Cross-Laminated Timber (CLT)
Large 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 together, with each layer rotated 90 degrees from the one below. The crosswise layup gives the panel strength in both directions, so it can serve as a floor, wall, or roof element in mass timber buildings.
The Full Picture
CLT is one of several mass timber products, alongside glulam beams and columns and nail-laminated timber. A panel is built from an odd number of layers, commonly three, five, or seven, of kiln-dried lumber boards. Alternating the grain direction of each layer reduces the swelling and shrinking that solid wood normally shows and lets the panel carry load across two directions, much like a concrete slab.
In North America, CLT manufacturing is governed by the ANSI/APA PRG 320 standard, which sets requirements for lumber grades, adhesives, and panel performance testing. Panels are cut by CNC machines in the factory, including openings for windows, doors, and mechanical penetrations, then shipped to site and craned into place. That prefabrication is a main reason CLT projects can erect quickly with less on-site labor and waste than cast-in-place concrete.
Building codes treat CLT through the mass timber construction types. The 2021 International Building Code added Types IV-A, IV-B, and IV-C, which allow tall mass timber buildings with specified fire protection and, in some cases, exposed timber. Wood chars at a predictable rate when exposed to fire, so designers size panels with extra thickness and often protect them with gypsum board to reach required fire-resistance ratings.
For contractors, CLT changes the preconstruction conversation. Panel fabrication slots, lead times, crane capacity, weather protection during erection, and connection hardware all need to be settled early, and the structural engineer, fabricator, and erector typically coordinate closely before the design is final. Acoustic treatment, moisture management, and the sequence of other trades around exposed timber surfaces also deserve early attention.
Real Examples
Common Misconceptions
People assume: Wood buildings burn down faster than concrete or steel ones.
Actually: Thick mass timber chars on the outside at a fairly predictable rate, and the charred layer insulates the wood behind it. Code-approved mass timber designs are tested and detailed to meet fire-resistance ratings.
People assume: CLT is the same as plywood.
Actually: Plywood uses thin veneers. CLT uses full dimensional lumber boards in thick panels, designed as structural elements, and is manufactured to its own standard (ANSI/APA PRG 320).
People assume: CLT can be used anywhere without special detailing.
Actually: Moisture protection during construction, connection design, acoustics, and fire protection all need deliberate detailing, and the permitted height and area depend on the code construction type and local amendments.
Frequently Asked Questions
What is CLT used for in construction?
CLT panels are used as floor, wall, and roof elements, and sometimes as elevator and stair cores. They are common in mid-rise and tall mass timber buildings and in education, office, and multifamily projects.
How is CLT different from glulam?
Glulam is made of lumber laminated with the grain running the same direction, so it works as beams and columns. CLT alternates grain direction layer by layer, so it works as a two-way panel for floors, walls, and roofs.
Is CLT fire resistant?
Thick timber chars at a predictable rate, and designers account for that with added thickness and, where required, gypsum board protection. Fire-resistance ratings are demonstrated through testing and recognized in the International Building Code mass timber provisions.
Who manufactures CLT in the US?
Several North American manufacturers produce CLT to the ANSI/APA PRG 320 standard. Availability, panel sizes, and lead times vary, so fabricator selection is usually an early preconstruction decision.
Does CLT cost more than concrete or steel?
It depends on the project. Material cost per square foot can be higher, while savings may come from faster erection, lighter foundations, and reduced site labor. Project-specific pricing from fabricators is needed rather than a general rule.