The junction between a party wall and an SFS external wall is a critical detail that, until now, has depended on engineering judgement. voestalpine Metsec’s latest full scale fire testing addresses this longstanding knowledge gap directly.
In modern light steel framing, designers have long understood how individual wall systems perform under fire exposure. Through wall SFS systems are supported by decades of standardised testing, and gypsum based party wall systems have equally well defined datasets. What has been missing, however, is clear evidence of how these systems behave where they meet.
A critical detail, historically untested
Standard methods such as BS EN 1364 1:2015 provide established benchmarks for isolated wall performance. They do not, however, address real world construction interfaces. As a result, design teams have traditionally relied on conservative assumptions when assessing the fire performance of internal wall to external wall junctions.
These assumptions have been safe, but untested. With regulatory focus on evidential performance and a sector wide push for accountable product information, the absence of junction level data has become increasingly difficult to justify. The industry has made it clear that tested junction details are a priority. Metsec’s recent test programme responds directly to this need.
Testing what designers actually build
During summer 2025 at the BRE, two large scale test specimens were constructed to replicate common party wall/SFS wall junctions, tested following the general principles of BS EN 1364-1:2015. These were not abstract laboratory setups, they were representative of real site conditions, including the actual through wall and party wall linings supplied by major UK manufacturers.
The intention was to create junction tests that reflect everyday detailing rather than idealised assemblies. Installation practices, interface treatments, and build up sequencing were all selected to be representative of real construction rather than optimised for laboratory performance. This ensured that the tests addressed the genuine behaviours and potential weaknesses that designers and contractors routinely consider when assessing junction fire performance.
By aligning the construction with both industry practice and standardised testing, the programme provides a reliable indication of how these junctions behave under fire exposure and offers meaningful evidence on which to base future design and specification.
Performance that removes ambiguity
Both junction configurations achieved 90 minutes fire resistance, matching the published performance of the individual through wall and party wall constructions. Thermocouples installed directly at the junction demonstrated no adverse heat transfer, confirming that the interface did not create a localised weakness or undermine stability or integrity. In practical terms:
• The junction did not reduce system performance.
• The wall interface behaved as a continuous, compliant fire resisting element.
For the first time, this removes the reliance on inferred performance and provides a test backed basis for design decisions.
Why this matters for designers and contractors
Fire compartmentation depends on the consistent performance of every part of a wall assembly, not just the main through wall build up but also the junctions where different systems meet. In practice, these interfaces can be the most uncertain elements of a construction, as they are often assembled from components tested separately and brought together on-site. Without specific evidence, designers have had to make conservative assumptions about how these junctions behave under fire exposure, ensuring safety but introducing ambiguity into design decisions, warranty submissions, and contractor responsibilities.
By testing representative party wall/SFS junctions directly, this programme provides a clear and auditable understanding of how the interface performs as part of the overall compartment line. It supports a more robust and transparent approach to specification, reducing the dependence on judgement based interpretation and offering designers greater confidence when assessing compliance. For contractors, it delivers clarity on the expected behaviour of the junction, reinforcing the importance of workmanship and detailing in achieving the performance demonstrated under test. In a regulatory environment increasingly focused on traceable evidence and accountable product information, this kind of tested junction level data becomes a valuable tool for all parties involved in design, assurance, and delivery.
A step forward in transparency and confidence
This testing programme marks the first published evidence of its kind for party wall/SFS junctions. It supports a more transparent, evidence driven approach to fire performance and sets a benchmark for how manufacturers should demonstrate system behaviour in real world applications.
For designers, it provides confidence. For contractors, it provides clarity and for the wider industry, it helps close a gap that has been open for too long. voestalpine Metsec’s work demonstrates a commitment not only to system performance, but to the transparency and accountability now expected across the construction sector.
