Steel

Definition and Material Composition#

Steel is a strong iron alloy commonly used as a structural material in buildings. In the context of the UK built environment, it is defined primarily by its carbon content—typically less than 2%—which distinguishes it from cast iron. The addition of varying proportions of other elements, such as manganese, chromium, and nickel, allows for the production of different grades with specific mechanical and chemical properties.

In cladding systems and external wall assemblies, steel serves both structural and aesthetic functions. It is valued for its high tensile strength-to-weight ratio, durability, and predictability under load. Unlike many other building materials, steel is non-combustible, a characteristic that defines its role within contemporary fire safety frameworks for high-rise and high-risk buildings.

Applications in External Wall Systems#

Steel appears in several distinct forms within an external wall system (EWS). Its most common application is as the primary structural frame—either as hot-rolled steel sections or cold-rolled light gauge steel framing (LGSF). Beyond the primary structure, steel is frequently used for:

• Supporting Rail Systems: Steel "helping hand" brackets and rails provide the sub-frame to which outer cladding panels are fixed.
• External Cladding Panels: Steel sheets, often coated with polyester powder (PPC) or plastisol, are used as the rainscreen itself, particularly in industrial and commercial settings.
• Fixings and Fasteners: Stainless steel is the industry standard for bolts, screws, and rivets to prevent galvanic corrosion between disparate metals.
• Cavity Barriers and Flashings: Galvanised steel is often utilised for fire-stopping components and weather-proofing details around window apertures.

Role in Cladding Testing and Assessment#

In the UK, the assessment of steel within a cladding system often focuses on its performance during an EWS1 (External Wall System) survey or a Fire Risk Appraisal of External Walls (FRAEW) conducted to the PAS 9980 standard. Because steel is classified as non-combustible (typically achieving a European Classification of A1 or A2-s1, d0), its presence is generally viewed differently by assessors than that of combustible materials like timber or certain composites.

Testing often involves verifying the gauge (thickness) and the coating of the steel. While the steel core is non-combustible, the coatings applied for aesthetics or corrosion resistance must be assessed for their contribution to the fire load. In repair and replacement scenarios, professionals examine the condition of the steel sub-frame to ensure that corrosion has not compromised its structural integrity, which would necessitate remediation before new cladding is installed.

UK Regulatory Context#

The regulatory landscape for steel in UK buildings is shaped by the Building Regulations 2010, specifically Approved Document B (Fire Safety) and the updates introduced via the Building Safety Act 2022. For "relevant buildings" over 18 metres in height (or 11 metres in certain contexts), the use of non-combustible materials in the external wall is a primary requirement. Steel is a fundamental material used to achieve compliance with these restrictive standards.

For buildings in England and Wales, the design and installation of steel components must also adhere to British Standards, such as BS EN 1993 (Eurocode 3) for structural design and BS EN 1090 for the execution of steel structures. In the context of remediation, the Golden Thread of information requires that the specific grade and specification of steel used in the facade be documented and maintained throughout the building’s lifecycle to facilitate future safety assessments.

Considerations for Building Owners and Managers#

Building owners, managing agents, and Resident Management Company (RMC) directors must understand that the presence of steel does not, in isolation, guarantee a specific safety rating for a building. While the material is non-combustible, the performance of the external wall depends on the entire assembly, including insulation, cavity barriers, and the method of attachment.

When commissioning reports or remediation works, stakeholders should be aware of the following distinctions:

Maintenance is another critical factor. While steel is durable, the protective coatings on steel cladding panels have a finite lifespan. Managing agents must oversee periodic inspections to identify "edge peel" or chalking of the coating, which can lead to substrate corrosion if left untreated.

Common Misunderstandings#

A frequent misconception is that all metallic cladding is inherently "safe" or identical in fire performance. It is important to distinguish between solid steel panels and Aluminium Composite Material (ACM) or Steel Composite Material (SCM). SCM consists of two thin layers of steel bonded to a core material; if that core is combustible, the panel’s fire performance differs significantly from that of a solid steel sheet.

Another misunderstanding involves the assumption that steel cannot fail in a fire. While steel does not burn or contribute fuel to a fire, it loses structural strength at elevated temperatures (typically starting around 400°C). Therefore, structural steelwork often requires passive fire protection, such as intumescent paint or boards, which must be verified during cladding inspections to ensure they have not been disturbed or omitted.

Related Terminology and Technical Links#

Understanding steel in cladding requires familiarity with several related terms:

Non-combustible: A material that does not contribute to a fire, typically rated A1 under BS EN 13501-1. Steel is the most common structural example.

Galvanic Corrosion: A process that occurs when two dissimilar metals (such as steel and aluminium) are in contact in the presence of an electrolyte (like rainwater), leading to the rapid degradation of one metal. This is a primary concern in EWS design.

Bimetallic Fixings: Fasteners designed to join different materials while mitigating the risk of corrosion, often featuring a stainless steel body with a hardened carbon steel point.

Cold Bridging: The transfer of heat through conductive materials like steel. In cladding design, thermal breaks are required to prevent steel components from transferring cold from the exterior to the interior, which can lead to condensation and mould.