Cross-laminated timber
Cross-laminated timber is a subcategory of engineered wood panel product made from gluing together at least three layers of solid-sawn lumber at angles to each other. It is similar to plywood but with distinctively thicker laminations. CLT was first used in a building in the 90's. Circular-CLT was first used in a building, The Urban Woods in The Netherlands, in 2024.
The grain of each layer of boards is usually rotated 90 degrees from that of adjacent layers and glued on the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. An odd number of layers is most common, but there are configurations with even numbers as well. Regular timber is an anisotropic material, meaning that the physical properties change depending on the direction at which the force is applied. By gluing layers of wood at right angles, the panel is able to achieve better structural rigidity in both directions.
CLT is distinct from glued laminated timber, which is a product with all laminations orientated in the same way.
History
CLT
The first patent resembling CLT was first developed in the 1920s by Frank J. Walsh and Robert L. Watts in Tacoma, Washington. Many sources however, date the first patent back to 1985 when it was patented in France. Significant developments were then made in Austria when Gerhard Schickhofer presented his PhD thesis research on CLT in 1994. Utilizing the theories he developed during his research, Schickhofer began working with three small sawmills and the Sawmillers Association, to start production of CLT. With help from some government funding, they were able to hand build a test CLT press and create the first few panels. At the same time, the first press system, utilizing water-based pressure, came on the market enabling Schickhofer and his team to think beyond the capabilities originally thought possible for CLT. After years of extensive research, Schickofer submitted the results to the Austrian and EU government bodies that dealt with the approval of materials for commercial products and in December 1998 it was approved. A period of substantial growth in production and projects soon followed in Germany and other European countries as a push for green buildings became more prominent. CLT was slow to take off in North America, but it has begun to gain momentum in more recent years.C-CLT
In 2024 C-CLT was introduced to the world as a result of a chainwide collaboration in The Netherlands. C-CLT, or Circular-CLT, is CLT made out of non-virgin wood. The Circular Cross-Laminated Timber panels, C-CLT, are made from demolition wood and discarded pallets, all sourced locally in The Netherlands. The outer layer is made of ash wood. The thick solid wooden panels that are created comply with European regulations, reduce CO₂ emissions and are of course circular; it is even 'up-cycling': low-quality wood becomes a high-quality building product. The plates are 10 to 15% stronger than traditional CLT. C-CLT was first used in june in a modern highrise building called The Urban Woods.C-CLT is the result of a comprehensive Dutch collaboration. The pallets are prepared by the local social enterprise. Then planks are made, connected with finger joints, and cross laminated into solid panels. The Urban Climate Architects delivered the design, and The Urban Woods treats the wood and builds modern modular homes from it. Researchers of the TNO institute conduct thorough research including metal parts detection, adhesive bonding, and strength classification.
Building codes
In 2002, Austria used Schickhofer's research to create the first national CLT guidelines. The International European Technical Assessments began to regulate the properties and design of CLT in 2006. Efforts to standardize CLT in Europe started in 2008 and by 2015 the first European product standard for CLT, EN 16351, was approved. Also in 2015, CLT was incorporated into the International Building Code in accordance with ANSI/APA PRG 320 and the National Fire Protection Association began to research and develop codes regarding the fire safety of CLT and other engineered woods. The 2021 revision of the IBC included three new construction types for mass timber buildings, Type IV-A, Type IV-B and Type IV-C. These new types enabled buildings using mass timber to be built taller and over greater areas than before.Manufacturing
The manufacturing of CLT is generally divided into nine steps: primary lumber selection, lumber grouping, lumber planing, lumber cutting, adhesive application, panel lay-up, assembly pressing, quality control and marking and shipping.During primary lumber selection, lumber will undergo a moisture content check and visual grading. Depending on the application, structural testing may also be completed. The moisture content check is conducted because the lumber that is typically used, can arrive with a MC of 19% or less, but lumber for CLT needs to have a MC of approximately 12% during manufacturing to avoid internal stress due to shrinkage. This test is also done so that adjacent pieces of lumber do not have an MC difference greater than 5%. In order to conduct an MC check, various hand-held or on the line devices can be used. Some are more accurate than others, as they check moisture content within the wood not just at surface level. Further research and development is ongoing to improve the accuracy of such devices. Temperature in the manufacturing facility is also checked and maintained throughout this process to ensure the quality of the lumber. Visual grading is performed so that any warping in the lumber is prevented from affecting the pressure the bond line can withstand. It also ensures that waning, defects in the wood due to bark or missing wood due to the curvature of the log, does not significantly reduce the available bonding surface. For a product to be considered an E-class CLT, visual grading must be considered for perpendicular layers while parallel layers must be determined by the E-rating. Products are classified as V-class if visual grading is used for both perpendicular and parallel layers.
