Cross-Laminated Timber: A Guide to Selection and Specification

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Cross-laminated timber (CLT) is a prefabricated, solid engineered mass timber panel. Because CLT is prefabricated, most components arrive ready to assemble and go together very quickly. CLT’s large-scale components enable faster construction, not only because of prefabrication, but because fewer joints are needed between elements. 

CLT also offers incredible strength. The crosswise arrangement of board layers lends integral structural stability to the panel, considerably increases dimensional stability and ensures uniform load transfer to all sides for excellent structural capacities.

CLT’s panel size varies by manufacturer, but typical widths are 2 feet, 4 feet, 8 feet and 10 feet, with a thickness of 20 inches or less and a length of up to 60 feet.

When selecting and specifying CLT, there are several key characteristics to consider, including allowable design capacities, grades, laminations and adhesives.

Allowable Design Capacities

There are five basic “E” grades, six “V” grades and three “S” grades for CLT products and layups. “E”, “V” and “S” indicate a CLT grade with layup of E-rated or machine stress rated (MSR) laminations, visually graded laminations or structural composite lumber (SCL) in the longitudinal layers, respectively. Visually graded or SCL laminations are used in the transverse layers for both “E” and “V” grades and “S” grades, respectively. Custom layups of CLT are also permitted, as stipulated in ANSI/APA PRG 320, Standard for Performance-Rated Cross-Laminated Timber. 

The allowable stress design (ASD) reference design capacities for CLT grades with layups of three, five and seven layers are shown in ANSI/APA PRG 320. The ASD reference design capacities for different CLT products, including custom grades and layups, are typically published in APA Product Reports ( or manufacturers’ literature. 

Trademarks and Acceptance

Chapter 10 of the 2018 National Design Specification® (NDS) provides design procedures, reference design values and other information for CLT, while engineering design of connections using dowel-type fasteners in CLT is covered in Chapter 12 of the 2018 NDS. Sections were also added to the 2018 and 2015 International Building Code and International Residential Code regarding CLT used as different structural elements. Clause 8 in the Canadian Standards Association CSA O86-19 provides design procedures, resistance values and other information for CLT used in Canada. Note: National Design Specification® is a registered trademark of the American Wood Council.


Any Softwood lumber species or species combinations recognized by American Lumber Standards Committee under PS 20 or Canadian Lumber Standards Accreditation Board under CSA O141 with a minimum published specific gravity of 0.35 are permitted for use in CLT, provided that other requirements specified in ANSI/APA PRG 320 are satisfied. SCL should meet the requirements of ASTM D5456, Standard Specification for Evaluation of Structural Composite Lumber Products, and have an “equivalent specific gravity” of 0.35 or higher. 

Lumber grades are required to be at least 1200f-1.2E MSR or visually graded No. 2 in the longitudinal layers and visual graded No. 3 in the transverse layers.

Moisture content is required to be 12 ± 3% for lumber and 8 ± 3% for SCL at the time of CLT manufacturing unless a lower moisture content is specifically qualified in accordance with the standard. 


In the U.S., adhesives used for CLT manufacturing are required to meet ANSI 405, Standard for Adhesives for Use in Structural Glued Laminated Timber, with the exception that some gluebond durability tests are not required. This is because CLT manufactured according to ANSI/APA PRG 320 is limited to dry service conditions, and some gluebond durability tests are designed for adhesives in exterior applications. In Canada, CLT adhesives must meet the requirements of CSA O112.10. In both the U.S. and Canada, CLT adhesives must meet ASTM D7247 for heat durability and CSA O177, small-scale flame test. In addition, CLT adhesives must comply with the requirements for elevated temperature performance in accordance with the full-scale compartment fire test specified in Annex B of ANSI/APA PRG 320.

Cross-Laminated Timber: A Guide to Selection and Specification 1


CLT shall be furnished and installed in accordance with the recommendations provided by the CLT manufacturer and the engineering drawing approved by the engineer of record. Permissible details shall be in accordance with the engineering drawing.


1. Materials, Manufacture and Quality Assurance—Product quality shall conform to ANSI/APA PRG 320, Standard for Performance-Rated Cross-Laminated Timber.

2. Trademarks—CLT products conforming to ANSI/APA PRG 320, Standard for Performance Rated Cross-Laminated Timber, shall be marked with CLT grade, CLT thickness or identification, mill name or identification number, the APA logo and “ANSI/APA PRG 320.” The top face of custom CLT panels with unbalanced layup used for roof or floor shall be marked with “TOP” stamp.

3. Protection for Shipment—Members shall be protected with a water-resistant covering for shipment.

Find more information on the selection and specification of structural engineered wood products in the APA Engineered Wood Construction Guide, available as a free download at

About APA –The Engineered Wood Association

Founded in 1933 and based in Tacoma, Washington, APA represents approximately 176 plywood, oriented strand board, glulam timber, wood I-joist, Rim Board and structural composite lumber mills throughout North America. Its primary functions are quality auditing and testing, applied research, and market support and development.

Learn more at

By Miller Wood Trade Publications

The premier online information source for the forest products industry since 1927.

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