CLT Box
floor slab
The timber box element is a prefabricated structural element combining several timber components. Both loadbearing and, depending on its make-up, thermally efficient, it allows fast, precise and highly prefabricated construction.
What is a CLT Box floor slab?
The timber box element is a prefabricated structural element made of a cross-laminated timber panel combined with two glued laminated timber flanges, forming a hollow section. The CLT panel takes the bending loads and provides bracing, while the glulam flanges increase the structural depth. The box geometry thus combines the span of a beam with the stiffness of a solid floor.
This make-up allows long spans without intermediate supports. The void between the flanges can be left open to take insulation in a passive-house roof, or filled with wood granules that provide acoustic mass in multi-storey floors. The rigidity of the box allows thinner sections.
This composite behaviour allows full prefabrication in the workshop, with service penetrations, rebates and sealing layers fitted before delivery. Assembly on site comes down to setting the elements in place and screwing them.
Manufacture relies on finger-jointed European spruce lamellas bonded with a formaldehyde-free polyurethane adhesive. The timber comes from Best Wood Schneider's own sawmills in Germany and Switzerland, whose chips feed the same producer's range of wood fibre insulation.
The box thus meets, in a single element, structural, acoustic and prefabrication requirements that would otherwise call for several separate layers. It is intended for floors in multi-storey buildings and for roofs of low-energy houses.
Installation & uses
The box is made to order in the workshop, sized to the project drawings, then delivered to the building to be installed directly. Elements are fitted panel against panel, joined with rebates and structural screws. Service penetrations and floor openings are prepared in the workshop, which reduces work on site.
The floor version takes a filling of wood granules between the flanges, added after installation to provide the floor's acoustic mass and fire protection. The roof version stays open to receive wood fibre insulation, blown or in boards slid between the flanges. A breather membrane or vapour control layer can be fitted in the workshop, depending on the package.
This degree of prefabrication makes the box suited to rapid construction, with a short installation time and workshop precision. It suits multi-unit housing, public buildings and extensions to existing buildings, in spans where a concrete slab would be a drawback.
The interior finish can be left exposed in stained natural timber, or concealed behind a suspended ceiling board. At end of life, the screwed connections allow the whole box or its components to be dismantled and reused.
About the producer
Best Wood Schneider is a German producer of structural timber, with its original sawmills at Eberhardzell and Meßkirch in Germany, and a site in Switzerland. The company covers the whole of timber construction: glulam, CLT, CLT-BOX, KVH and laminated timber, under PEFC certification.
The producer runs a closed supply chain, with the chips from sawing feeding its own range of wood fibre insulation. This integrated operation aims to make use of the whole log, from structural timber to insulation, without leaving the processing site.
Technical performance
ETA-21/0336 (European Technical Assessment for CLT BOX); CLT EPD (June 2024, shared with CLT); service classes 1 and 2 to EN 1995-1-1; airtightness tested to EN 12114
FAQ
Is the CLT Box floor slab loadbearing?
Yes. The box is a structural element: CLT panel in C24 and glulam flanges in GL 24h, covered by European Technical Assessment ETA-21/0336. It falls within service classes 1 and 2 as defined in EN 1995-1-1, meaning interiors and sheltered parts.
What is its embodied carbon?
−492 kgCO₂eq per m³ over A1-A3, the balance of +189 in fossil emissions and −682 in biogenic sequestration. Over the full cycle, from A1 to C4, the figure comes to +336 kgCO₂eq/m³. The biogenic carbon content is 187 kg of carbon per m³. Source: CLT EPD of June 2024.
Which box height should I choose?
From 160 to 460 mm, for an effective width of 900 to 1,200 mm and lengths of 2.30 to 16 m. From a height of 440 mm, the lengths produced range from 8 to 16 m. Each box has two flanges.
What happens to it at end of life?
The box can be dismantled, recycled and reused, under PEFC certification; the filling granules can also be recovered. The EPD assumes a reference service life of one hundred years, without maintenance.
Alongside
Same layer, same use · three materials to compare.
Compressed urban excavation loam, unfired. BC Materials · Brussels.
Whole shells, 15-35mm · biobased insulating sub-base (λ=0.106)
Loadbearing block in Belgian recycled aggregates + low-carbon cement. Roosens · Familleureux.