Safe and sustainable construction for all our communities

Overview

We are calling on the MHCLG to legislate for a safety framework, which focuses on the use of timber structures, including the use of timber in external walls.

Preliminary research demonstrates safe solutions are feasible, despite timber being combustible, and this research could be built upon to create statutory design guidance. 

There is currently no statutory design guidance for timber based systems, in England and Wales.

This is needed as a matter of some urgency, due the demands on the construction industry now, to play its part in helping resolve the housing crisis, as well as the industry's role in the wider embodied carbon reduction agenda.

There are important lessons learnt from recent external wall fires.

Therefore there is also an urgent need for a new testing regime coupled with a science-based approach to the fire safe design of external walls, for all materials, including timber.

Current Government consultation on external wall construction

Arup are formulating their response to the consultation led by the Building Safety Programme at the MHCLG, who are currently seeking views on proposals to change the current requirements of the Building Regulations. These in effect ban the use of combustible materials in and on the external walls of certain buildings and in specified attachments to the external walls. 

This latest consultation sets out proposals regarding changes to building types covered, height threshold, list of exemptions, attachments such as blinds, shutters and awnings, and a proposal to specifically ban the use of metal composite panels with a polyethylene core in and on the external walls of all buildings.  The deadline for responses to this consultation is the 25th of May 2020.

Why we are publishing this article

This latest consultation has triggered a petition by the Architects! Climate Action Network (ACAN), under the banner “Join us in opposing the Government’s proposal to ban the use of safe structural timber in the external wall of buildings with a floor above 11m”. We present some considerations in this article, on why we will not be signing this petition, despite our ongoing commitment to designing timber buildings as part of our role in reducing carbon emissions. 

The urgent need for a UK Government led safety framework for timber buildings 

To ensure the overall fire safety of timber buildings, we call on the MHCLG to produce a safety framework based on significant research, robust fire testing and pragmatic design guidance accounting for the unique aspects of timber in fire events, such as the additional fire load it contributes, and the effects of glue-softening in laminated timber components.  

Preliminary research demonstrates safe solutions are feasible, despite timber being combustible, and this research could be built upon to create statutory design guidance. This is needed as a matter of some urgency.  

Is this latest consultation focused on timber structures only? 

This consultation focuses on all combustible materials and in the external wall only. 

But it has the potential to particularly impact CLT construction systems, which have an important role to play in reducing embodied carbon and addressing the housing crisis.  CLT construction systems are a viable and economic option for the construction of low to mid-rise dwellings particularly. But these systems tend to include the use of structural timber panels in the external walls. 

Hence there is some concern the importance of CLT materials in helping resolve the housing crisis, as well as the role these materials could play in the wider embodied carbon reduction agenda, may be lost through this consultation which focuses on fire safety. 

But, both the housing crisis and the Net Zero agenda, see below, are pressing issues for the construction industry. Hence the urgency in the need for an independent Government led safety framework. 

Net Zero Solutions

The pressing need for the construction industry to create safer buildings to protect all our communities is clear. The construction industry is also tasked with leading on Net Zero Solutions for the Built Environment: playing our part in achieving an overall balance between emissions produced and emissions taken out of the atmosphere. 

Driving forward on Net Zero Solutions has implications for fire safety; formulating a response to this timely and worthwhile Government consultation, brings these implications to the fore. 

However, there has been conflict between the reduction of operational carbon and maintaining fire safety standards, for several decades now; so, this is not new or unchartered territory. 

One manifestation of the conflicting demands here in the UK, was the rise in sales of building insulation, and over-cladding systems, throughout 2000-2017; and there is a point of view that this has in part led us to some of the more recent serious fires, and ultimately the need for this latest consultation request from MHCLG.

There needs to be clarity that the consultation focuses only on external walls 

This recent consultation focuses on the external wall only and not the wider structural system. It deals with specific building uses – buildings with a storey over 18m above ground level that contain one or more dwellings, or are classified as institutional use (Hospital, home, school or other similar establishment, where people sleep on the premises). This consultation asks if this scope should be extended to include other sleep risk buildings such as hotels and hostels, and if the height threshold should be dropped to 11m. 

Just as the Architects Climate Action Network (ACAN) has been communicating recently (https://www.architectscan.org/safe-timber), we too share the desire to ensure the wider industry understands this consultation focuses on the external wall only, not the structural frame. 

Timber is a viable structural system and is currently permitted under the regulations 

We note that timber systems remain regulated for in the Building Regulations (2010) as amended, and the Construction (Design and Management) Regulations 2015, both of which continue to allow timber as a structural form to be used - subject to satisfactory evidence that the functional requirements set out in the above Regulations can be met.

At this time the only restriction made in the Regulations, applies to the external wall of some sleep risk buildings over a certain height, that rely on high degree of internal fire compartmentation. 

It is our opinion however that timber remains viable as an option for the structural system in all buildings, regardless of height.

There should be a science-based approach for the fire safe design of external walls

The ACAN state they want to: â€œTake a science-based approach. Use BS8414 as the base for fire safety compliance, which was found to still be fit for purpose in the Hackitt Review.” 

We too support a science-based approach to formulating robust and resilient construction systems, including timber, for use in external walls. However, we do not advocate the retention of BS 8414. 

It is important to note what Dame Hackitt states, regarding BS 8414, in her final report, which occurs only on Page 93, Section 7.9:

“The revised text should also have the effect that assessments in lieu of tests relating to the likely fire performance of external cladding systems (measured against the BS8414 test) would only be carried out by competent staff working for an organisation that is accredited to run the BS8414 test itself”

We find no conclusion within her Report, that BS 8414 “remains fit for purpose”.

