Cross laminated timber (CLT) is an engineered wood product made by stacking layers of lumber boards at right angles to each other and bonding them with structural adhesive. The result is a large, solid panel that can serve as walls, floors, and roofs, functioning much like a concrete slab but made entirely from wood. CLT has become one of the fastest-growing structural materials in modern construction, used in buildings ranging from single-family homes to high-rises of 18 stories or more.
How CLT Panels Are Made
Manufacturing starts with kiln-dried softwood lumber, typically spruce, pine, or Douglas fir. Workers arrange the boards in layers, usually three, five, or seven, with each layer oriented perpendicular to the one below it. This cross-layering is the key innovation. A single board is strong along its grain but weak across it. By alternating grain direction, CLT distributes forces in both directions, giving the panel strength and stiffness comparable to reinforced concrete for many applications.
The layers are bonded under hydraulic pressure using structural adhesives. Finished panels are then precision-cut to specification using computer-controlled routers, with window openings, door cutouts, and connection points machined at the factory. Panels typically arrive on-site ready to assemble, with moisture content around 10%, well below the 16% threshold at which the wood maintains its full structural strength ratings.
Why Builders Choose CLT
Speed is one of the biggest draws. Because panels arrive pre-cut and ready to install, on-site construction moves significantly faster than with poured concrete or steel. A study cited by the USDA found that overall project savings of about 15% are achievable when CLT replaces conventional building materials, factoring in reduced labor time, lighter foundation requirements, and fewer deliveries. Crews can erect the structural shell of a multi-story building in weeks rather than months.
CLT panels are also much lighter than equivalent concrete elements. A CLT floor panel weighs roughly one-fifth as much as a concrete slab of similar span, which means smaller foundations and lower transportation costs. This weight advantage opens up construction on sites with poor soil conditions or limited crane access.
Material costs have stabilized in recent years. CLT panel prices have held fairly steady at around $42 to $44 per cubic foot through 2024, though transportation from the manufacturing plant adds to the final price. The economics improve further when you account for the compressed construction schedule and reduced on-site labor.
Carbon and Environmental Benefits
CLT’s environmental case rests on two mechanisms: trees absorb carbon dioxide as they grow, and that carbon stays locked in the wood for the life of the building. A USDA Forest Service comparison of a mass timber building against an equivalent steel structure found that the timber version produced 198 kilograms of CO₂ equivalent per square meter of floor area, compared to 243 for steel. That’s a 19% reduction in carbon emissions from manufacturing and transportation alone.
The carbon storage effect is even more striking. The same study found that approximately 2,757 tonnes of CO₂ equivalent were stored within the mass timber building’s wood products. When researchers factored in this stored carbon, the wood products actually showed negative net emissions, meaning the structure captured and retained more carbon dioxide than was released during its entire manufacturing and transportation process.
How CLT Handles Earthquakes
CLT panels are extremely stiff in their own plane. During seismic events, the panels themselves barely deform. Instead, movement and energy absorption happen at the connections: the metal brackets, hold-downs, and fasteners that join panels to the foundation and to each other. This is a deliberate design strategy. Engineers concentrate flexibility at predictable points, making the building’s seismic behavior easier to control and repair after an earthquake.
Research published in the Journal of Structural Engineering found that CLT shear walls respond to lateral forces through a combination of sliding and rocking at their base connections. When perpendicular walls are connected to the main shear wall, performance improves dramatically. In testing, adding a perpendicular wall increased maximum load capacity by 30% to 70% and deformation capacity by 19% to 59%, depending on wall dimensions. These connected wall systems also dissipate more energy during repeated earthquake cycles, reducing the cumulative damage to the structure.
Building Code Allowances
The International Building Code now includes three construction types specifically designed for mass timber. Type IV-A buildings can reach 18 stories, Type IV-B allows up to 12 stories, and Type IV-C permits nine stories. Each type has different requirements for how much of the timber structure must be protected by noncombustible coverings like gypsum board. The higher you build, the more encapsulation is required.
These code provisions opened the door for CLT in urban mid-rise and high-rise construction, markets previously dominated by concrete and steel. Projects in cities like Milwaukee, Portland, and Vancouver have demonstrated that tall mass timber buildings can meet the same safety and performance standards as their conventional counterparts.
Sound and Thermal Performance
Bare CLT panels provide modest sound insulation on their own. A three-layer panel (roughly 4 inches thick) achieves a sound transmission class (STC) rating of only 32 to 34, well below the 50 or higher typically required for apartment walls and floors. This means CLT buildings almost always need additional acoustic treatment.
Adding a single layer of gypsum board to one side bumps the rating to 36 to 38. The real gains come from decoupled assemblies: sandwiching mineral wool insulation between the CLT panel and a separate gypsum layer, or mounting finishes on resilient channels. A CLT wall with mineral wool insulation and gypsum board on both sides reaches STC 55 or above, which is comfortable for multi-family residential use. Floor assemblies with proper topping and ceiling treatments can exceed both STC 50 and impact insulation class (IIC) 50, meeting code minimums for sound isolation between dwelling units.
Thermally, wood is a better insulator than concrete or steel, but CLT walls still need additional insulation to meet energy codes in most climates. Researchers at Oak Ridge National Laboratory have developed high-performance insulation panel systems for CLT that can achieve code-required levels with commonly available insulation materials. The wood itself contributes some thermal resistance, so the added insulation layer can often be thinner than what a concrete or steel structure would need.
Moisture: The Critical Factor
Wood and water have always been a complicated relationship, and CLT is no exception. CLT is manufactured for dry service conditions, meaning a moisture content below 16%. Above that threshold, the panel’s structural ratings may need to be reduced. Above 20%, the conditions for fungal decay become possible if warmth and oxygen are also present.
Panels typically ship at around 10% moisture content, plus or minus 2%. The vulnerable period is during construction, before the building is enclosed and protected from rain. Many builders use temporary coverings or schedule CLT installation during dry weather windows. Once the building envelope is sealed and climate-controlled, maintaining safe moisture levels is straightforward. Long-term monitoring of CLT buildings in humid southern climates has shown that properly detailed structures maintain healthy moisture content well below the 20% decay threshold.
Where CLT Is Headed
CLT is no longer an experimental material. It has codified height limits, established pricing, tested seismic and acoustic performance data, and a growing network of North American manufacturers. The practical ceiling for most projects today is the 18-story limit set by Type IV-A construction, though hybrid systems combining CLT with concrete cores are pushing into taller territory. For builders and developers weighing their options, CLT offers a rare combination: a structural material that builds faster, stores carbon, and performs competitively on cost, while producing buildings that look and feel distinctly different from anything concrete or steel can deliver.