The creation of Vert is a twofold story of achieving environmental positives in our over-hectic, overheating urban centres. One aspect is about what Vert does, the other is about how it’s made and, crucially, what it’s made of. Fundamentally, Vert is about urban greening, bringing natural havens into our town and city spaces. It is effectively a giant – 10mx10mx10m– demountable pergola, a frame supporting hemp netting up which can twine fast-growing plants bedded in planters, specifically chosen for their resilience and wildlife appeal. 

The result is an oasis of greenery that can be set up in areas where it’s impractical to root trees and other planting. It provides habitats for insect life and consequently a potential food source for birds and other animals. For human urban dwellers, it delivers an area of welcome shelter, shade and daily contact with nature, with all the wellbeing benefits that this is increasingly proven to bring.

Coming up with the Vert concept was a joint endeavour of German industrial designers Diez Office (DO) and urban-greening specialists OMC°C. The third lead partner in developing the project and bringing it to this year’s London Design Festival is the American Hardwood Export Council (AHEC), which introduces the other critical element: the material Vert is constructed in. 

AHEC’s role is to develop the market for responsibly grown but underused U.S. hardwoods, which involves supporting projects that broaden their range of applications. By growing and widening the use of U.S. timber, with all the carbon and wider environmental benefits that can bring to manufacturing and construction, AHEC also helps ensure the long-term resilience of the forest resource. 

In recent years, the European market has homed in on a handful of U.S. hardwoods – a fashion trend which has bred familiarity and perhaps a reluctance to try something different. But for the long-term future of the forest, the biodiversity it supports and its wider environmental values, AHEC believes we must use the full spectrum of species. Using a favoured few risks putting these under supply stress, not to mention limiting overall hardwood availability and market choice. It is increasingly recognised that we need to use more timber and fewer energy-intensive, high-embodied-carbon materials to help combat climate change. But they have to be sustainably sourced. That, AHEC argues, means using what the U.S. hardwood forest sustainably provides. It means working in tune with nature. 

Vert brings these strands together. It is constructed from red oak, one of those U.S. species which is less widely used in Europe, yet is hugely abundant, comprising 18% of the US hardwood forest. Moreover, it is red oak in an engineered, or mass timber, format: glulam. 

AHEC has previously initiated an LDF installation trialling red oak glulam. Timber Wave comprised a web of slender laminated, finger-jointed elements, creating a curving portal around the entrance of the V&A museum. Vert goes further, pushing the product’s technical boundaries to highlight its structural potential still further. 

Helping bring this aspiration to reality were three other key members of the Vert team: pioneering engineered wood developers and fabricators Neue Holzbau (NH) of Switzerland; German structural engineer and partner on previous OMC°C projects Bollinger + Grohmann; and London architects Forward Studio.

Neue Holzbau brought to the table their extensive experience in developing new engineered wood solutions. “We act as a problem solver for timber constructors, contractors and installers,” says the company’s CEO for International Business, Reto Schneider. Although the bulk of engineered or mass timber is in softwood, NH also has extensive experience in using hardwoods. “Hardwood has been integrated in our production as standard for nearly 20 years. We bond beech, ash, oak and many other ‘speciality woods’,” Schneider says. Making engineered timber products in hardwood rather than softwood, he adds, enables manufacturers to work with greater material efficiency, given its higher density and strength-to-weight: “We can use smaller cross sections in the supporting structure and produce very efficient, material-friendly components.”

Forward Studio founder Fred Pittman worked with AHEC on Timber Wave while at AL_A architects and its tulipwood CLT structural experiment Multiply while at Waugh Thistleton. Working with Diez Office on Vert, he contributed his architectural perspectives and experience on these projects, advising on such issues as buildability and after-use. He agreed with NH on the appeal of using hardwoods for engineered wood, notably the opportunity they provide to do more with less timber. The bulkiness of softwood engineered wood products can be a deterrent to specifiers. 

Pittman explains: “In large, hangar-type buildings, it may be less important, but in an office or a classroom, those extra centimetres can count. Working in hardwood allows for finer section sizes and a lighter structure. It adds a different dimension and is also visually richer. NH showed us two glulam columns rated for the same kN force, one hardwood, one softwood – the difference in section was eye-opening.”