The basis of BS 8414 Fire Performance of External Cladding Systems needs to be reviewed 

Our concern is that the scientific basis of BS 8414, and its associated acceptance criteria in BR135, requires detailed reconsideration, as they do not provide a sufficiently robust process to assess the performance of external walls in real building fires. 

For example, the timber crib heat source in the test is extinguished at 30 minutes after ignition, and the time at which temperature is assessed in the test, is over a duration of 15 minutes only. Both these time frames are substantially less onerous than the typical fire exposure a structural system should resist, particularly in high rise building fire scenarios.

As part of Arup's ongoing research into the behaviour of various forms of external wall construction in fire, we present below some temperature time graphs - taken from the most recent results produced in an MHCLG sponsored BS8414 test.  In this particular test, which we present as an example only, the system contained stone wool insulation with HPL panels (https://www.gov.uk/government/publications/fire-test-report-mhclg-bs-8414-hpl). We have used this, as we could not find publicly available CLT test data for us to produce the mark ups provided below.

In the Figures, the 600°C internal fire spread failure criteria from BR135 is marked with the black dashed line; and the 15-minute point after the start time with the blue dashed line. Two graphs taken from the published test results are overmarked. The top graph shows the temperature measurements in the centre of the rainscreen. The lower graph shows the temperature in the centre of the cavity.

All these criteria come from the separate publication BR135. BR135 contains the classification system for fire performance of cladding on multi-storey buildings; and results from any BS8414 fire test, must currently be considered on that basis: 

“Failure due to internal fire spread is deemed to have occurred if the temperature rise above Ts of any of the internal thermocouples at level 2 exceeds 600 °C, for a period of at least 30 s, within 15 min of the start time, ts”

Where Ts is the start temperature and ts is the start time.  

The resulting “green box” indicates where the performance is considered against the assessment criteria only, and therefore in that context this tested HPL assembly met the BR135 performance criteria. 

But we direct your attention to the temperature profiles beyond the current limit of this 15 minute “green box” . All those behaviours, including the peak temperatures observed in the test results presented here, are currently ignored under the current BS 8414/BR135 testing regime.

These temperature results will vary from system to system, and therefore what occurs in the limits of the “green box” or outside those limits. 

This is just one simplistic illustration, of why we will not propose the retention of BS 8414, nor the assessment criteria set out in BR135, as part of our response to this consultation regarding combustible materials in and on the external walls of certain buildings.

Other relevant parameters 

Our response does incorporate our view that the height of a building is not the sole parameter when considering how to regulate materials for use in external walls.  

The available firefighting access and facilities; and the form of evacuation regime; are just two critical features in deriving how to mitigate the risk the construction of external walls pose to people, in the event of a fire.  

Regarding timber used in external walls, substantial full-scale testing will be a very important way for designers to assure the public, building owners, those who have a duty of care towards occupiers/tenants/residents, the statutory approving authorities, the insurance industry, and the fire and rescue services, that timber can be safely relied upon in the event of fire.  

Evidence based design 

Overall, for external walls, and indeed for the wider structural system and compartmentation, formed of timber, fundamental research is now needed to improve our understanding. 

This should address the fire performance of the material itself, and when it is used in complex systems, the creation of a body of certified active and passive fire protection systems needed to protect compartments formed from timber construction, the creation of connection and fixing details proven as robust in the event of fire.  

In other words, a whole body of knowledge is now required for designers to make the substantially greater progress needed on Net Zero Solutions, and so enable the production of even more fire safe timber buildings. 

The need for bespoke firefighting techniques and therefore preparing for the fire characteristics in these buildings, will be a substantial additional mitigation measure, to assure the public. 

We can build on lessons learnt from the recent fires in external walls, and demonstrate as an industry we are now committed to bringing competency and quality to any construction form, upon which we rely, including timber. This will help alleviate the current perception of fire safety risk, held by many of the stakeholders reliant on our industry. 

The regulatory regime changes in Scotland regarding timber fire safety 

ACAN call for: “Align legislation with the Scottish approach. This will encourage a common regulatory approach throughout the UK improving clarity and safety.”

We support this call. In 2019 the Technical Handbooks in Scotland were updated to acknowledge in Section 2.07 Alternative approaches, some of the parameters that have emerged from research into fire safe timber in the last few years (https://www.istructe.org/journal/volumes/volume-96-(2018)/issue-1/fire-safety-design-in-modern-timber-buildings/). There are other references demonstrating the research on these topics available.

The Technical Handbooks, refer to parameters including the additional contribution exposed combustible materials make to fire load density, assessing the impact of delamination of CLT, and the need to maintain overall stability without reliance on intervention from the fire and rescue services.   

This acknowledgement enables robust design proposals from designers.  Therefore, like ACAN, we too support the spirit of the Technical Handbooks.  

We therefore call for new statutory design guidance in England and Wales, that also considers timber performance in fire.   

Concluding remarks 

This latest MHCLG consultation process addresses all combustible materials, not just timber. 

However, we will be strongly encouraging MHCLG to urgently create and publish a safety framework to enable timber construction, and so ensure the strong need for this material by the construction industry can be implemented pragmatically, and safely. 

The Net Zero objective, can in our view, be integrated with the life safety requirements in any building, this means timber buildings too; both within the current and hopefully within any new regulatory framework. 

This will accelerate the construction industry's ability to produce safe and sustainable construction for all our communities, and shape a safer world.