Cross-laminated timber

Sustainable off-site construction

Although in use for a number of decades in central Europe, the format has only recently broken through to large-scale implementation in the UK. Avanti have been experimenting with CLT since 2006, with our first CLT building, Ickburgh Special Needs School in Hackney, now on site in partnership with Eurban and McLaren Construction. The 9 week on-site programme for the erection of the three levels of the 5,140m2 building is a key driver for the choice of material. Often main contractors find it difficult to price in the benefit of speed of delivery, however McLaren have been able to allow for minimized weight, accelerated programme, early first fix, certainty of outcome and lighter site management to enable the CLT solution to outcompete traditional approaches.

However, these are not the only benefits associated with CLT. Our client, the London Borough of Hackney, was impressed by CLT’s sustainability benefits. Naomi Shaw from Eurban explains further:

“All CLT used in the UK by Eurban is sustainably forested, and carries Programme for the Endorsement of Forest Certification (PEFC) or Forest Stewardship Council (FSC) accreditation. The off-site manufacturing process ensures that there is zero waste as any off-cuts are recycled into other timber products. The panels themselves are re-usable or recyclable (e.g. as wood pellets). The precision of the CLT envelope allows high air-tightness values to be achieved without additional VCL.

Furthermore, significant amounts of CO2 are locked within the fabric of the building, and the fabric contains much less embodied energy than comparable steel or concrete structures. An added benefit is lower weight, which reduces foundation size.”

With building energy performance moving eventually towards zero carbon targets, the overall impact of the embodied energy in a building’s materials and construction processes accounts for a growing proportion of carbon footprint. Sadly there are is no accepted methodology regarding embodied carbon performance which means that the planning and Building regulations process does not automatically reward CLT. This would be a meaningful step forward and the Royal Institution of Chartered Surveyors and other organisations are looking at a unified approach.

Another step-change for the industry is allowing for the coordination process to attain the level required to allow advance procurement of high-precision components. This is especially relevant to highly bespoke, heavily serviced projects such as Ickburgh School where each service route needs to be incorporated in CLT panel designs. Typically, these projects tend to go quickly to site following Stage E (with the ultimate client reserving the right to make changes up to the last possible moment) to allow for a lengthy primary structure build – which can be counter-productive when applied to off-site construction. Hopefully, with experience, clients and main contractors will see the benefit in balancing upfront cost and time against on-site gains.


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