Terminology

• CLT: cross-laminated timber; odd number of lumber layers glued with alternating grain
• Layup / ply: number and orientation of layers (3-, 5-, 7-, and 9-layer panels)
• Major / minor strength direction: parallel and perpendicular to the outer layers' grain
• Stress grade: PRG 320 grades E1, E2, E3 (machine stress-rated outer layers) and V1, V2 (visually graded); E1 most available
• Appearance grade: visual or industrial face quality, set by the manufacturer
• NLT / DLT: nail-laminated and dowel-laminated timber; lumber on edge, fastened with nails or hardwood dowels
• MPP: mass plywood panel, made from veneer layers
• Char rate / char layer: depth of wood that burns away in a fire; remaining section carries load
• Encapsulation: noncombustible protection such as gypsum board over mass timber
• Construction types IV-A, IV-B, IV-C, IV-HT: mass timber construction types with different protection and height limits
• Spline and lap joints: panel-to-panel connections along edges
• Self-tapping screws: long structural screws for panel connections

Standards

Sizes

• Panel thickness: 4 in. to 12 in. common; up to about 20 in. possible
• Panel width: 4 ft to 10 ft; length 16 ft to 64 ft, limited by press size and shipping
• Layups: 3-layer for light walls and partitions; 5-layer and up for bearing and shear walls

Grades

• Stress grade from ANSI/APA PRG 320 sets design capacities for 3-, 5-, and 7-layer layups
• Specify appearance grade for exposed faces; industrial grade where faces are concealed

Test standards

• ANSI/APA PRG 320: performance-rated CLT manufacture and qualification
• AWC NDS: CLT design values and char-based fire design (Chapter 16)
• ASTM E119: fire-resistance testing of mass timber assemblies
• ASTM E90 and E492: sound transmission of walls and floors
• ICC-ES or APA product reports for each manufacturer's panels and connectors

Materials

• CLT: most common wall panel; two-way strength and dimensional stability
• NLT and DLT: walls less common; DLT avoids metal fasteners for CNC cutting
• MPP: veneer-based panels with high strength in thinner sections
• Glulam: columns, beams, and wall posts paired with CLT
• Gypsum board encapsulation: Type X layers where exposure is limited by construction type
• Connectors: steel angles, hold-downs, self-tapping screws, and proprietary concealed connectors
• Moisture protection: factory-applied sealers, temporary membranes, and site tenting

Detailing

• Define exposed versus encapsulated faces early; construction type sets the allowable exposed area
• Coordinate MEP chases and openings for CNC cutting; field cutting is slow and costly
• Protect panels from rain during erection; keep panel ends off wet slabs and seal end grain
• Detail stair and elevator cores in CLT or concrete; check whether timber cores are allowed at the building height
• Acoustic separation: CLT walls alone rarely meet dwelling-unit targets; add furred gypsum or resilient layers
• Exterior: continuous WRB, air barrier, and insulation outboard of the panel; panels must stay dry in service
• Fire: char-based design or tested assemblies; seal panel joints and penetrations

Uses

Specific uses

• Bearing and shear walls in mid-rise housing, offices, and schools
• Stair and elevator cores in low- and mid-rise mass timber buildings
• Exposed interior walls for biophilic design
• Prefabricated wall panels for fast enclosure
• Mixed systems with glulam columns and CLT floors

Pros and cons

Pros

• Prefabricated and precise; fast erection with small crews
• Lighter than concrete, reducing foundation and seismic loads
• Exposed wood finish and lower embodied carbon
• Fire resistance through predictable charring

Cons

• Higher material cost than light-frame wood
• Sensitive to moisture during construction
• Acoustic performance needs added layers
• Limited domestic manufacturing; long lead times and early design freeze
• Exposed area and height limited by construction type

Manufacturing and cost

Construction

• Lumber dried, graded, finger-jointed, and laid up in alternating layers with structural adhesive
• Pressed in hydraulic or vacuum presses; cut to shape with CNC routers including openings and connector pockets
• Panels shipped in erection sequence and craned into place; connected with screws and steel connectors
• Delegated engineering by the supplier for connections and panel layout is common

Main manufacturers and suppliers

• Mercer Mass Timber
• Kalesnikoff
• SmartLam North America
• Nordic Structures
• Sterling Structural
• Vaagen Timbers

Trade associations: WoodWorks, Wood Products Council and APA, The Engineered Wood Association.

Typical costs

• CLT panels, material only: $40 to $90 per sq ft of panel
• CLT walls, finished and installed: $60 to $120 per sq ft of wall

Sources

• Canadian Wood Council: cross-laminated timber, https://cwc.ca/en/how-to-build-with-wood/wood-products/mass-timber/cross-laminated-timber-clt/
• APA: CLT selection and specification guide, https://www.apawood.org/guides-tools-training/technical-document-library/construction-guides/e30x-clt-engineered-wood-excerpt-cross-laminated-timber-clt-selection-and-specification/
• WBDG: UFGS 06 17 19 cross-laminated timber specification, https://www.wbdg.org/FFC/DOD/UFGS/UFGS 06 17 19.pdf
• STRUCTURE magazine: CLT, a guide to selection and specification, https://structuremag.org/?p=20196
• WoodWorks: concealed spaces in mass timber, https://www.woodworks.org/wp-content/uploads/presentation_slides_Romanowski_Concealed_Spaces_Wood_Design_08.2025.pdf
• RDH: economics of mass timber in construction, https://www.rdh.com/wp-content/uploads/2022/12/Economics-of-Mass-Timber-in-Construction_2020-02-11.pdf
• Wikimedia Commons: Многослойная клееная деревянная панель Райн Александр Дмитриевич.jpg, by Райн Александр Дмитриевич (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:Многослойная_клееная_деревянная_панель_Райн_Александр_Дмитриевич.jpg