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Waste wood project promises cheaper, greener CLT

Words:
Colin Rose

UCL researchers say reusing secondary timber from demolished buildings could construct up to 10,000 homes a year

Prototype CLST panels in use as table tops at Poplar’s Chrisp St Exchange co-working space.
Prototype CLST panels in use as table tops at Poplar’s Chrisp St Exchange co-working space. Credit: Christopher Pendrich

Demolition in the UK produces around 1 million tonnes of waste timber waste a year – more than half of it solid wood. A team at University College London has been working on reusing it to make cross-laminated secondary timber (CLST), which could build up to 10,000 homes annually. 

No one is doing this yet. Sited near forests, the major European CLT producers have plentiful primary sources. But while Austria has 47% forest cover, in the UK it’s only 13%, so here secondary timber is more available locally at low cost. You might even be paid to take it away. 

UCL’s goal is to fully test and certify the product for use in line with conventional CLT. Initial lab testing showed no significant difference between the compression stiffness and strength of CLST and a control. Modelling the defects typically found in secondary timber suggested only a small effect on panel stiffness in compression and bending. Calculations into the properties of panels containing timber of significantly lesser quality found very little drop off in performance when configured correctly. 

In a pilot project, floorboards were reclaimed from a 1970s housing block. As with much timber from demolished buildings, its retail value was not of interest to salvage brokers. At the Remakery in Brixton, the boards were de-nailed, planed and ripped down – and jointed to make layers which were laminated in a hydraulic press.

Specimens of cross-laminated secondary timber for lab testing.
Specimens of cross-laminated secondary timber for lab testing. Credit: Alejandro Romero Perez de Tudela

Working with Poplar HARCA and architects Seán and Stephen, these panels became table tops for a co-working space. This protoptype was a one-off, labour intensive process; and while automation promises big efficiencies, there is little point in competing head-to-head on scale with the big manufacturers. But two alternative routes could set the product apart. 

One is to drive the circular economy agenda, by maximising the amount of recovered wood that is used in the product and by designing panels for disassembly and adaptation – to extend the building’s life – and for upgrade and reuse when the building is deconstructed. As well as providing negative- or low-carbon building components, the economic value of the material would be maintained at end-of use. This is of particular relevance to buildings with a known lifespan, or ones that are likely to change function over their life, and to any client with a long-term financial interest in the site.

The second route is working in a more tailored way with architects and clients who want to use CLT but who are looking for something distinct and bespoke. The uniqueness of used building timbers offers the chance to get away from the ubiquity of spruce walls and soffits, to instead work with texture, relief and weathered finishes

 

Waste wood at a recycling plant. The material is typically chipped and made into products like MDF or animal bedding, or incinerated for energy.
Waste wood at a recycling plant. The material is typically chipped and made into products like MDF or animal bedding, or incinerated for energy.

The climate change agenda can only strengthen the business case for CLST as environmental taxation rises, labour taxation falls, and production costs and regulation of whole life carbon add value to such products. Additionally, the technology underpinning CLT is getting cheaper as its use increases.

Now, CLST has to be proved in structural applications. Timber from old buildings is often of better quality than new wood, while strength grading is more complex with reclaimed timber. Researchers at Edinburgh Napier are looking into this. Investigations into mechanical performance of panels made from a mix of primary and secondary timber will be followed by research into fire performance, bonding strength and logistics of collection and fabrication.

The extent of waste generated by the construction industry offers huge possibilities. CLST presents a business case for reusing or upcycling materials at an industrial scale: turning mixed, low-value materials into a high-quality, standardised component. It’s the kind of product urgently needed to meet the demands of the construction industry, while vastly reducing its environmental impact. 

Colin Rose works at Reed Watts Architects. Read more on his collaborative research on CLST

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