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Firestop Systems in CLT Construction

This article will introduce new options for certifying firestop and joint systems installed in CLT walls or floors.

Timber building construction

October 10, 2022

By Matthew Schumann, industry manager, Building Materials, Built Environment COU  

The model building codes now feature significant new requirements regarding the design, construction and commissioning of larger structures using mass timber construction methods. The mass timber industry and other stakeholders developed these requirements to advocate for the use of wood solutions as a sustainable construction alternative to steel, masonry or other types of lightweight construction.   

The concept of mass timber construction has also become attractive to the architectural design community for other reasons, including sustainability and aesthetics. As a result, a number of mass timber projects have been constructed in diverse locations such as Cleveland, OH; Milwaukee, WI; and Portland, OR. While the concept of mass timber construction is not new and has roots in the mill construction practices of 19th century New England, its recent renaissance indicates that we’ll be using this construction method for some time to come. 

The acceptance of mass timber construction means that changes in supporting construction elements must also push forward. One key area undergoing rapid change is the field of firestopping of through penetrations and membrane penetrations. UL Solutions customers and colleagues have directed feedback to our Certification Staff indicating the need for changes to support certification of various fire-stopping products in mass timber construction — specifically, cross-laminated timber, or CLT. This feedback reflects the frequent use of manufacturer-provided engineering judgments or test reports to gain acceptance of the proposed solutions for penetrations in CLT construction. With more projects underway, manufacturers recognized that third-party certification would provide a better solution, placing the certified firestop systems in CLT on par with the several thousand existing solutions for masonry or gypsum construction. The common message UL Solutions received from contractors, code authorities and building owners was that third-party certifications were preferred over constant reliance on engineering judgments. This consensus in opinions from different stakeholders strongly supported UL Solutions’ efforts to address this paradigm shift with UL Product iQ® and our certification paths for fire stopping in CLT. You can use Product iQ, available at, complimentary of charge with a one-time registration. 

We have updated the XHEZ guide information in Product iQ to include the options for a CLT floor or wall assembly by assigning the letter G for floors and O for walls in the alpha-alphanumeric identification system. These letters are the second alpha component in the system title and differentiate CLT construction from other specified construction types such as concrete floors, framed walls or bulkheads.  

Successful code proposals resulted in the 2021 International Building Code (IBC)  addressing CLT by introducing new construction types for mass timber, a new requirement for CLT to undergo testing and feature a label for compliance with the 2019 ANSI/APA PRG 320 standard, and additional information about methods of noncombustible construction and how to determine hourly ratings for the materials specified. This led to revisions to the UL Solutions XHEZ guide information published on July 11, 2022, to add explanatory language about mass timber, 2021 IBC and 2018 NDS requirements for CLT and the rules for noncombustible protection. This clarifies that certification of firestopping materials in code-permitted constructions is both possible and permitted.   

The result of these changes is that when the CLT is not third-party-certified, the UL firestop system language will indicate the requirements for the CLT and any additional protection required to meet the various ratings specified in the individual system. This information will be described in the individual systems in a similar manner to the current descriptions for other types of walls and floors.    

New firestopping systems featuring CLT construction will appear in Product iQ® before 2023. Expect updates to the XHEZ guide information as firestopping products addressing CLT assemblies enter the market.    

These updates to the XHEZ guide information are a result of UL Solutions’ collaboration with firestopping manufacturers and the wood industry. For the regulatory community, our customers and the building industry, this updated information ensures that UL certifications reflect the current pace of today’s rapid building innovation. 

UL Solutions Updates Firestop System Numbering Scheme

This article explains the latest number system for UL Solutions firestop systems.

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