CLT & Glulam – Char Depth Calculations

Mass Timber (MT) construction, including Cross-Laminated Timber (CLT) and glue-laminated (glulam) structural members, are growing in popularity, with advances in assembly fire testing and code requirements furthering fire and life safety compliance options.

Where a CLT or glulam element is required to carry a fire-resistance rating (FRR), there are two general options available: (1) comply with a tested or listed system, or (2) calculate the FRR using the “char depth” method.  For the latter option, a member is structurally over-sized to allow for a calculated portion of the member to be sacrificial, and burn and char if exposed to a fire event.  The charring creates a natural insulation for the remaining intact portion of the wood, allowing for structural integrity to be maintained for a certain period of time.

The use of the char depth calculation method is permitted through Section 722 of the 2015 IBC, which references Chapter 16, Fire Design of Wood Members, of the 2015 National Design Specification (NDS) for Wood Construction, with a commonly used and validated char rate at 1.5 – 1.8 inches/hour. If this option is pursued, it should be coordinated between the structural engineer of record and the fire/life safety consultant on the project.

In addition to the char depth calculation approach, tested systems are also available for use in such construction, with more and more becoming available each year. When using such test reports or listings, compliance with the details of the tested system should be reviewed for application into the design.

We have been involved in several mass timber projects and are looking forward to seeing more of this building method in the future! If you have any question on Mass Timber in construction, please reach out to our office at to find out how we can assist.