Note (September 2026): Earlier versions said CLT houses need no extra sound or heat insulation and that CLT is inexpensive; CLT alone does not meet usual sound insulation ratings and its upfront costs can exceed those of conventional materials, so these statements have been corrected.
CLT (cross-laminated timber) panels are popular because they are prefabricated, strong in two directions, much lighter than concrete, renewable when sourced from managed forests, and able to keep carrying loads in a fire for 90 minutes (REI 90). The 2021 International Building Code allows mass timber buildings up to 18 stories.
Key Takeaways
- CLT is made of at least three layers of solid-sawn lumber glued at right angles to each other, which gives it stiffness in both directions.
- Europe standardized CLT with EN 16351 in 2015; in North America it is made to ANSI/APA PRG 320 and was added to the International Building Code in 2015.
- The 2021 International Building Code created construction Types IV-A, IV-B and IV-C, permitting mass timber buildings up to 18 stories.
- Main drawbacks: upfront and transport costs, sound insulation that needs extra layers, floor vibration design, and moisture protection during construction.
- As of September 2026, Ascent MKE in Milwaukee (25 stories, 86.6 m, completed 2022) is described as the world’s tallest mass timber structure.
Cross-laminated timber panels are a modern material for building that opens up new possibilities in solid wood housing construction. CLT is not automatically cheaper than concrete or steel, because panel production and long-distance transport can add upfront cost, but its use in construction has grown steadily, and the demand for it on https://clt-rezult.com/en/clt-panel/clt-paneli/ is constantly growing.
The basics
Cross-laminated timber (CLT) is an engineered wood panel made by gluing together at least three layers of solid-sawn lumber, with the grain of each layer usually turned 90 degrees to the layers next to it. An odd number of layers, arranged symmetrically, is most common, and the number of layers depends on the loads the panel has to carry. The lumber is dried to about 12% moisture content before gluing and the layers are pressed together (around 14.5 psi in a vacuum press, 40 to 80 psi in a hydraulic press), which keeps swelling and shrinkage low; the finished panels still have to be protected from the weather during shipping and construction.
Tall timber buildings were long held back by the available wood products and by building codes. CLT changed that: the Stadthaus in Hackney, London, completed in 2009, reached nine stories using only CLT framing, and the 2021 International Building Code added mass timber construction types that permit buildings up to 18 stories. CLT panels are fully prefabricated, with openings for doors and windows cut by computer-controlled machines in the factory, so on-site assembly is fast. Shorter schedules can bring cost savings, fewer accident risks and less disruption to neighbors, although those savings have to be weighed against panel and transport costs.
CLT panels are used for walls, floors and roofs, and panel size is limited mainly by transport and site storage. The panels are also combined with other materials: the 25-story Ascent MKE tower in Milwaukee uses concrete, steel and mass timber, with CLT forming most of its floor slabs.
Key Features of CLT Panels
The following characteristics explain much of the popularity of CLT, together with the caveats that come with each one.
- High fire resistance.
- The risk of board shrinkage is reduced, not eliminated, by drying the lumber and gluing the layers crosswise. CLT alone does not meet the usual sound insulation ratings, so decoupled gypsum board, a thicker panel or a second panel with an air gap is normally added, and whether extra thermal insulation is needed depends on the panel thickness and the local energy code.
- CLT has a low thermal conductivity of about 0.10 W/m·K, compared with roughly 0.4 to 2.5 W/m·K for other common building materials, and its layers of wood also act as thermal mass.
- Building sites can be safer because most of the fabrication, including precision cutting, is done in the factory rather than on site.
- Because CLT is much lighter than concrete, smaller lifting machinery is needed on site and crews can be smaller. The panels are still set and connected to a structural engineer’s design, so installers need more than carpentry skills and familiarity with power tools.
- CLT has been shown to perform well under seismic forces, and its lower weight reduces the loads on foundations; how a building behaves in an earthquake still depends on its engineered design and connections.
Finally, CLT is renewable and stores carbon, provided the timber comes from well-managed forests. That environmental case, together with fast prefabricated construction, is a major reason the material has become so popular.
What Is Cross-Laminated Timber and How Does It Differ From Glulam?
Cross-laminated timber is a mass timber panel product. Mass timber is the wider family of large structural wood components made from lumber or veneers bonded with adhesives or mechanical fasteners. CLT is similar in principle to plywood, but its layers (called lamellae) are much thicker.
| Product | How it is made | Typical use |
|---|---|---|
| Cross-laminated timber (CLT) | At least three layers of boards glued, each layer usually at 90 degrees to the next | Walls, floors, roofs, stair and elevator cores |
| Glued laminated timber (glulam) | Layers of lumber glued with all grains running the same way | Beams and columns, including curved members |
| Dowel-laminated timber (DLT) | Softwood boards locked together with hardwood dowels, no glue or metal | Cross-laminated panels where adhesive-free wood is wanted |
| Nail-laminated timber (NLT) | Parallel boards fastened with nails | Floors, roofs, walls, elevator shafts |
A Short History of CLT
A patent resembling CLT was developed in Tacoma, Washington, in the 1920s, although many sources date the first CLT patent to 1985 in France. Modern CLT grew out of research in Austria, where Gerhard Schickhofer presented his PhD thesis on the material in 1994 and worked with small sawmills to start production. Austrian and EU authorities approved the product in December 1998, and Austria published the first national CLT guidelines in 2002.
Production and projects then spread across Germany and other European countries as green building gained ground. Mass timber in general became more popular from 2012 onward, driven by concern about the sustainability of building materials and by interest in prefabrication and modular construction. CLT was slower to take off in North America, where it has gained momentum more recently.
How Are CLT Panels Made?
