Cross-Laminated Timber (CLT)

Cross-laminated timber (CLT) is an engineered wood product made from layers of solid timber boards glued together at right angles to one another, typically in three, five, or seven layers. This cross-lamination gives the panels structural strength and stiffness in both directions, creating a material that can be used for walls, floors, and roofs in buildings up to 10 storeys or more.

Developed in Austria in the early 1990s, CLT has transformed how architects and engineers think about timber in construction. It is not a niche eco-material - it is a legitimate structural alternative to concrete and steel for a growing range of building types, from housing to offices, schools, and mixed-use developments.

Major manufacturers include Stora Enso, KLH, Binderholz, and Mayr-Melnhof. UK-based manufacturing is emerging, with facilities from companies like Innovaré reducing the transport footprint for domestic projects.

CLT production begins with kiln-dried softwood boards - typically spruce, pine, or larch - sourced from sustainably managed forests. The boards are graded for structural quality, planed, and then arranged in alternating layers perpendicular to each other.

The layers are bonded using structural adhesive (most commonly polyurethane-based, though some manufacturers offer formaldehyde-free options) and pressed under hydraulic pressure. The resulting panels can be manufactured up to 16 metres long, 3.5 metres wide, and 500mm thick.

Panels are then precision-cut using CNC machines to create openings for windows, doors, services, and connections. This off-site fabrication means CLT arrives on site ready to assemble, significantly reducing construction time and on-site waste.

CLT has a strength-to-weight ratio that makes it competitive with concrete for many building types. A CLT panel weighs approximately one-fifth of an equivalent reinforced concrete slab, which can reduce foundation requirements and enable construction on sites with poor ground conditions.

CLT offers a significant embodied carbon advantage over concrete and steel framing. The timber stores carbon absorbed during tree growth, while the manufacturing process is relatively low-energy compared to cement production or steelmaking.

A typical CLT panel stores approximately 700-900 kg CO2 per cubic metre of timber. When this biogenic carbon storage is accounted for (module A1 in EPD methodology), CLT structures can achieve dramatically lower - sometimes negative - embodied carbon figures compared to concrete or steel alternatives.

Fire safety is the most frequently raised concern about CLT buildings, but the evidence is well-established. CLT chars at a predictable rate (approximately 0.65mm per minute) and the charred layer acts as insulation, protecting the unburnt timber behind it. Structural engineers design for this by specifying additional sacrificial timber thickness.

CLT buildings routinely achieve 60, 90, and 120-minute fire resistance ratings when properly designed. Full-scale fire tests - including those by the BRE and various European research institutes - have demonstrated that CLT structures can meet the fire safety requirements of tall buildings.

CLT is not the right choice for every project. These are the genuine constraints to evaluate during design.