Metal

Definition and Material Characteristics#

In the context of the UK cladding industry, "metal" refers to a broad category of inorganic, typically ductile and conductive materials utilised in the construction of external wall systems (EWS). The most prevalent metals in contemporary British architecture are aluminium and steel, though zinc, copper, and stainless steel are also utilised for specific aesthetic or performance requirements. Within a cladding system, metal may serve as the primary aesthetic rainscreen, the structural framing (mullions and transoms), the sub-frame (brackets and rails), or the internal liner trays.

Unlike masonry or timber, metal cladding components are typically manufactured off-site to precise tolerances. Their integration into a building fabric is determined by their physical properties, including thermal expansion coefficients, tensile strength, and corrosion resistance. In the UK, these materials are often finished with coatings such as Polyester Powder Coating (PPC) or Anodised finishes to provide durability against the temperate, often maritime, climate.

The Role of Metal in External Wall Systems#

Metal components are rarely installed in isolation. In a modern rainscreen system, a metal panel—whether it is a solid plate, a honeycomb core panel, or a composite—functions as the outermost layer of a multi-part assembly. This assembly typically includes a ventilated cavity, thermal insulation, a moisture barrier, and a structural backing wall. The relationship between the metal element and these sub-components is a primary focus during building fabric assessments.

The structural role of metal extends to the support mechanism. Aluminium or galvanised steel "helping hand" brackets and T-sections form the grid that transfers the wind loads from the external panels to the building’s primary structure. The choice of metal for these components is dictated by the need to manage electrolytic corrosion (galvanic action), which can occur when dissimilar metals are placed in contact in the presence of an electrolyte, such as rainwater.

Regulatory Context in England and Wales#

The regulatory framework governing the use of metal in cladding systems has undergone significant refinement, particularly following the introduction of the Building Safety Act 2022 and subsequent amendments to the Building Regulations 2010. For buildings over a certain height—specifically those with a floor at least 18 metres (or approximately seven storeys) above ground level—the requirements for external wall materials are stringent.

Under Approved Document B, the focus is placed on the reaction to fire classification of the materials used in the external wall construction. Metals are generally classified as non-combustible (Class A1) or of limited combustibility (Class A2-s1, d0) according to BS EN 13501-1, provided they do not incorporate combustible cores or significant quantities of organic coatings. However, the regulatory context distinguishes between the metal element itself and the overall system performance. The presence of metal does not, in itself, determine the regulatory status of a wall system; rather, it is the interaction of all materials, including insulation and cavity barriers, that is assessed by competent professionals.

Testing and Assessment Procedures#

When metal cladding is scrutinised during an EWS1 (External Wall System) survey or a PAS 9980 appraisal, the objective is to identify the specific type of metal and the nature of the entire build-up. Solid metal panels, such as 3mm solid aluminium, are distinguished from Metal Composite Materials (MCM).

Testing often involves site interventions where a small section of the cladding is removed to inspect the cavity. Professionals look for:

• The thickness and grade of the metal.
• The presence of any filler or core material within the panel.
• The condition of the metal sub-frame and fixings.
• Evidence of corrosion or fatigue in the metal sheets.

Laboratory testing may be required if the composition of a metal-faced panel is unknown. Methods such as bomb calorimetry or furnace testing are used to establish the calorific value and reaction to fire classification of the component parts.

Responsibilities for Building Owners and Managers#

For Resident Management Companies (RMC), Right to Manage (RTM) directors, and managing agents, the presence of metal cladding requires a structured approach to asset management. It is a common requirement for these stakeholders to maintain a "Golden Thread" of information regarding the materials used in their buildings. This includes O&M (Operation and Maintenance) manuals, fire certificates, and technical data sheets provided by the original manufacturers.

Where documentation is missing, the burden of proof falls on the building owner to demonstrate that the metal systems installed meet current safety standards. This often necessitates commissioning a Fire Risk Appraisal of External Walls (FRAEW) conducted by a qualified fire engineer. Building managers must also ensure that the maintenance of metal surfaces—such as cleaning regimes to prevent the buildup of atmospheric pollutants—is carried out to preserve the material's integrity and warranty status.

Common Misunderstandings#

A frequent misunderstanding in the industry is the conflation of "metal cladding" with "combustible cladding." While some historical composite panels featuring metal skins and polymer cores have been the subject of remediation, solid metal panels (such as solid steel or aluminium) are fundamentally different in their physical properties and fire performance. It is incorrect to assume all metal-faced systems require replacement.

Another misconception relates to the "non-combustibility" of finished metal. While the base metal (aluminium or steel) is non-combustible, the coatings (PPC) or the adhesives used in honeycomb panels contain organic content. Regulations provide specific allowances for these coatings up to certain thickness limits, but they must be accounted for in the overall fire strategy of the building.

Related Terminology and Material Variations#

The following table outlines common metal-related terms encountered in technical specifications:

Closely related terms include "Sub-structure," referring to the metal framework behind the panels, and "Thermal Bridge," which describes the tendency of metal components to conduct heat out of the building, a factor that must be mitigated by thermal breaks in the design phase.