Using the results from lumber selection, the grouping step ensures the timber of various categories are grouped together. Lumber used for the major and minor strength directions are grouped primarily relying on MC and visual grading. Within the major strength direction, all lumber is required to have the same engineering properties so that panel limitations can be determined. Likewise, all lumber for the minor direction must have a single set of properties. Higher quality lumber can also be grouped so that it is reserved for areas in which fasteners are installed to maximize fastener effectiveness. For aesthetic purposes, some lumber will be set aside so that the outermost layer of a panel is visually appealing. Timber that does not fit into either category may be used for different products, such as plywood or glued laminated timber.
The planing step improves the surfaces of the timber to reduce oxidation which increases the effectiveness of the adhesives. Approximately is trimmed off the top and bottom faces and is trimmed off the sides to ensure a flat surface. In some cases, when the lumber edges are not glued because they have an acceptable width tolerance, only the top and bottom faces are planed. It is possible that planing may increase the overall moisture content of the timber due to the drying variations throughout the wood. When this occurs, bonding suitability should be assessed and reconditioning may be necessary.
The timber is then cut to a certain length depending on the application and specific client needs. Cut-offs from the longitudinal layers may be used to create the transverse layers if the same specifications are needed for both the parallel and perpendicular layers.
Application of the adhesive occurs shortly after planing to avoid any issues affecting the surface of the lumber. Applying the adhesive is most often done in one of two ways: a through-feed process or side-by-side nozzles. In the through-feed process extruder heads distribute parallel threads of adhesive along the piece of lumber in an airtight system to avoid air gaps in the glue that could affect bonding strength. This is typically used for phenol resorcinol formaldehyde or polyurethane-reactive adhesives. For PUR adhesives, the layers of lumber may be misted to help with curing. The side-by-side nozzle option is commonly reserved for CLT layers that are formed in advance and works by installing the nozzles along a beam that will travel along the length of the lumber and apply the adhesive. To avoid additional manufacturing costs, adhesive is typically only applied to the top and bottom faces of the lumber, but edge-gluing can be done if necessary.
Panel lay-up is performed next and involves laying the individual pieces of lumber together in preparation for assembly pressing. In accordance with ANSI/APA PRG 320, at least 80% of the surface area between layers must be bound together so that the bond is effective. In order to meet this standard, manufacturers are tasked with finding the most efficient way of laying the lumber. This time between spreading the adhesive and applying pressure is known as assembly time and must fall within the time targeted by the specific adhesive used.
Assembly pressing completes the adhering process with either a vacuum press or a hydraulic press. Vacuum pressing generates a pressure of approximately, which is not always enough to address the warping potential or surface irregularities. To accommodate this, lumber shrinkage reliefs can be cut in the longitudinal direction. These reliefs release the stress in the lumber and decrease the risk of cracking due to drying. They must however, have a maximum width and depth so that the bonding area and panel strength are not impacted significantly. Using a vacuum press may be more beneficial in some circumstances, because they can press more than one CLT panel at once and can be used for curved elements. A hydraulic press on the other hand, generates a greater pressure, ranging from, and applies it to specific faces of the panel. For this reason, panels may need to undergo both vertical and side clamping pressing.
Once assembly pressing is complete, the CLT panels undergo quality control machining. Sanders are used to sand each panel down to the desired thickness with a tolerance of, or less if specified by a project. The CLT panels are then moved to a multi-axis numerically controlled machine that makes precision cuts for doors, windows, splices, and connections. Any minor repairs that are necessary at this stage are completed manually.
To meet the requirements of ANSI/APA PRG 320 and ensure that the correct product has been specified, delivered, and installed, CLT panels must be marked to identify a variety of information. This includes the grade, thickness, mill name, agency name or logo, ANSI/APA PRG 320 symbol, manufacturer designations, and a top stamp if it is a custom panel. These markings must be stamped at intervals of or less so that when longer pieces are delivered to site and cut, they still display the necessary information. Further markings may be added to demonstrate the main direction of loading and the zones designed to receive connections. During shipping and construction, the CLT panels must be protected from the weather to maintain their structural integrity.