CLT: the architects friend

Materials World magazine
1 Mar 2015

Simon Frost speaks to two UK architects to find out why they are throwing their weight behind cross-laminated timber, and how the construction industry is responding.

It’s no surprise that stories about cross-laminated timber (CLT) often focus on the tallest structures in the pipeline. Superlatives sell papers – the world’s oldest dog and fattest chicken are safe bets for a headline writer chasing clicks. But though the prospect of ‘plyscrapers’ that dwarf today’s timber buildings is one to look forward to, architects have plenty more immediate reasons to be excited about CLT. 

Described by Paul Karakusevic, Partner at Karakusevic Carson Architects, as a ‘CLT evangelist’, one of the UK’s early adopters and prominent champions of CLT construction is Andrew Waugh, founding co-Director of Waugh Thistleton Architects (WTA). ‘We now design and aim to build all of our projects in timber,’ he tells me. ‘Cement is one of the biggest polluters in the world, and we’ve been looking at how we can reduce its use in our buildings for many years. So when we first started working with CLT in 2003, we were inspired by the sustainable benefits of the material, but also incredibly excited by the accuracy and precision, and its acoustic and thermal performance. It maintains an equilibrium in terms of humidity as well, which makes a CLT building a very natural place to live.’ 

In the 12 years since the Shoreditch-based outfit began working with CLT, the material’s prominence in global architecture has steadily increased, as has the physical scale of the projects. WTA’s Stadthaus, a nine-storey residential high-rise in Hackney, was the tallest timber building in the world when completed in 2009, overtaken three years later by the 10-storey Forté apartment building in Melbourne, Australia. Construction is now underway on the next record breaker – a 14-storey apartment block in Bergen, Norway. Across the Atlantic, SOM is developing plans for an unprecedented 42-storey, 80% CLT skyscraper.  

But it’s not all about height – WTA is now working on what it claims will be the largest CLT development in the world, in Dalston, London – 28,000m2 of timber will be used for an 11-storey apartment building, 121 flats and 3,460m2 of commercial build. Throughout Europe, Australia and North America, CLT has become particularly popular in the construction of schools, public buildings and low- to mid-rise apartment blocks. 

Knock-on effect

With increased architectural interest, changes can be seen in the construction industry. ‘It used to be really tough to find a contractor who would take a CLT project on,’ says Waugh, ‘but now it’s the total opposite – we find that contractors are really keen to work with it, because it’s such an accurate material, so straightforward to build with.’ Karakusevic echoes Waugh’s sentiments. ‘Using CLT shortcuts about 15 different trades because it’s all pre-fabricated – you get the walls, floors and ceilings delivered in one package, in a very open, transparent, pre-engineered and costed way, so the main contractor knows exactly what they’re in for,’ he says.

The integral role that computer-aided design (CAD) plays in CLT construction is key to what makes this most ancient of building materials so modern.
From the virtual model of a building, a cutting list is created and each component is computer numerical control-cut to exact specification and delivered on site to be built, as Karakusevic puts it, ‘like a big cardboard model’. By shifting so much of the process off-site, construction is far quicker and less obtrusive than that of masonry or timber frame. The CLT that comprised Karakusevic Carson Architects’ Bridport House, a 100% social rent redevelopment in Hackney, London, took just eight weeks to erect. Karakusevic estimates that the same building would have taken 18–20 weeks to build by traditional means. 

Work to be done

Although double the height of the building it replaced, Bridport House was just 10% heavier, which was especially important as the building sits atop a Victorian storm relief sewer, so concentrated point loads had to be avoided. And CLT’s light weight reduces the carbon cost of its transport, too. 

But sustainability, Waugh believes, is still not the impetus to architects that it should be. ‘The architects’ profession is not leading the sustainability debate at all, it’s following. You only have to look at the architects that we as a profession celebrate and give awards to – it never has anything to do with sustainability or the architects’ understanding of their impact on the environment. But climate change, fossil fuels, materials running out – these are issues that architects should be running with. Unfortunately, it’s still totally marginalised here. In the USA, architects have a much more progressive attitude towards embodied carbon and sustainability strategy in design – but if I walk into a room with UK architects and start talking about sustainability, they fall asleep. It’s awful.’ 

Homegrown buildings

Although CLT has been steadily gaining ground in the UK for the last 15 years, we still haven’t started to manufacture it ourselves – we import it, mostly from Austria, Switzerland and Scandinavia, detracting from CLT’s green credentials.
This could be about to change. Peter Wilson, Director of the Wood Studio at Edinburgh Napier University, stated in December 2014 that two unnamed companies are close to investing in a CLT manufacturing plant to be built in Scotland, using Scottish timber. He expects decisions to be made before Easter 2015, after which a manufacturing plant could be operating within 18 months. A domestic plant would add an output for the sustainable forestry industry and could even make use of ash trees felled to stem the spread of chalara dieback. 

The Wood Studio has been conducting feasibility studies and practical research for several years, including a survey of Forestry Commission-managed land to ascertain whether enough material would be produced over the next 25 years to sustain an industrial plant, and a review of the demand for CLT. Both studies indicated that a homegrown CLT plant is a viable pursuit.  

The odd couple

Another change that Waugh believes architects need to embrace more widely is the relationship between architecture and engineering. ‘Engineers still describe working with architects as a problem-solving exercise – the architect gives them a building plan and expects them to resolve the structural problems. It’s not a creative process,’ he says. ‘But as a practice, we work with engineers at the design stage, talking to them as part of the design team. The idea that we can just design buildings without a full understanding of how they’re constructed is appalling, but unfortunately that’s largely the way the architectural profession is at the moment.’  

As a technology that is still in its infancy, it’s natural that tweaks are needed for CLT construction to reach its full potential, but the next few years will likely be a period of fast development. Waugh neatly summarises something that will increasingly work in the material’s favour, especially as the drive towards emissions targets intensifies. ‘It soaks up carbon as it grows, stores it when it’s cut and it’s replenishable. You can’t say that about cement or steel.’ Touch wood, the UK will be building houses with its own trees before long. 

Fire resistance

While it’s natural to associate wood and fire, the fire-resistant properties of CLT have been repeatedly and successfully tested. ‘Timber frame and solid timber are very different when it comes to fire,’ says Andrew Waugh. ‘Yes, there was a CLT building at Nottingham University that burnt down during construction last year, but it is actually being rebuilt with timber because it retained its structural integrity incredibly well.’ The moisture content of timber means that faces of CLT panels exposed to fire char predictably and slowly. The insulating char layer protects the core of the panel, allowing it to maintain its structural integrity for longer than steel. In tests recently commissioned by CLT producer Stora Enso and carried out by fire engineering and safety consultant Markku Kauriala, all load-bearing CLT elements tested reached a minimum classification of REI 60 (resistant to fire with respect to load-bearing capacity, integrity and insulation for at least 60 minutes), with more than half achieving REI 90.