Cross-Laminated Timber (CLT)
Engineered wood panels built from crosswise glued layers of lumber.
Quick Answer
Cross-laminated timber (CLT) is an engineered wood panel made by gluing layers of dimensional lumber with each layer rotated 90 degrees to the one below. This crosswise layup gives the panel strength and stability in two directions, so it can serve as a structural floor, wall, or roof element in mass timber buildings.
The Full Picture
CLT is manufactured by stacking boards in layers, typically three, five, or seven, with adjacent layers oriented perpendicular to each other, then bonding them with structural adhesive under pressure. The alternating grain direction limits shrinking and swelling and gives the panel two-way structural behavior, which is what distinguishes it from glulam beams, which are laminated with the grain running in one direction.
In North America, CLT is manufactured under the ANSI/APA PRG 320 standard, which defines panel grades, performance requirements, and quality control. Panels are large, produced in a plant to specified sizes, and cut by CNC machines with openings and connection details before shipping. Panel size is limited mainly by manufacturing and transport constraints, so designers coordinate dimensions with the manufacturer.
Structurally, CLT is used as floor and roof slabs, load-bearing walls, shear walls, and elevator or stair cores, connected with screws, steel brackets, and hold-downs. Fire performance relies on charring: the outer layer of wood chars at a predictable rate and insulates the interior, so designers add sacrificial thickness or protect the panels with gypsum board as the code requires. Moisture protection during shipping and construction is important, since exposed wood can swell or stain.
Acoustics, vibration, and floor topping are design considerations. Bare CLT floors transmit impact and airborne sound, so assemblies typically add toppings or resilient layers. Panels are generally priced by volume or area and by manufacturer lead time, and early coordination with the manufacturer and engineer affects both cost and schedule.
Real Examples
Common Misconceptions
People assume: CLT and glulam are the same product.
Actually: Glulam beams are layered with the grain running parallel, so they work mainly in one direction. CLT panels alternate grain direction, giving panel behavior in two directions.
People assume: CLT can be left unprotected during construction.
Actually: Exposed CLT can absorb moisture, swell, or stain, so weather protection, drainage of rain from floors, and drying plans are normal parts of the construction process.
Frequently Asked Questions
What standard governs CLT in the US?
ANSI/APA PRG 320, Standard for Performance-Rated Cross-Laminated Timber, covers CLT manufacturing, grading, and qualification, and building codes reference it.
How many layers does CLT have?
Panels typically have an odd number of layers, commonly three, five, or seven, though other layups exist depending on the manufacturer and design.
How does CLT perform in fire?
The outer wood chars at a predictable rate, protecting the unburned interior, which keeps carrying load. Designs account for char depth or add protection such as gypsum board, per the code.
Can CLT be used for floors and walls?
Yes. It is used as floor, roof, and wall panels, including shear walls and cores, with connections and acoustic or topping layers designed for the project.