How it works

Speeding up the transition to sustainable and accessible living, that is Leko’s mission.

With this in mind, our teams perfected a completely new wood construction system using crossed structural boards, allowing for buildings that are three times sturdier and three times more insulated. It’s a new way of building that’s more responsible, more durable and more profitable.

Current systems

There are many different types of constructive systems, notably wood-frame, glued or non-glued cross-laminated timber (CLT or MHM), and its insulated derivatives (CLTi).

Wood-frame constructions are mostly used in individual housing. This system is composed of beams framing a number of posts with insulating material placed between them, offering good thermal insulation. Mechanically speaking, the frame is secured on its side (low inertia in terms of bracing), which means oriented strand boards are needed for bracing, adding a significant amount of glue. The frame is assembled using (very flexible) shear loaded nails. Although manufacturing processes have evolved a great deal, the process has only partially been automated, and prefabricated frames are still largely hand-crafted. Excessive use of glue and poor mechanical properties make this system unsuited for new, low-carbon multi-story wooden buildings offering a high level of comfort.

CLT poses environmental problems due to the large amounts of glue it contains, and the long drying times cause difficulties during production. These are also solid panels that require a significant amount of over-insulation. Although they offer great mechanical strength, they work along two axes of inertia, one stronger, one weaker, which can cause warping. The glueless version of these load-bearing panels, called Massiv Holz Mauer (MHM), offers an interesting alternative, but uses a lot more wood than regular CLT for mechanical properties that essentially fall short.

Insulated CLT is halfway between CLT and MHM and consists of different layers of wood, which are placed crosswise and glued in place at a 90° angle, just like CLT. Unlike CLT, though, CLTi panels are not solid: there is a space between the boards for insulation material to be inserted, ensuring great mechanical strength and good thermal insulation. The problem with this solution, similar to wood-frame constructions and CLT, is that the use of glue makes them more prone to brittle fracture.

Leko offers a competitive, effective solution

The technology of crossing wood and assembling it with a mechanism of interlocking spikes was first released to the public domain after 3 years of research. It took another two years of development in Leko’s laboratories for this technology to grow into Leko’s certified and industrialised crossing wood constructive system, a unique, multi-layered structure offering architects, builders and developers a fast and profitable way of creating custom energy-efficient buildings.

A comparison of the different constructive systems showed that there is a need to improve the assemblies in order to make them less polluting and more resilient. The panels with the best mechanical properties are glued panels, where the wood is joined by a glue line at the interface of the different components. The use of glue does slow down the production process, however, as the glue needs time to dry, and it also has a significant environmental impact.

The research carried out by Stéphane Girardon in his thesis on improving the mechanical properties of screwed down wood-to-wood connections by prepping interfaces (« Amélioration des performances mécaniques des assemblages bois sur bois vissés par préparation des interfaces ») offers some interesting ideas that were used by the teams at Leko to develop the Leko crossing wood constructive system.

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Spiked interface

The principle of the spikes is to create resistance on the interface of the wood elements by creating obstacles. By choosing a spike-shaped obstacle, we can leverage the largest possible surface area, increase the rigidity of the connection, and keep it feasible. This results in a triangular groove machined along the length of the wooden board, square-based teeth in the areas with the “spikes”, and another triangular groove on the transversal side of the board.

Spikes

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Assembling crossed boards

This type of assembly, using spikes on crossed boards, is a mechanical assembly. The goal was to keep the boards together and to limit their degrees of freedom. To achieve that goal, the assembly is divided into two parts, an upper part where the crossbeam has spikes and the post does not, and a lower part where the post has spikes and the crossbeam does not. The connecting plane is therefore completely blocked, and to keep the assembly in place, four screws are inserted in the four corners of the assembly.

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Layered design

By assembling the boards crosswise, layers can be created, whereby every layer is oriented at a right angle to the last, as in CLT. The difference with CLT is that the layers mostly consist of insulation material sandwiched between the framing timber. This layering system allows for the panels to be built using a type of additive manufacturing, with successive layers being added like in a 3D printer. This means it is possible for the assembly to be robotized, using automation algorithms, with the support of anticipatory logistics. Wood to Industry 4.0 standards.

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40% thinner

Crossed wood panels, which consist of 3 to 10 layers depending on their application (inner or outer wall), serve both a structural and a functional purpose. Thanks to their mechanical properties, these high-performance 2-in-1 panels ensure a vertical load distribution and bracing within the structure they make up, but they also serve various functional purposes, for instance with regards to thermal insulation, acoustics, fire resistance etc. This integrated design is based on the principles of “Integrative Design” of the Rocky Mountain Institute, which are intended to make efficient buildings cheaper by reinventing the way they are built.

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The Leko®-structure

The Leko system is an innovative HEQ wood construction system that is prefabricated off-site in the Leko®-factories and optimised for on-site assembly. LEKO technology allows for houses and residential buildings with a very high energy efficiency and a very small carbon footprint to be built. Leko-walls are 3 times stronger than wood-frame walls, 3 times more insulating than CLT walls and 40% thinner than conventional walls.

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Assembly principle

The spike assembly is a mechanical assembly, meaning it’s the geometry of the different components that allows for the forces to be transmitted through contact. Consequently, the direction of the different longitudinal and transverse grooves can be used to block a number of degrees of freedom. As the boards are assembled in the plane of the interface, the assembly has a maximum of three degrees of freedom: two translational ones (Tx, Ty), and one rotational one (Mz).

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By blocking these three degrees of freedom, the system can be forced to work in both directions and can therefore be used as bracing, like a CLT panel, for example. Machining the grooves at a 90° angle allows for three options.

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An abundant and under-valued local resource

Leko’s new crossing wood technology aims to transform an abundant resource that often goes to waste and literally goes up in smoke, called fuel wood. This ‘green gold’ is produced by thinning hardwood forests, which is necessary to allow the forest to keep growing. In the Greater Region, nearly 20% of the wood harvested each year, or 2 million m³, comes from thinning, providing enough wood to build 10 000 houses every year (5 million m² of Leko walls). The annual increase of the forest area is still larger than what is harvested.

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1. GIRARDON S., “Amélioration des performances mécaniques des assemblages bois sur bois vissés par préparation des interfaces”, Thèse de doctorat de l’Université de Lorraine, 285 pages, (2014).

2. https://www.rmi.org/insights/reinventing-fire/reinventing-fire-buildings/