Waste not, want not is an idiom that we rarely heed. In fact, humankind—and the building industry at large—excels at wasting stuff. The world’s largest landfill sits in the middle of the Pacific Ocean for crying out loud! Construction and demolition (C&D) debris in particular, comprising concrete, wood, asphalt shingles and pavement, gypsum, metal, brick, and related materials, remains a major global problem.

According to the latest EPA figures, of the 600 million tons of C&D debris that were generated in the U.S. alone in 2018, just under 145 million tons went to landfills. (The remaining 455 million tons were directed to “next use,” which includes the production of aggregate as well as incineration for energy generation, among other markets.) And once you isolate wood within the C&D category, the margins separating next-use and landfill get much thinner.

Now, as much wood debris from C&D sites as the U.S. literally throws away annually (about 70% of the total amount, roughly equaling 27 million tons), barely 3% of it is recirculated for manufactured products. Aiding this heedless waste is the fact that the U.S. and its territories boast decent forest coverage throughout its varied climate zones (36% cumulatively, with greater concentration in the east, southeast, and northwest), so inventory of virgin product remains relatively steady.

Across the pond in the UK, matters are somewhat different. For waste timber alone, approximately 56% is incinerated for energy, 32% is down-cycled into chips for non-structural particle-based products, 9% is incinerated for no purpose or goes to landfill, and 3% is exported. (The country’s cumulative forest coverage stands at 13%.) So, what’s to be done to shift this paradigm and achieve greater circularity in the marketplace?

Secondary mass timber

One viable solution is to leverage waste streams in different ways and redirect them to produce engineered structural timber. More specifically, take recovered wood from demolition projects (as well as offcuts and other waste from construction sites) and upcycle it into cross-laminated secondary timber (CLST) and glue-laminated secondary timber (glulamST) of varying dimensions.

A research collective comprising innovators from University College London’s (UCL) Circular Economy Lab and the research start-up UK CLT have devised a novel way to do just that.

Last September, at the London Design Festival, the research collective, along with UK-based modular builder Portakabin, presented its prototype CascadeUp. The modular structure, modestly sized (3.5m high by 2.5m wide by 2m deep) to the scale of a large powder room, was made from 100% waste timber from demolition sites. GlulamST comprises the structure’s framing while CLST is used for wall and floor panels.

“Taken at scale, this is an approach that can boost local economies and drive new employment in reclamation and manufacturing sectors close to urban areas,” UK CLT founding partner Dr. Colin Rose told Timber Development UK last month.

A senior research fellow at UCL and former practicing architect, Rose says the genesis for this latest endeavor began with a question: “How do you enable reuse for a much wider array of materials that are out there?”

The reclamation industry, as he describes it, does a good job with “higher-value, heritage elements,” but is shrinking and “not tackling the breadth of materials being discarded,” he says. What doesn’t exist yet, at least at any significant scale, is the market presence of third-party businesses that can effectively complete the value chain between waste streams and reuse streams, between demolition and design. Making the business case for this, which includes convincing stakeholders (e.g., developers, demolition contractors, architects, and engineers) that there is in fact value in these waste products, is the main challenge.

If the gap in the market were obvious from the start, “somebody would have already filled it,” Rose says. In a concerted effort to bridge that divide between research and a practice that can turn a profit, Rose says he wants UK CLT to be a beacon of sorts, and “a symbol of what needs to change” for circular economies to thrive.

Promoting regional (circular) economies

“People want to be at the forefront of circular economies,” Rose says. As an example, he points to upward trends for the reuse of structural steel in London, and the ways in which that industry is becoming less wasteful. This practice has indeed achieved greater scalability, and in turn impacted global economies and the regulatory landscape. UK CLT’s goal is to do for timber what many developers have managed to for steel, which includes reusing (not recycling) vast amounts and heavily reducing the embodied carbon budgets of new buildings made with reclaimed product.

Timber still doesn’t benefit from such reuse pathways. “There is so much timber waste out there,” Rose says. He does feel, however, that the economic, regulatory, and cultural conditions are in place for more favorable outcomes.

To make it work, start local. All wood used to produce CascadeUp was sourced from two demolition sites and a community recycling site within London. And in keeping current protocols for the manufacturing, strength grading, and assembly of mass timber products, UK CLT is working with only reclaimed softwoods.

Further, to maintain as transparent a supply chain as possible, the start-up partnered with Madaster to create traceable product passports. This extra step ensures that every board foot of reclaimed wood benefits from a digital record, which can be used upon future disassembly, new reuse paths, and eventually recycling or end-of-life pathways.

Testing for structural integrity

“One challenge of reusing secondary timber for structural purposes is the potential degradation of its mechanical properties,” reads an excerpt from the 2024 paper in Engineering Structures, co-written by Rose and fellow researchers. The paper’s authors further cite studies into how secondary timber performs compared to primary timber in terms of elasticity and strength. The conclusions are what you might expect.

Owing to aged surfaces, contaminants, nail holes, and other defects, secondary timber has regularly demonstrated a lower modulus of rupture (MoR) grade than primary timber products. Even after extensive de-nailing, planing, jointing, and further refurbishment, the secondary product simply does not meet the specified structural loads that primary engineered timber can support. Consequently, there are also measurable volumetric losses when converting reclaimed wood into usable product.

“Although some properties of secondary timber may be compromised when compared to primary timber,” Rose and his co-authors note, “the increasing use of high-performance engineered wood products, such as cross-laminated timber (CLT), provides other opportunities for structural reuse of secondary timber.”

They mention the promise of CLST to be used in low- and mid-rise buildings, which can be constructed using “lower strength” and “smaller dimension” timber. When developed and tested on a smaller scale, CLST “showed similar compressive strength and stiffness to a control made with primary timber, and minor defects had a small effect on the stiffness.”

Rose also points to emerging technologies that are enabling more efficient detection and automated removal of nails and other metal fasteners in recovered wood products. For the purposes of CascadeUp, however, an industrial metal detector was used to identify any foreign substances in the wood, but all metal pieces were manually removed.

What’s next?

Rose still sees himself as having one foot in the lab and the other in his young business. For the time being, he likes UK CLT’s stature as something of a proving ground for how to achieve circularity in this space. Economically, it’s a tough nut to crack. But the demand is there. Rose says that since debuting CascadeUp and setting up his company’s website, he began receiving “many inquiries” about product. “They’re not coming to me offering timber, but they do want me to supply mass timber made in the UK out of secondary sources.”

This supply-and-demand conundrum is noteworthy, because it pretty much makes the case for UK CLT’s business plan. Customers want the product, and given the current rates of wood waste from C&D debris, there is no doubt about having enough supply. The link missing from this prospective value chain is a middleman capable of sourcing, processing, and delivering said product. Someone to supply the supply, as it were.

Rose stresses that his company’s CascadeUp prototype demonstrates a modular approach to mass timber components as well as their use in volumetric modular buildings. He envisions configuring a larger catalogue of glulamST beams and columns, CLST wall and floor panels, and timber cassette roofs. This “kit of parts” could be applicable in many markets, from temporary buildings to housing to education and healthcare, manufactured and assembled at smaller scales than the headline-dominating mass timber behemoths we see rising in one city to the next.

Rose is confident there is a market for this. The tricky part might well be educating others in the supply chain about viable alternatives, and then inserting oneself into the equation. “Deconstruction and reuse processes need to be much more integrated with industry practices,” he says. One trend he finds encouraging is the increased use of pre-demolition audits on the part of developers. These and similar tools are increasingly geared “not just towards waste management, but to actually achieving reuse.”

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