Tall mass timber is an industry term to identify mass timber buildings, constructed of mass timber elements, that exceed current height limits for wood buildings set by the International Building Code (IBC). Mass timber includes any product currently permitted for use in Type IV construction, such as Cross Laminated Timber (CLT), Structural Composite Lumber, glued-laminated timber, and large section sawn lumber.

Because of the unique structural and fire resistance characteristics of solid timber walls and floors (CLT) and mass timber structures, new provisions have been approved for the 2021 Edition of the IBC for tall mass timber buildings up to 18 stories.

The committee's work can be followed at this link: ICC Ad Hoc Committee on Tall Wood Buildings

Tall Mass Timber Fire Testing

Identically furnished, multistory, one-bedroom apartments constructed of exposed, partially exposed, and unexposed (protected) five-ply cross-laminated timber (CLT) passed a series of five rigorously monitored fire tests. The purpose of the tests was to provide data that helped inform recommendations the ICC Ad Hoc Committee on Tall Wood Buildings (TWB) proposed for the 2021 International Building Code.

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North America's Tallest Wood Building

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Fire Resistance

The International Building Code (IBC) requires all buildings to perform to the same level of fire safety, regardless of the materials used in construction. Repeated fire resistance testing has demonstrated how mass timber structures meet or exceed IBC requirements.

The fire performance of mass timber is fundamentally different from dimension lumber. The inherent fire resistance of mass timber is due to the charring that occurs on the exterior of mass timber components in a fire event. This charring creates a protective barrier for the inner structure of mass timber and commonly self-extinguishes.

Successful ASTM E119 fire resistance testing is one reason the ICC recognized mass timber construction (Cross Laminated Timber) in the 2015 IBC. At that time, two-hour resistance was required by code, while actual testing demonstrated resistance of 3 hours and 6 minutes. This exceptional performance is essential for occupant and first responder safety and compares favorably with any other material.

More recent 2017 testing conducted at the federal government’s ATF research laboratory, the world’s largest research facility for fire investigations, has added substantially to the body of science on mass timber. Five tests were performed on full-scale, multi-story mass timber apartment buildings at the federal lab. The results have confirmed the earlier findings. Importantly, several tests were conducted that assumed the unlikely event of a sprinkler system failure, while others confirmed the effectiveness of sprinkler systems in mass timber fire events:

A mass timber structure fully protected by two layers of 5/8-inch type X gypsum wallboard. The fire burned itself out after 3 hours with no significant charring on the protected mass timber surfaces.
A mass timber structure with 30% of the CLT ceiling exposed. Test concluded after 4 hours, with apartment contents burned out and CLT self-extinguished due to char that protected the underlying mass timber.
A mass timber structure with 2 CLT walls fully exposed. Apartment furnishings and contents consumed after 4 hours and CLT self-extinguished after formation of protective char surface.
A fully exposed mass timber structure with sprinkler system. A single sprinkler quickly contained the fire.
A fully exposed mass timber structure with sprinkler system. Fire allowed to grow to flashover – 23 minutes – before being quickly extinguished by sprinkler.

Fire resistance testing for any material under the IBC must be rigorous and clearly demonstrate compliance with the code. Experience and testing indicate that mass timber technologies provide ample safety margins for building occupants and first responders in any fire event.

Tall Mass Timber Toolkit

Understanding the Tall Mass Timber Code Changes: A Toolkit for Fire Officials follows the International Code Council's approval of code changes allowing for the construction of tall mass timber buildings up to 18 stories for the 2021 edition of the International Building Code. This document provides a recommended sequence for review that should give a comprehensive understanding in an efficient and most-easily understood manner. The toolkit provides detailed comparisons of new types of timber construction and differences from traditional dimension lumber, and offers an overview of the primary fire protection features which will now be required for mass timber buildings.

2021 IBC Approved Code Change Resources

Building Officials Guide to Tall Mass Timber Code Changes

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17 Tall Mass Timber Code Changes Recommended for Approval

IBC Section 602.4 Type IV construction (G108-18)
IBC Section 703.8 (new) Tested noncombustible protection contribution (FS5-18)
IBC Section 722.7 (new) Calculated noncombustible protection contribution (FS81-18)
IBC Section 703.9 (new) Sealing of adjacent mass timber elements (FS6-18)
IBC Section 718.2.1 Fireblocking materials (FS73-18)
IBC Section 403.3.2 and IFC Section 914.3.1.2 High rise sprinkler water supply (G28-18)
IFC Section 701.6 Owner’s responsibility (F88-18)
IFC Section 3308.4 (new) Fire safety during construction (F266-18)
IBC Table 504.3 (G75-18) Allowable height in feet
IBC Table 504.4 (G80-18) Allowable number of stories
IBC Table 506.2 (G84-18) Allowable area
IBC Section 3102.3 Special construction (G146-18)
IBC Appendix D Section D102.2.5 Fire Districts (G152-18)
IBC Sections 508.4.4.1 and 509.4.1.1 (new) Fire barriers at separated occupancies and incidental uses (G89-18)
IBC Section 2304.10.1 (new) Fire resistance of connections (S170-19)
IBC Section 1705.5.3 (new) and Table 1705.5.3 (new) Special inspection provisions (S100-19)
IBC Section 110.3.5 (new) Inspection of protection for connections (ADM35-19)

2021 IBC, 2018 Group A, Tall Mass Timber Proposals Review Guide

Type of Construction Comparison document

Tall Mass Timber Type of Construction Comparison

What the Fire Service Needs to Know about Tall Wood Buildings, Firehouse.com

Accommodating Mass Timber Buildings — the Right Way to Change the Building Code, Firehouse.com

Examples of Tall Mass Timber Buildings

Tall Mass Timber Buildings are growing in popularity because of their strength, resilience and efficiency.

See Examples of Tall Mass Timber Buildings

Tall Mass Timber – Fire Safe and Code Compliant

In 2016, the International Code Council (ICC) appointed a balanced committee of building officials, fire officials, architects, fire protection engineers, and industry experts to examine and if appropriate propose building code changes to allow for tall mass timber buildings. The changes create three new construction types in the 2021 International Building Code allowing tall mass timber buildings to reach a maximum of 18 stories.

However, despite years of objective, rigorous testing which informed these code decisions, some still question the safety of tall mass timber – particularly fire safety.

In his recent article in Fire Engineering, Raymond O’Brocki, Manager of Fire Service Relations for the American Wood Council (AWC), reviews these code changes to dispel safety concerns, and ensure confidence in mass timber as well as the ICC’s independent and reliable code development process.

In response to fire safety concerns regarding increased height for wood buildings, including dependence on fixed fire protection systems and the need for additional training and pre-incident plans, O’Brocki notes that:

The fire resistance rating for the tall mass timber structure is three hours in buildings above 12 stories and two hours in all others.
The only specialized firefighter training needed to respond to a tall mass timber building is training in high-rise operations. These buildings will perform as any high-rise building in the event of a fire.
The tall mass timber code changes include a section on owner’s responsibility which requires a third-party inspection of all passive fire protection annually. This would not require any special knowledge of a fire inspector to review the third-party inspection report and is identical to reviewing annual fire alarm and sprinkler reports during the annual inspection.

Our nation’s building codes have never been stronger than they are today. As construction innovations evolve, the safety of firefighters and first responders remains AWC’s top priority. We are confident that ICC will also continue to strengthen research and testing when determining code changes – and tall mass timber is no exception.

To learn more about the recent tall mass timber code changes and fire safety requirements, read Raymond O’Brocki’s Fire Engineering article, “Tall Mass Timber Buildings and Fire Service Concerns.”