing agents, and directors of Resident Management Companies (RMC) or Right to Manage (RTM) companies, the presence of combustible materials is a matter of long-term asset management and legal compliance. Understanding the combustibility of a building's fabric is essential for securing insurance, facilitating property transactions, and ensuring the safety of occupants.

When combustible materials are identified within an external wall, the fiduciary responsibility involves appointing a qualified fire engineer or building surveyor to conduct a proportionate assessment. The objective is to determine if the materials, as installed, meet the functional requirements of the Building Regulations. If they do not, a strategy for repair or replacement must be developed. This process requires a neutral, evidence-based approach that considers the building’s height, occupancy type, and existing fire safety measures, such as internal sprinklers or fire alarms.

Common Misunderstandings#

A frequent misunderstanding is the assumption that 'combustible' is synonymous with 'dangerous' or 'illegal'. In the UK, many combustible materials remain permitted for use in specific building types and heights, provided they are installed as part of a system that has been proven to adequately resist the spread of fire. The regulatory focus is on the risk presented by the material in its specific context, rather than its mere presence.

Another common misconception relates to 'limited combustibility'. While A2-rated materials are often grouped with A1 materials in regulatory permissions, they are technically capable of producing a small amount of heat and smoke. Conversely, materials rated Class B are often described as 'fire retardant' or 'flame resistant'. While these treatments can slow the spread of flame across a surface, they do not change the underlying combustible nature of the base material.

Related Terminology#

• Reaction to Fire: A measure of how a material responds to heat and flame, specifically its contribution to fire growth.
• Fire Resistance: Different from combustibility, this refers to a construction element’s ability to maintain its structural integrity and prevent fire passage from one compartment to another for a set period.
• Calorific Value: The amount of energy released as heat when a material is completely burned; this is used to determine A2 classifications.
• Thermoplastic vs. Thermosetting: A distinction in how combustible polymers behave; thermoplastics (like polyethylene) melt and drip when heated, whereas thermosetting plastics (like phenolic foam) tend to char.
• Cavity Barrier: A component designed to close a void within an external wall to prevent the passage of fire and smoke, often essential when combustible materials are present in the assembly.