The fact you don’t need as much material, also narrows the price differential between hardwood and softwood engineered timber. This would be still more the case, argues Pitman, if timber were costed on its capabilities rather than just volume: “We talk about timber in terms of cost per m3 – why not talk about cost per kN?” 

NH has its own laboratory, but also drew on the support of researchers at Bern University of Applied Sciences to develop red oak glulam. The same type of adhesive was used as for other hardwoods, but with certain ‘parameters’ to suit the species.

The red oak was tested alongside NH-made glulam in U.S.white oak, tulipwood, European oak, and thermo-treated red oak. In the finger joint tensile strength testing under standard EN408, tulipwood performed best at 39.5 N/mm2, followed by red oak at 34.6 N/mm2, white oak at 31.9 N/mm2, and European oak at 30.4 N/mm2. 

Although heat treatment makes timber more durable and particularly suited to exterior use, it also makes it more brittle. Consequently, all samples of the thermo-treated glulam failed below the minimum test time.

In delamination tests all tulipwood glulam specimens passed. The red oak and the thermally modified red oak had one failure at the 30-minute closed time but passed at a closed time of 60 minutes. European oak passed one test at the longer time whereas white oak failed at both times, a fact attributed to its lower density. 

The test results for red oak are described by Pittman as ‘incredible’. NH were equally impressed: “As well as bonding and pressing pressures, tests covered the behaviour of the joinery and performance of joining technologies. The red oak surprised everyone,” says Mr Schneider. “It performed significantly better than both European and American white oak.”

The finished Vert structure is on quite a scale. The planting starts 3m above ground so people can walk underneath, then extends to the full 10m height of the framework. Engineering the structure so that it fulfilled its urban-greening purpose most effectively and was at the same time highly stable, was a demanding process. Bollinger + Grohmann’s  Frankfurt director Philipp Eisenbach explains: “In the planning phase, our focus was not just to use sustainable materials but to achieve a material saving. We also designed the whole structure for simple assembly and non-destructive disassembly to allow multiple use and increase the structure’s lifespan. At the same time, we had to guarantee structural stability.”

During the concept phase, the structure was analysed in a ‘parametric environment’ to evaluate its behaviour while manipulating aspects including height, width, and the number and size of the planting nets. “We paid a lot of attention to the plants’ wind behaviour and the structural responses, as load-case wind is the governing impact to the whole system,” says Eisenbach. The result is a structure that is  both efficient and safe.

To facilitate ease of disassembly, Vert is secured to steel base plates rather than a more invasive foundation. As for the red oak glulam elements, the base V columns comprise 360mm x 190mm sections; the horizontal double beams are 380mm x 110mm; horizontal beams 400mm x 190mm; and A-Frames 190mm x 190mm.

The timber is connected using NH’s GSA Technology joint system in galvanised steel. “This is an in-house developed system we’ve been using over 20 years,” says Schneider. “It comprises glued-in threaded rods that allow maximum loads to be passed through small timber cross-sections.”

The glulam is finished in a Saicos water-based impregnation and UV coating, while the deck in thermally modified red oak, supplied by Thermory, is left unfinished. “Due to the structure being outdoors and the use of galvanised steel connections, NH suggests that a structural engineer should check Vert before reuse after two years,” says Pittman, “But it’s anticipated that the timber would last far beyond this.”

As for the longer-term future of red oak glulam, the partners in the Vert project see it being employed in a wide variety of structural applications. Pittman says: “You can see its application in office buildings, with British Council Office structural grids from 9m to 15m. There are also real possibilities in industrial architecture, urban infrastructure, schools, sports, and hospitality projects.”

NH also sees ‘great untapped potential’, as Schneider explains: “We were all pleasantly surprised by red oak glulam’s quality and performance and the finish is marvellous. And as we only have one planet, we must take care of the wood resource and use those species that occur in large quantities. Following Vert, we want to discuss the next steps with AHEC to take our red oak progress further.” 

Read more about the testing to demonstrate red oak's suitability for Vert in this report.