Insurers claim a hybrid approach is the secret to delivering mass timber buildings, but what type of structures work, and how can architects ensure projects are underwritten? Stephen Cousins investigates
Increasing reliance on engineered wood for mass timber construction is an inevitable consequence of the drive towards net zero as design and construction teams turn away from carbon-intensive materials and methods.
Materials like cross laminated timber (CLT), laminated veneer lumber (LVL) or glulam are low embodied carbon, lightweight, versatile and offer a high degree of precision, but they don’t easily fit into long-established construction classes – and prohibitive regulations, including the ban on combustible cladding in new buildings over 18m, stifle their application.
The mass timber dilemma goes to the heart of established insurance principles of risk assessment, and as a result multi-storey projects often struggle to secure cover or find themselves back at the drawing board looking to concrete or steel as an alternative.
A research paper, Insurance Challenges of Massive Timber Construction, carried out by RISCAuthority on behalf of 24 insurers including Aviva, Axa and Zurich, argues that the best way forward for the industry is to take a hybrid approach to building structure.
Exploiting a combination of conventional and modern building materials and methods, it says, helps limit financial losses to satisfy the needs of insurance risk control as well as meeting carbon reduction targets.
Insurers are reluctant to underwrite mass timber projects mainly because timber lacks the inherent resilience of steel and concrete. Combustibility is a key concern, but according to the report, water damage is more important, as leaks tend to occur more often and timber is more susceptible to damage to finishes, delamination and structural deterioration.
Jim Glockling, director of RISCAuthority and technical director at the Fire Protection Association, says: ‘In a multi-storey environment, escape of water is a massive area of loss. It can’t be underestimated how important that is to the insurer. Masonry and concrete are resistant to escape of water; massive timber isn’t.’
This is confirmed by Andrew Waugh, director at engineered timber pioneer Waugh Thistleton Architects: ‘Increasingly we find insurers are primarily concerned about long term water ingress – leaking taps and flooded bathrooms. This means demonstrating that leak detection systems are installed, detailing adequate water proofing, and ensuring that our client hires a competent contractor.’
A fundamental maxim of insurance is ‘you can’t insure what you can’t quantify’, and a shortage of research and knowledge on the impact of fire or water on composite wood materials adds to the perception of risk.
The report states that massive timber building designs are being proposed ‘at a form and scale that is running ahead of current scientific understanding, testing and research’, which therefore cannot fulfil insurers’ requirements for information.
Glockling comments: ‘If an LVL or glulam beam has had water dripping on it for a number of years through a leaky roof, how easy is it to adjudicate on whether it’s still structurally capable or not?...you still can’t get people to sign off that a certain amount of fire or water damage in mass timber buildings isn’t meaningful to structural integrity, which generally means structure has to be replaced, adding to the expense.
Designers must understand that simply aiming to achieve compliance with building regulations is pretty meaningless to the insurer
The research highlights various hybrid measures implemented on successfully-insured projects. These include using concrete cores in mass timber buildings to vertically route services and house plant and electrical intakes, which can improve building stability, reduce combustible void challenges and make access safer for firefighters.
Similarly, locating all bathrooms and kitchens within concrete cores can reduce the potential for escape of water damage and can support built-in drain-to-safe features.
Furthermore, building the first floor in concrete can protect against arson or accidental fire during construction and improves resilience to flood. Alternating CLT floors within concrete or steel-framed buildings can preserve a higher level of ‘insurance relevant’ compartmentation, says the report, improving building stability under fire and supporting fire fighting activities.
‘Light timber structures are really a loose collection of voids, so it’s difficult to argue that a CLT version of that same building wouldn’t be better,’ says Glockling, ‘especially when structures are designed to be more tolerant of escape of water, either by installing specialist devices, or designing the structure to shed to a safe place.’
He adds that to realise insurable mass timber structures designers must first understand that simply aiming to achieve compliance with building regulations ‘is pretty meaningless to the insurer’.
Current regulations and the framework for the specification of construction materials, methods and safety systems, are developed to ensure structural stability for enough time to complete evacuation, but after that time has elapsed, there is no further expectation for the building to resist fire.
The RISCAuthority study details 26 insurance-relevant design features that affect overall building insurability, divided into six principal categories covering building occupancy and use, scale, structure and fabric, other risk factors, and fire and water mitigations.
‘Architects should take those opportunities to address these within the RIBA design phases and build in whatever measures they can,’ says Glockling.
Construction issues such as long term movement or visual cracking can be avoided if a contractor team with sufficient knowledge and experience of pulling together different materials is appointed.
Greg Cooper, managing director of Derby-based Hybrid Structures, explains: ‘We have the experience, knowledge and test data; we know what works on projects, what details meet requirements in terms of robustness, and how things are manufactured and go to site. It’s all about bringing together a full turnkey solution, rather than looking at everything in isolation.’
Concise detailing of junctions, connections, and interfaces to ensure they meet acoustic, fire and durability requirements during construction and operation is key, he adds.
Unfavourable market conditions experienced by engineered timber in recent years have galvanised a number of other initiatives striving to unlock construction through research, design innovation or cross-sector collaboration.
Recent research carried out by the Timber Accelerator Hub, run by the Alliance for Sustainable Building Products, has investigated the barriers preventing wider uptake of mass timber, how to overcome difficulties obtaining insurance and negative perceptions about fire performance and prohibitive regulation.
The New Model Building project, led by Waugh Thistleton Architects with UCL and Buro Happold, aims to create a pre-warrantied standard design for a six-storey mass timber housing block.
The UK needs to move quickly to catch up with other countries already backing engineered timber construction, says Waugh: ‘The rest of the world is changing its building codes to promote the use of engineered timber and is using public procurement to encourage the construction of timber housing, schools and hospitals. Building in timber is still the only viable way to reduce the carbon burden of construction.’
CASE STUDY: 38 Berkeley Square
An innovative hybrid steel/exposed CLT structure was developed for the recent construction of 38 Berkeley Square, a nine-storey office block in Mayfair by architect Piercy & Co.
Through a first principles ‘deterministic’ approach to proving fire performance, including detailed calculations and upfront design, the project team was able to meet the requirements of the London Fire Brigade, approved inspectors and insurers.
Specialist contractors, including structural timber contractor B&K, were engaged during pre-tender stages, Sweco Building Control regularly assessed design proposals, and a timber calculations specialist crunched the numbers to demonstrate that the building would not present an increased fire risk.
CASE STUDY: 6 Orsman Road
6 Orsman Road in Shoreditch, designed by Waugh Thistleton Architects, is a six-storey commercial office supported on a steel frame with a CLT core and floor slabs. Steel takes the principal gravitational loading, while the slabs and core act in diaphragm to take shear forces.
The project achieved a 70% reduction in carbon emissions over traditional steel and concrete and its weight was reduced by 60%. Construction was also significantly faster and less expensive than the concrete steel equivalent.