Why is secondary timber still secondary?

Maximising the use of bio-based products is key to meeting current sustainability targets. Yet according to sustainability consultancy Metabolic, using bio-based products for half of new housing in Europe would require production of engineered timber to increase nearly fivefold, and roundwood by four and a half times.

Given the improbability of an uplift in production on that scale, as well as the current rapid growth in global demand and the UK’s own limited timber harvest, using the country’s waste timber to make ‘cross-laminated secondary timber’ – CLST – would seem to make a great deal of sense.

Rather than its current fates of combustion for energy recovery, chipping for MDF, dumping in landfill, and other low-grade, non-circular activities, incorporating this plentiful material into supply chains would benefit the construction industry, as well as the country’s economy and environment. And it would also seem to be a highly achievable ambition – so why isn’t it happening?

Two figures with particular expertise in the area, both currently at UCL’s Department of Civil, Environmental and Geomatic Engineering, are Colin Rose and Jonas Breidenbach. Over the last decade, the former has penned a series of pioneering papers on systematising the reuse of timber, and employing recovered timber in construction, while also spending time in private practice working in related areas, for instance on Reed Watts’ acclaimed Roundhouse Works in Camden, with its cladding of reclaimed timber sleepers.

Together with Julia Stegemann, he is also a founding partner of the research and development-led company UK CLT that focuses on local manufacture of CLST and ‘glued-laminated secondary timber’ (GlulamST), turning ‘circular economic theory into practical action, in pursuit of a sustainable and regenerative built environment’, in the words of its website.

Jonas Breidenbach has been working for two years as a research fellow at UCL, exploring secondary timber quality and quantities in the UK and CLST’s implementation within a circular economy. His background spans structural timber design, international development, business administration and carbon capture. He is also a trained carpenter, and brings that expertise and passion for timber to his role as the third partner at UK CLT.

I spoke to both to get an update on a sector, and proposition, that has so much going for it, but seems to be taking its time to transfer onto the country’s building sites.

Is it fair to say that, since your initial research, CLST hasn’t quite got the traction it deserves?

CR: In part that’s just life – development and partnerships are needed to turn ideas into practical proposals. Research wouldn’t be playing its role if it wasn’t describing what might be possible in future. But waiting after my initial research was not just an accident. I did think that there would be growing interest in this topic over time, and there’s certainly much more talk about the circular economy today. 

What do you ascribe that to?

CR: The London Plan has helped ensure engagement – there’s a lot more discussion of physical reuse hubs, and support for and adoption of material passports and digital reuse platforms. Reuse of steel in particular has been taking off. That’s in part due to its value, but also to the introduction of a standard for its structural reuse, and established companies creating a supply chain. We feel timber has the same potential, having both inherent value and throughput; we’re just a few years behind steel. In Europe, there’s been work on standards and the circular economy under the aegis of CEN/TC 350, and a specific standard for structural reuse of timber is under review in Norway.

Have your own understandings of CLST evolved?

CR: In the wake of my PhD research, I tended to describe it in terms of a direct equivalent of CLT, which is probably the easiest way to enable and encourage reuse of materials – coming up with products that you can procure and use in the same way as new equivalents, so designers and contractors don’t have to deal with difficult supply chains, or worry about it looking and performing differently. I still think that’s the right principle – and in many cases CLST does share many of CLT’s properties, structural or otherwise – but I also feel there’s also an increasing appetite for materials that illustrate and tell the story of circularity.

There was a lot of publicity for your CascadeUp pavilion at last year’s London Design Festival – the first project constructed entirely from CLST and glulamST. Do you feel things are moving again?

CR: Yes – the pavilion also made appearances at the UCL Festival of Engineering and, more recently, at the entrance to Futurebuild London, and has been really effective in helping us to raise awareness, getting people thinking about what’s possible. We’re increasingly being asked by companies if they can introduce circularity within their own operations, both in their sourcing and reuse of materials, so they can become circular businesses. 

JB: The reality of a secondary product with different qualities from the original – ones that lessen its suitability for certain purposes but also ones that add value – always brings challenges and opportunities. There’s definitely interest. We’ve had lots of organisations approaching us, attracted by the idea that this material can be sourced and manufactured locally, and be used not far from its point of origin. They include prominent architectural practices in London, housing associations and developers, all of them expressing interest in working with our products.

And what’s your pitch to those people?

