Cross-laminated timber (CLT) stacks layers of lumber at right angles and bonds them into panels that are light enough to crane into place and strong enough to carry multi-story loads. The panels deliver acoustic separation, fire resistance, seismic performance, and thermal insulation in a single product, and they install fast with little on-site waste. That combination makes CLT a practical alternative to concrete, masonry, and steel on larger projects. Specifying it responsibly starts with standards, and builders who hold envelope and structural products to recognized benchmarks apply the same logic through compliance requirements for residential building products.
What ANSI/APA PRG 320 Covers
In the United States and Canada, CLT design standards (the NDS in the U.S. and CSA O86 in Canada) require certified products to meet ANSI/APA PRG 320, the Standard for Performance-Rated Cross-Laminated Timber. The standard governs how panels are manufactured, tested, and marked, and it anchors the design values, adhesive requirements, and durability criteria that engineers rely on.
How Certification Works
Certification is not a one-time sticker. Accredited agencies audit manufacturing plants, witness tests, and re-verify production on a schedule, so a PRG 320 mark means the panel in the yard matches the panel that was tested. Buyers can verify certificate numbers through the APA directory, and the supply-side momentum visible when manufacturers expand product offerings for North American demand keeps certified CLT competitive with imported panels.
Characteristics Evaluated
| Characteristic | Requirement in PRG 320 | Risk if Not Met |
|---|---|---|
| Design properties | Derived from NDS and CSA O86 lumber values | Incompatible design values |
| Adhesive heat durability | ASTM D7247 plus CSA O177 flame test | Bond failure in a fire |
| Moisture durability | Vacuum-pressure-soak aging tests | Delamination over time |
| Fire performance | Species-based char and bond testing | Unpredictable fire behavior |
The standard also covers panel grades, layup configurations, and allowable tolerances, so two panels that look alike on a truck can differ meaningfully in what they are rated to carry. Reading the grade stamp is the first step in any CLT submittal review.
Design Property Compatibility
Design capacities published in PRG 320 come from lumber properties recognized in the NDS and CSA O86. Imported lumber can carry different strength, stiffness, and density values that those documents do not recognize, which makes the published design values for foreign-species panels incompatible with North American calculation methods. Specifiers face the same situation they handle when a product category rule decides which roof coating materials meet the local code: the numbers have to come from a recognized source.
Lumber Properties and Published Values
Every engineered product pairs material properties with design assumptions. When the species changes, so do the allowable stresses, the fastener values, and the connection details. A panel built from species outside the NDS and CSA O86 system leaves the engineer to derive values with no authoritative basis, which pushes the risk onto the project.
Why Compatibility Matters
Incompatible design values do not fail at the desk; they fail at the connection. Shear, bearing, and uplift at panel edges depend on properties assumed during design, and mismatched assumptions produce joints that work on paper and crack in service.
Imported panels are not inherently weaker; they are simply outside the system that North American codes recognize. A manufacturer that wants to sell them here can qualify the product through the same testing and gain the same recognition, but until that happens the engineer is designing without a net.
Adhesive Heat Durability
CLT panels are only as strong as the bonds between layers, and those bonds have to survive elevated temperatures. PRG 320 requires adhesives to pass heat durability testing so they do not melt or lose shear strength when a fire heats the panel face.
ASTM D7247 and the CSA O177 Flame Test
Heat durability is evaluated with ASTM D7247, the standard test method for shear strength of adhesive bonds in laminated wood products at elevated temperatures, together with the CSA O177 flame test. Together they confirm the bond holds when it matters most. Most adhesives used in panels manufactured outside North America have never been evaluated this way, and an untested adhesive is a life safety question, not a paperwork gap.
The Life Safety Stakes
In a structure fire, the char layer protects the panel interior, but that protection assumes the adhesive still holds the unburned layers together. A bond that softens at moderate temperatures lets layers separate early, changes the panel’s structural response, and puts occupants and firefighters at risk. Durability claims need proof over time; automated transmissions reshaped North American trucking only after years of service validated them, and CLT adhesives deserve the same evidence standard.
What Happens When Bonds Soften
Softened bonds show up as delamination along glue lines, visible as separated layers at panel edges. Early detection is difficult once panels are installed, which is why the qualification test happens before the panel ships.
Moisture Durability and Accelerated Aging
Moisture is the slow killer of wood products. PRG 320 requires CLT to pass the same accelerated aging tests used for structural glued laminated timber, which has decades of proven performance around the world.
Vacuum-Pressure-Soak Testing
The vacuum-pressure-soak cycle forces water into the panel under pressure, then dries it, repeating the cycle to simulate years of wet-dry exposure in weeks. Panels that survive with limited delamination and adequate wood failure pass. Most imported CLT has not been evaluated to this standard, so its long-term behavior is unknown rather than proven.
Face-Bond Criteria
PRG 320 sets the same face-bond criteria as glulam: a minimum percentage of wood failure and a maximum allowance for glue-bond delamination after aging. These are among the most stringent requirements for CLT in the world, and they exist because a panel that delaminates loses stiffness, strength, and fire resistance together.
Production Capacity and Supply Decisions
Supply decisions shape what gets specified. Domestic producers are expanding production capacity for North American industry across many product lines, and the same logic that favors locally certified goods applies to structural timber: shorter supply chains, audited plants, and standards enforcement that stops at the border.
Weather exposure during construction is part of the moisture story. Panels that sit uncovered on site for weeks absorb moisture at edges and cutouts, and the durability rating assumes the panel starts service at the moisture content the manufacturer specifies.
Fire Performance and Char Behavior
CLT performs well in fire because wood chars predictably. The char layer insulates the unburned core, and engineered panels maintain structural capacity while the fire consumes the sacrificial layer. Char rate and bond behavior depend on species and adhesive, which is why certification ties fire performance to tested materials.
How CLT Behaves in Fire
In a standard fire test, panels are loaded to design capacity and exposed to the time-temperature curve. The panel keeps carrying the load until the remaining section falls below the required strength, giving designers a calculable fire resistance that competes with noncombustible construction in many assemblies.
Species Knowledge and Char Rates
Char rates are published for the North American species used in certified panels. Less common imported species may lack that data, and species knowledge can disappear when a supply chain vanishes; the American chestnut story shows how quickly material expertise can be lost when a species leaves the market.
Connections and penetrations deserve the same scrutiny as the panel itself. Hold-downs, brackets, and the sealants around openings all contribute to the assembly’s performance, and the manufacturer’s installation instructions cover details that the material standard does not.
Specifying Certified CLT
The practical test for any CLT purchase is documentation. A certified panel carries a PRG 320 mark, a certificate number, and a manufacturer’s declaration that ties the lot to tested production.
Questions to Ask a Supplier
- Ask for the PRG 320 certificate number and confirm it through the APA directory.
- Request the species and grade of every layer, and confirm they appear in the NDS or CSA O86.
- Ask which adhesive was used and whether it passed ASTM D7247 and the CSA O177 flame test.
- Request the moisture durability test report, including wood failure and delamination results.
- Confirm the design values match the standard you are engineering to, and get them in writing.
Documents to Request
- PRG 320 certificate and APA directory listing.
- Test reports for adhesives and moisture durability.
- Published design values matched to the NDS or CSA O86.
- Manufacturer installation instructions and handling guidance.
A complete submittal includes the certificate, the test reports, the design values, and the installation instructions. Industries that build credibility through shared standards, the way the power sweeping industry did through association training and certification, show what buyers should expect: consistent, audited quality rather than claims.