CLT manufacturing is generally described as nine steps:
- Lumber selection: boards are checked for moisture content (about 12% is targeted, with no more than 5% difference between neighboring pieces) and graded visually or by stiffness (E-rating).
- Grouping: boards with matching properties are grouped for the major and minor strength directions, and the best-looking boards are kept for the outer faces.
- Planing: about 2.5 mm is taken off the top and bottom faces and 3.8 mm off the sides so the glue bonds well.
- Cutting: boards are cut to length for the order.
- Adhesive application: usually polyurethane-reactive (PUR) or phenol resorcinol formaldehyde (PRF) adhesive, applied soon after planing.
- Panel lay-up: layers are laid crosswise; ANSI/APA PRG 320 requires at least 80% of the surface between layers to be bonded.
- Pressing: a vacuum press (about 14.5 psi) or a hydraulic press (40 to 80 psi) completes the bond.
- Quality control and machining: panels are sanded to thickness within about 1 mm, then CNC machines cut openings, splices and connections.
- Marking and shipping: panels are stamped at intervals of 8 feet (2.4 m) or less with grade, thickness, mill and PRG 320 marks, and protected from the weather in transit.
Because so much is done in the factory, site work on a CLT project looks different from a poured-concrete job; see our guide to construction formwork types for the temporary molds that cast-in-place concrete needs.
Building Codes and Standards for CLT
- Europe: European Technical Assessments began regulating CLT in 2006, and the first European product standard, EN 16351, was approved in 2015.
- North America: CLT entered the International Building Code (IBC) in 2015, in accordance with the ANSI/APA PRG 320 product standard.
- Taller buildings: the 2021 IBC added three mass timber construction types, IV-A, IV-B and IV-C, allowing taller and larger buildings, up to 18 stories.
Local adoption of model codes varies, so the permitted height and fire protection for a CLT project depend on the code in force where it is built.
Is CLT Fire Resistant?
CLT is combustible, which is why it receives a class D fire rating. Its performance once a fire has started comes from charring: a char layer forms on the surface and slows the fire from reaching the inner wood. CLT is classified REI 90, meaning it can keep its load-bearing capacity and integrity for 90 minutes in a fire, while unprotected steel loses its load-bearing capacity after about 15 minutes. The rating of a real wall or floor depends on the panel thickness and any protective lining the designer specifies.
Famous CLT and Mass Timber Buildings
| Building | Location | Completed | Height | Role of CLT |
|---|---|---|---|---|
| Stadthaus | Hackney, London, UK | 2009 | 9 stories, 30 m | First building framed only in CLT, including stairs and elevator shaft |
| Forte Living | Melbourne, Australia | 2012 | 10 stories, just over 32 m | 759 CLT panels made in Austria from European spruce |
| Mjøstårnet | Brumunddal, Norway | March 2019 | 18 stories, 85.4 m | Glulam load-bearing structure; CLT for stairwells, elevator shafts and balconies |
| Ascent MKE | Milwaukee, Wisconsin, USA | August 2022 | 25 stories, 86.6 m (284 ft) | Hybrid of concrete, steel and mass timber; CLT mainly for floor slabs |
CLT is also used outside housing: the Mistissini Bridge in Quebec (160 m long, completed 2014) uses locally sourced CLT panels with glulam girders. In 2024 a Dutch collaboration introduced Circular CLT (C-CLT), made from demolition wood and discarded pallets, first used in The Urban Woods building in the Netherlands.
CLT Pros and Cons at a Glance
| Advantages | Disadvantages |
|---|---|
| Prefabricated panels shorten site schedules | Production and long-distance transport can raise upfront cost |
| Much lighter than concrete, so smaller foundations and lifting equipment | Fewer engineers and contractors with CLT experience in some markets |
| High in-plane and out-of-plane strength; good seismic performance | Needs added layers to meet sound insulation ratings |
| Low thermal conductivity (about 0.10 W/m·K) and airtight construction | Standard floor vibration test methods do not fit CLT well, so design relies on engineering judgment |
| Renewable and stores carbon when sourced from managed forests | Must be kept dry during shipping and construction |
Who Should Consider CLT?
CLT suits projects where speed, weight and carbon matter: mid-rise apartments, offices, infill sites with tight access, and modular building. It is a different workflow from site-built concrete, so the team needs designers, engineers and installers with mass timber experience. For background on how building types differ, see residential and commercial construction; for planning a prefabricated schedule, construction management software helps coordinate factory deliveries with site work.
Faster assembly with smaller crews is one reason builders facing construction labor shortages look at CLT, and developers tracking embodied carbon often cover it in a sustainability report.
Frequently Asked Questions
What does CLT stand for?
CLT stands for cross-laminated timber, an engineered wood panel made of at least three layers of lumber glued with the grain of each layer usually at right angles to the next.
How tall can a CLT building be?
The 2021 International Building Code permits mass timber buildings up to 18 stories. Taller hybrid towers exist: Ascent MKE in Milwaukee reaches 25 stories and 86.6 m using concrete, steel and mass timber.
Is CLT as strong as concrete?
CLT is not a like-for-like substitute for concrete, but its cross layers give it in-plane and out-of-plane strength and stiffness comparable to a reinforced concrete slab at a much lower weight.
Does a CLT house need insulation?
CLT has low thermal conductivity (about 0.10 W/m·K), but CLT alone does not meet usual sound insulation ratings, and the need for thermal insulation depends on panel thickness and the local energy code.
How long does CLT last in a fire?
CLT is classified REI 90, meaning it keeps its load-bearing capacity and integrity for 90 minutes in a fire, because a protective char layer forms on the surface.
What is the difference between CLT and glulam?
CLT is a panel with layers glued crosswise, used for walls, floors and roofs. Glulam has all layers running the same way and is used for beams and columns.
