Why understanding risk is crucial for high-rise fire safety

Regulations must focus on the fire performance of cladding systems as a whole.

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Managing fire risk in tall buildings is a complex science, and no cladding system should be permitted to avoid proper scrutiny.

In the immediate aftermath of the tragic fire at Grenfell Tower last year, an independent review was established to examine thoroughly every aspect of fire safety regulation, enforcement and practice within the construction industry. The final report from the Review of Building Regulations and Fire Safety, led by Dame Judith Hackitt, is due imminently. Among a number of critical topics, it will consider which materials are suitable for high-rise applications, and what assessments and tests should be used to identify
safe constructions.

Non-combustible materials are not a guaranteed solution

There is a school of thought which argues that the best way to rule out fire risk in high-rise facades is by demanding that all systems are “non-combustible” or of “limited combustibility”. While this approach may seem logical in principle, it has significant flaws.

For example, materials are currently classified as “non-combustible” or “limited combustibility” based solely on small-scale tests carried out on tiny quantities and sometimes isolated samples of materials. No consideration is given to how the different products will perform when combined as part of a complete system.

In practice, material classification alone is not a guarantee of fire safety. The correct design of cavity barriers, for example, is every bit as important as the insulation or cladding products used. It is how everything is put together – how it works as a system – that counts.

Despite this, current regulations, automatically approve systems in which both the insulation materials and the external cladding are classified as non-combustible or of limited combustibility. This means a major part of the construction may never be scrutinised, even though significant volumes of combustible materials may be present, including thermal breaks, sealants, vapour barriers and tapes.

Relying on the stand-alone performance of products, without considering the many other factors that can allow a fire to spread is therefore simplistic and does not fully address all the risks involved.

One way to clearly assess how a whole facade system would be expected to perform in a real fire is to subject it to a rigorous, recognised large-scale test.

A full and rigorous test

BS 8414 is the test used to assess the fire performance of complete facade systems under current regulations. It is designed to replicate a fire starting inside a room, breaking out through a window and exposing the external facade to fire. To do this, the complete facade system is erected on a test wall at least eight metres in height (the equivalent of a three-storey building). It is then exposed to flames from a large combustion chamber at its base for 30 minutes.

The test evaluates the rate and extent of fire spread within a facade system (under performance criteria set out within BR 135). For the system to be classifiable, flames cannot reach the top of the wall at any point during the 60-minute duration, and temperatures must not rise above a set limit.

BS 8414 provides a high level of confidence that systems that have passed are safe to use in high rise and high-risk buildings. The independent fire safety engineering consultancy Tenos recently published research which showed that it was more onerous than the other internationally used large-scale test, NFPA 285.

In addition, the report concluded that no cladding system that has passed BS 8414 has ever made a significant contribution to the spread of a facade fire, that has been publicly reported on and that Tenos could uncover.

In contrast, some of the most high-profile high-rise fires of years have occurred in buildings with non-combustible insulation in their facades, proving that fire safety is a more complex issue than materials alone, and that the system in its entirety must be designed and tested appropriately.

The capacity problem

But while BS 8414 is rigorous, it is important to note that the test results only apply to the precise design and specification that was tested. Changing details such as the cavity width or brand of product, would require another test. With the vast array of available products and potential small variations in design, it would be impossible to test everything at a large scale.

For this reason, desktop studies were introduced as another acceptable route to compliance. In theory, this route would allow a qualified fire engineer to inspect the proposed cladding system and make a judgement about whether it would be expected to pass BS 8414. Unfortunately, the lack of common rules or accreditation requirements for assessors have led to concerns being voiced about this approach. 

In an interim report published last December, the Review of Building Regulations and Fire Safety made it clear that substantive reform is needed in this area, and sensibly suggested that it should only be possible for desktop studies to be undertaken where sufficient and relevant test evidence is available.

Intermediate testing

A far better solution could be to base judgements on relevant large-scale
tests, supplemented by a standard intermediate-scale system test of the proposed cladding system. Trials have suggested that there can be good correlation between the results achieved at intermediate scale and at large scale.

Requiring desktop studies to include test data at an intermediate scale on the actual system being analysed could make this route to compliance far more robust, but also realistic to achieve.

BS 9414

A further robust route based on large-scale testing is currently being developed by the British Standards Institute. BS 9414 is an extended application (EXAP) standard that will set a very clear process by which minor variations can be made to systems that have passed large-scale testing. This will include clear parameters for acceptable changes from the original design and specification, together with precise test data requirements.

To ensure accuracy, both BS 9414 and any future desktop studies must be supported by a mandatory qualifications and certification scheme for assessors. In addition, a public register should be kept of both these qualified assessors and all approved systems. 

Lessons learned

It is the responsibility of every person within the construction industry to ensure that the events of 14th June 2017 are never repeated. This extends from the initial designs, to the care taken when installing systems and safety measures, to the long-term maintenance and upkeep of our buildings.

It is also vital that our future regulatory structure acknowledges it is the performance of complete cladding systems, rather than individual components, that ultimately determines their fire safety and ensures that each system’s fire performance is rigorously assessed before it is considered acceptable to be fitted to a building.

This will require testing to BS 8414 and a reformed “desktop study”-type process, and this regime should be extended to include all systems.

Lawrence Cody is standards and regulatory consultant at the European Phenolic Foam Association (EPFA).