Cross-laminated timber (CLT)

What is cross-laminated timber (CLT)?

Cross-laminated timber is an engineered timber panel made from multiple layers of kiln-dried, solid-sawn lumber boards glued together at perpendicular angles. The perpendicular grain orientation—typically three to seven layers with the outermost layers aligned in the same direction—creates a solid, straight, rectangular panel with exceptional stiffness and two-directional load-bearing capacity. This construction method counteracts the natural tendency of solid wood to warp, cup, or shrink, resulting in highly dimensional stability and precision that rival reinforced concrete and steel.

CLT originated in Austria in the 1990s and has become central to mass-timber construction worldwide. A typical residential CLT panel consists of five layers: approximately 22 mm + 22 mm + 22 mm + 22 mm + 22 mm, arranged with alternating grain direction. The material achieves a smooth surface suitable for exposed interior finishes and accommodates CNC-routed openings for doors, windows, and mechanical systems.

How is cross-laminated timber manufactured?

CLT production follows a precise nine-step process: lumber selection and sorting by grade, moisture adjustment to 12%, planing for flat surfaces, cutting to size, adhesive application, panel lay-up, assembly pressing, CNC machining for openings, and quality marking.

Lumber enters the mill at controlled moisture content; typical target is 8–14% to minimize internal stress after pressing. Boards are visually graded to eliminate warp, then sorted by structural class. Planed surfaces receive structural adhesives (typically polyurethane or melamine-formaldehyde) applied by through-feed extruders or nozzles, covering at least 80% of bonding surface between layers. Vacuum presses (generating ~14.5 psi) or hydraulic presses (40–80 psi) hold layers under pressure for curing. Final steps include precision machining for openings and marking at 8-foot intervals with grade, thickness, mill name, and production date.

What are the structural properties of CLT?

CLT exhibits high in-plane and out-of-plane strength and stiffness, with two-directional load-bearing capacity, superior seismic performance, and dimensional stability that make it suitable for walls, floors, and roofs in residential and commercial buildings. The perpendicular layering transfers loads from one direction to an adjacent layer, distributing stress evenly and allowing the panel to function as both beam and column without additional bracing.

Typical strength values increase with panel thickness and number of layers. A five-layer panel around 120 mm thick achieves a bending moment capacity comparable to reinforced concrete slabs of similar span. CLT panels can span 4–7 meters for residential floor loads; longer spans require thicker elements or reinforcement. The material handles seismic forces effectively due to its ductility and ability to dissipate energy through panel connection details.

How does CLT compare to glued-laminated timber (glulam)?

The key difference is layer orientation: CLT alternates grain direction perpendicular to each adjacent layer, while glued-laminated timber (glulam) aligns all laminations in the same direction. This makes CLT suitable for two-directional loads (floors, walls), whereas glulam excels in single-direction beams and arches. Glulam remains the material of choice for large-span roof beams and exposed architectural elements, while CLT serves as a panel system for complete wall-and-floor assemblies.

CLT panels are manufactured as sheets (typically 10 feet wide, up to 60 feet long), allowing prefabrication of an entire story's structural system in a single factory run. Glulam beams require on-site assembly and are best for clear-span applications. Both materials are renewable, but CLT's two-dimensional strength makes it the preferred solution for residential multi-story buildings and other engineered mass-timber alternatives.

What are the thermal and fire-performance characteristics of CLT?

CLT provides good thermal insulation (0.12–0.13 W/mK), contributing to passive-house and low-energy design goals, and achieves 60–90 minute fire resistance through the charring of outer layers, which then insulate the inner structure. The solid mass of CLT also provides excellent thermal mass and acoustic damping, reducing both operational heat loss and sound transmission between units in multi-family housing.

Fire behavior in CLT differs from solid timber: exposed outer layers (typically 22 mm) char at a predictable rate (~1.5 mm per minute in standard tests), creating a carbonized exterior that protects inner layers from oxygen and slows combustion. A five-layer panel 120 mm thick typically achieves a fire resistance of 60 minutes without additional treatment. For higher ratings (90+ minutes), thicker layers or intumescent coatings may be applied. This performance makes CLT suitable for residential and commercial multi-story buildings in regions with stringent fire codes.

Where is cross-laminated timber commonly used?

CLT is widely used in residential and commercial multi-story construction for load-bearing walls, floor-ceiling systems, and roof assemblies in buildings up to 10+ stories, as well as in single-family homes and hybrid systems combining CLT with concrete or steel.

In residential applications, CLT works particularly well for families seeking passive-house certification due to its low thermal conductivity and inherent air-tightness when properly detailed. Prefabrication reduces site labor, making CLT economical for multi-unit housing. Commercial and hybrid projects often use CLT floors with concrete or steel cores, combining the speed of wood assembly with the durability of mineral structures. The material's precision and pre-cut openings minimize waste and rework, lowering total project cost and environmental impact. Manufacturing capacity has expanded rapidly across Europe, North America, and increasingly in Central Europe, with industrial-scale plants now operational in Slovakia and the surrounding region.