Joints

Definition and Functional Context#

In the context of the UK building envelope sector, joints are the points where two or more building components meet and are connected. While the primary cladding panels or masonry units form the majority of the external wall surface area, joints constitute the critical interface where these elements interact. They are not merely gaps between materials but are engineered components designed to accommodate thermal movement, structural tolerances, and environmental pressures.

The integrity of a cladding system is often defined by the performance of its joints. In a rainscreen system, for instance, joints are categorized as either open or sealed. Open joints are designed to allow a managed amount of air and moisture through the outer layer to facilitate pressure equalization, while sealed joints utilize gaskets or sealants to provide a continuous barrier. The precise specification of these interfaces determines how the building fabric manages wind loading, moisture ingress, and thermal expansion.

Role in External Wall Systems (EWS)#

Joints serve several fundamental roles within an external wall system. Structurally, they allow for the natural expansion and contraction of materials. Because cladding materials such as Aluminium Composite Material (ACM), High-Pressure Laminate (HPL), and natural stone have different coefficients of thermal expansion, the joints must be sized to prevent buckling or mechanical failure as temperatures fluctuate.

Furthermore, joints are central to the weatherproofing strategy. In face-sealed systems, the joint is the primary line of defence against water ingress. In drained and ventilated systems, the joint geometry is engineered to direct water away from the internal cavity. From a fire safety perspective, joints represent potential points of discontinuity. The detailing of how a cladding panel meets a window frame, or how two storeys of cladding are separated, involves the integration of fire barriers and cavity barriers. These components must be correctly aligned with the joints to ensure the compartmentation strategy of the building is maintained as intended by the original design or subsequent remedial specifications.

Significance in Testing and Assessment#

During the lifecycle of a building, joints are a primary focus for surveyors and façade engineers. In the context of PAS 9980 assessments and EWS1 surveys, the condition and specification of joints are scrutinized. Professionals typically examine whether the joint widths match the design intent and whether the materials used within the joint—such as backer rods, silicone sealants, or EPDM membranes—have degraded over time.

Physical testing often targets the joints as they are perceived as the most vulnerable points for air and water leakage. Testing methodologies, such as those prescribed by the Centre for Window and Cladding Technology (CWCT), involve spraying the joints with water under specific pressures to verify their performance. In fire performance testing, such as BS 8414 large-scale tests, the behaviour of joints is critical; the way a system responds to heat often depends on whether the joints remain intact or allow flame spread into the cavity behind the cladding panels.

Regulatory and Process Context in England and Wales#

The regulatory framework in England and Wales, primarily governed by the Building Regulations and the Building Safety Act 2022, places significant emphasis on the continuity of the building envelope. Regulation 7(1) requires that building work be carried out with adequate and proper materials and in a workmanlike manner. For joints, this means that the installation must strictly adhere to the manufacturer’s instructions and the fire professional’s specifications.

Under the gateway regime managed by the Building Safety Regulator (BSR) for higher-risk buildings, the detailing of joints is a matter of significant technical oversight. The 'Golden Thread' of information is expected to include specific details of joint construction, including the types of sealants used and the placement of cavity barriers at joint interfaces. During remediation projects, if a cladding system is being replaced, the local authority building control or an Approved Inspector will typically verify that the jointing details comply with current Approved Document B (Fire Safety) and Approved Document L (Conservation of fuel and power) requirements.

Considerations for Building Owners and Managers#

Building owners, Managing Agents, and Residents' Management Company (RMC) directors must view joints as a maintenance-critical element. Unlike the cladding panels themselves, which may have a long service life, the sealants and gaskets used in joints are often 'consumable' components with shorter lifespans, typically ranging from 15 to 25 years depending on the material and environmental exposure.

When commissioning a building survey, it is important to ensure that the scope includes a specific review of the jointing condition. Failure of joint sealants can lead to secondary issues such as dampness, mould, and the degradation of insulation materials within the wall cavity. Furthermore, during cladding replacement projects, the 'interface detailing'—how the new cladding joints meet existing windows, doors, and rooflines—is frequently where technical complexities and cost variations arise.

Common Misunderstandings#

A frequent misunderstanding is the belief that all joints in a cladding system should be hermetically sealed. In many modern rainscreen systems, 'open' joints are a deliberate design feature intended to allow the cavity to breathe and drain. Attempting to seal these joints with aftermarket products can impair the ventilation of the building, leading to interstitial condensation and long-term structural decay.

Another misconception is that the fire performance of a joint is solely dependent on the sealant used. In reality, the fire resistance of a joint is a result of the entire assembly, including the substrate, the fixing method, and the internal cavity barriers. Replacing a sealant without considering the wider system architecture may not achieve the intended safety performance. Finally, it is often assumed that if a joint looks intact from the ground, it is functioning correctly; however, microscopic tears in sealants or the misalignment of internal gaskets can only be identified through close-up inspection or professional testing.

Related Terminology#

Understanding joints requires familiarity with several related technical terms:

• Cavity Barrier: A component installed within the cladding cavity at joint interfaces to close the gap in the event of a fire.
• Backer Rod: A cylindrical foam material inserted into a joint to control the depth of the sealant and prevent three-sided adhesion.
• Shadow Gap: A deliberate aesthetic and functional joint design where a dark recess is created between panels.
• Flashings: Thin pieces of impervious material installed at joints to prevent water penetration.
• Substrate: The base surface or structural wall to which the cladding and its jointing system are attached.

Proper management of these interfaces is essential for the long-term durability and regulatory alignment of any high-rise or complex building envelope in the UK.