Rory Bergin from HTA Design says that with timber under fire in the complex testing landscape post-Grenfell, specifiers are being wrongly deterred from choosing the material for taller buildings, while being asked for more sustainable designs
Current amendments to Building Regulations place restrictions on the use of combustible materials in the external walls and balconies of multiple-occupancy residential buildings with a floor 18 metres or more in height above ground level. The ongoing review of the regulations by MHCLG has led to consideration of this residential building height threshold being lowered from 18 metres to 11 metres. Some authorities and clients, like the GLA, have already begun the proscription of combustible materials from use in external walls in buildings, regardless of their height, within their affordable housing programme.
Just at a point in time when we need to be using more sustainable materials, there is fear and confusion across the sector about what is acceptable and what isn’t when it comes to timber and combustibility. This confusion is partly driven by a bizarre interpretation of combustibility in facades which separates windows from walls, and treats them differently to the point where you cannot have a plastic interlayer in a glazed balcony balustrade but you can have a full-height plastic window overlooking it.
The way forward seems to be fire testing for products and systems. As architects, we are not experts on fire performance, and we shouldn’t try to be, so we increasingly demand tested ‘systems’ from manufacturers to enable us to use them in buildings, particularly those considered to be riskier, but increasingly for all buildings. The problem with setting out fire guidance for buildings above a certain height is that residents and insurers of buildings which may be close to that height will look to those standards and say, why can’t we have them for our building? Are we less safe because we don’t meet that higher standard? This behaviour has led to the GLA refusing to fund affordable dwellings of any height with combustible materials in the facades because they fear politically motivated headlines, and not because there is any reasonable risk attached to those buildings. We have had timber used safely in the structure of buildings for hundreds of years in the UK.
Meanwhile we are being pushed by planning policy and the imperative of mitigating climate change to reduce the embodied CO2 in our designs.
The RIBA 2030 Climate Challenge proposes that we should be reducing the CO2 content of our buildings from an average of 1200 kg CO2E/m2 down to 625 kg CO2E/m2 by 2030. Timber construction is a major weapon in our battle to get there, and the Government has signalled that in the run up to the 2025 Future Homes Standard, that they will look at regulating this.
Testing times
Instead of manufacturers collaborating to offer the industry a joined-up approach where a fully-tested wall system can be offered to the market, (and which could be insured easily as a tested product), we have firms going in the opposite direction and not taking liability for fire performance of their products. Without tested systems being offered to designers, the onus is pushed on to the design team to sort out who is responsible for fire performance of walls and floors. If no performance test is available then the design team should demand it, often at considerable cost and time delay. Architects’ PI insurance doesn’t generally cover us for accepting liability for the fire performance of untested systems.
Timber itself is not the problem – any failure of wood-based systems is usually down to piecemeal procurement, design competence and fragmented testing evidence – the Structural Timber Association has recently undertaken a whole raft of testing on this. To prevent timber frame buildings from prematurely failing to meet the statutory guidance (that is designed to protect life), designers must understand the complete design and construction process. Ultimately, we still have limited joined-up testing information available for engineers and architects to use. The protection of a building or asset is something that insurers are now becoming more concerned about; this raises a wider conversation.
The problem with taking the ‘test everything’ approach is that every project is different. Manufacturers reasonably ask: what version of our system should we test – the one designed to meet GLA targets, the one designed to meet current Building Regulations, the one designed to meet the 2021/22 Part L Regulations, or the one designed to meet Part L 2025 (The Future Homes Standard)? If every manufacturer was to test every product against every possible use of it, we would need many more testing centres, and a large number of SME suppliers would go out of business.
An existing solution
However, the fire performance of loadbearing timber frame walls and floors from inside the building needs to be tested, plus the acoustic performance of compartment walls and compartment floors, and this can be done now – without considering all the variants for different regulations. There was industry guidance on this, which has been withdrawn following the Grenfell fire. Even where manufacturers have done this testing however, they won’t take design liability for fire performance, leaving a vacuum in design responsibility that architects are expected to fill.
In addition, problems with performance of any kind, be it fire, acoustic, thermal, air tightness or water tightness, tend to happen at the interfaces between materials, so we would have to test every interface between every material. The ‘math’ demonstrates pretty quickly that with two types of timber frame, three types of insulant, and two external wall materials, joining two types of party walls gives you 24 tests. If you add in the varying requirements for changes to Building Regulations it comes to 72.
Unless testing houses can come up with a quicker and cheaper way of testing, this is not going to be possible.
I think that the answer to the problem is for the manufacturers of timber frame and other framing systems to collaborate and work together to develop a set of standard approaches which can be tested. Then fire consultants can use those as a basis to demonstrate that individual buildings are equivalent to, or demonstrably better than, a specific set of tests. Otherwise we are going to go in circles for years, and we are not going to be able to use timber frame to build more sustainable buildings – right at the point where we need it most.
Rory Bergin is partner, Sustainable Futures at HTA Design