CR: That CLST is easy to adopt – it doesn’t force you to do anything very different, but it delivers lower embodied carbon, zero resource extraction, and reduced waste. It retains value by being designed for disassembly and reuse in future. We’re working with smaller-scale, modular dimensions, so it’s more handleable, and potentially more adaptable and reusable, than 16m-long CLT panels. Circularity in timber is different to a steel column becoming another steel column: with timber it’s a transformational process that can add value.

JB: But it’s becoming clear that, beyond sustainability issues, there’s also increasing interest in the journey – the narrative – of materials. And secondary timber can provide that narrative. In most cases, supply chains are this unknown black box, involving materials sourced from far away, which also raises social fairness issues and environmental questions. Secondary timber has a known, short supply chain – we can say where everything has come from. So, in effect, a guilt-free, local specification is what we offer.

To be clear, our products don’t look like a load of old planks stuck together, but there are things we can do with the relief, texture and finish that differentiate them from a standard CLT panel made from spruce, and that offer is attractive to many people. And, as mass timber becomes more common as a construction material, this sort of cutting-edge area will be a way for projects to stand out from the crowd.

So what are the immediate challenges?

CR: There are still challenges to overcome around finance and insurance. At the moment, for secondary timber, there’s no standard for strength grading, which means you have to do more work to understand how it will perform structurally. As experience is gained, codes and guidance will catch up to provide further confidence to specifiers, and we can get going with non-structural uses in the meantime.

In terms of bringing insurers on board, there are already some smaller companies engaging with timber in the circular economy who see this particular issue, rightly I think, as primarily involving a ‘perception of risk’, and are starting to take a more commonsense approach. The challenges around cost are the same faced by most startups doing something innovative at a small scale. Regulation of embodied carbon and more stringent circular economy policies will help level the playing field for sustainable startups. We want to inspire policymakers to consider what they can do to enable circular businesses to succeed – hopefully having a ripple effect beyond UK CLT’s own impact.

Is there a risk that you will always be undercut by products coming out of Europe?

CR: Primary timber has scale on its side in a big way – so we need to head towards that kind of scale, which would be possible if we were to collect a large proportion of what’s being currently discarded, with major investment in infrastructure and logistics. In some ways we’re at an advantage – we don’t have to manage forests, fell trees, kiln-dry logs, transport goods overseas and so on. But then our feedstock isn’t all in one place – we have to collect different batches with different size pieces, and process them to get the metals out and reach consistent section sizes. So swings and roundabouts. It will take a series of springboards to get to the scale at which we want to operate.

We’re not the only people looking at this opportunity, which I find encouraging. At a recent conference where we presented results of our work, there were groups from Denmark, Norway, Germany, Austria and elsewhere who were working on similar topics. There’s good progress in North America too. It’s becoming a global movement with a potent mix of academics and businesses.

JB: We’re not aiming to compete pricewise with large-scale CLT manufacturers producing material in bulk in the near future, but we are convinced that some of that difference will be made up by people willing to pay a bit more to have a product with a story. For example, for the CascadeUp project we ‘mined’ from a demolition site in Mayfair, Central London, and we found some really beautiful pieces of old timber. In instances like that, as we said earlier, you know exactly where the material comes from, and it’s fascinating that some of that timber was about 300 years old, maybe older, and had already been growing for maybe 100 years before being harvested and used to construct the original building. So you have timber that has already lasted 400 years, and we’re giving those timbers another life instead of just chipping them and using them for energy recovery – it’s a beautiful thought.

Where do you see things in five years

CR: I’d like to get to the point where UK CLT has a hub where the main operations take place, but also has spokes on a smaller scale – perhaps movable micro-factories deployed on sites close to large-scale regeneration. I see this as a more nimble and lower-risk way to grow than investing in a single enormous factory, and we’re seeking funding to develop the necessary equipment.

JB: I get the impression there will be a shift of perception among buyers, and in society as a whole, with people starting to value where materials are coming from rather than feeling that ‘recycled’ equates to a second-class product. The perception will shift towards appreciating and enjoying sustainable, reusable products as first-class, and possibly even better, though I’m probably a little bit biased!

CR: But why wait five years? We need to start delivering projects now to create these changes. Sure, some things need more time bubbling away – or boiling – before we can capitalise on them at scale. But with the various research projects going on, I hope we’ll get something in place for the strength grading of secondary timber before too long, so people can rightly have more confidence in its structural capacity. And CLST could well start to be a cheaper option too, as governments make it more expensive to use carbon- or resource-intensive materials – that could take more than five years though!