What Is Cross-Laminated Timber?
Cross-laminated timber — commonly known as CLT — is an engineered wood product made by stacking layers of solid-sawn lumber at right angles to one another and bonding them together under pressure with structural adhesives. The result is a rigid, dimensionally stable panel that can be used for floors, walls, and roofs in buildings of significant scale.
First developed in Austria and Germany in the early 1990s, CLT has since become one of the most discussed construction materials of the 21st century, particularly as architects and engineers seek alternatives to concrete and steel with lower embodied carbon.
How CLT Is Made
The manufacturing process follows a precise sequence:
- Timber grading and sorting — Softwood lumber (commonly spruce, pine, or fir) is kiln-dried and graded for structural consistency.
- Layer arrangement — Boards are laid in alternating perpendicular layers — typically 3, 5, 7, or more layers depending on the required panel thickness and load-bearing capacity.
- Adhesive bonding and pressing — Structural adhesive is applied between layers, and the panel is pressed under high pressure until fully cured.
- CNC machining — Finished panels are cut to precise dimensions using computer-controlled routers, incorporating openings for windows, doors, and services.
This factory-precision approach means CLT panels arrive on site ready for rapid assembly — a significant advantage over cast-in-place concrete.
Structural Properties
The cross-laminated structure gives CLT impressive and somewhat counterintuitive properties for a wood product:
| Property | CLT Performance |
|---|---|
| Compressive strength | High — suitable for multi-storey load-bearing walls |
| Tensile strength | Good in both axes due to cross-lamination |
| Fire resistance | Chars predictably; core remains structurally sound |
| Dimensional stability | Low shrinkage and warping compared to solid timber |
| Thermal mass | Moderate — better than steel, less than concrete |
Fire Performance: Addressing the Biggest Concern
The question most people ask first is: isn't wood flammable? CLT's fire performance is actually well understood and often misunderstood. When CLT burns, the outer layer chars at a predictable rate (typically around 0.7mm per minute), and this char layer acts as an insulator, protecting the structural core. This predictable charring behaviour allows engineers to design CLT elements with known fire ratings — often achieving 60 to 90 minutes of fire resistance without any additional protection.
By contrast, unprotected steel loses structural integrity rapidly at elevated temperatures. Many CLT buildings are now approved under building codes that previously required concrete or steel for structures above a certain height.
Environmental Credentials
CLT's sustainability case is compelling, though nuanced:
- Carbon sequestration — Timber locks in atmospheric carbon during the tree's growth. That carbon remains stored in the building for its lifetime.
- Lower embodied carbon — CLT production requires significantly less energy than cement or steel manufacturing.
- Responsible sourcing matters — The environmental benefit depends heavily on whether the timber comes from sustainably managed forests (look for FSC or PEFC certification).
- End-of-life potential — CLT panels can be disassembled and reused, or combusted for bioenergy at end of life.
Notable CLT Projects
CLT has been used in landmark projects around the world, demonstrating its versatility across building types — from educational facilities and offices to residential towers and sports arenas. The material performs particularly well in mid-rise construction (typically 5 to 18 storeys), where its strength-to-weight ratio offers structural efficiency that concrete can't match on comparable timelines.
Is CLT Right for Your Project?
CLT suits projects where speed of construction, low weight, sustainable credentials, and aesthetic warmth are priorities. It requires careful detailing around moisture management, connections, and acoustic performance. For designers and developers willing to invest in the upfront coordination, CLT consistently delivers buildings that are faster to construct, lighter on the environment, and warmer in character than their concrete equivalents.