Legal and Regulatory

Tall Mass Timber Buildings Now Possible Under 2021 IBC Code Changes

The ICC approved 17 changes to the 2021 editions of the IBC and International Fire Code, allowing for mass timber buildings up to 18 stories. With the addition of three new mass timber construction types, this is the first time significantly new construction types have been added to the code.
By Kenneth Bland
January 7, 2020
Legal and Regulatory

The International Code Council (ICC) has approved 17 changes to the 2021 editions of the International Building Code (IBC) and International Fire Code, allowing for mass timber buildings up to 18 stories. With the addition of three new mass timber construction types (Type IV-A, IV-B, and IV-C), this is the first time in the history of the modern building code that significantly new construction types have been added to the code.

Building Materials

The primary building material that makes tall mass timber (TMT) buildings possible is cross-laminated timber (CLT). CLT is manufactured from dimension lumber (nominal 2x lumber) laid side-by-side or mass plywood panels of a specified width. Laminations of lumber are typically laid perpendicular to each other to form panels of various thicknesses that are bonded together using heat resistant adhesives that cure in large hydraulic presses. CLT commonly consists of an odd number of laminations.

These solid wood panels can be anywhere from 6 inches to 20 inches nominal thickness and 60 feet long. Typical CLT panels will be 6 inches to 14 inches nominal thickness. The panels are fabricated off site, transported onto the construction site and assembled in a manner that is efficient and remarkably fast. CLT panels can be used as floor, wall, or roof building elements supported by glued-laminated beams and columns.

While CLT is relatively new to the American marketplace, it has been available in Europe since the late 1980s and its use has increased significantly between 2000 and 2010. The first patent for CLT was issued in France in 1985. The first CLT projects in Germany and Switzerland were in 1993. The first multi-story residential CLT building was built in Styria, Austria in 1998. There are more than 500 CLT buildings in the United Kingdom.

The tallest TMT building in North America is Brock Commons in Vancouver, British Columbia. It is an 18-story residential building used for student housing on the campus of the University of British Columbia. The tallest CLT building in the United States is Carbon 12 in Portland, Ore., an eight-story condominium building. The T3 building in Minneapolis is a seven-story office building constructed with nail-laminated timber (NLT). NLT is another form of mass timber permitted to be used on TMT buildings.

Safety Testing

CLT has demonstrated that it is incredibly durable under fire and blast test scenarios. Fire testing at the Bureau of Alcohol, Tobacco, and Firearms lab demonstrated that CLT can withstand fires of intense heat (23 megawatts) fueled by a residential fire load for up to three hours with only isolated charring of the CLT panels and no failure of the structure. In one of the fire test scenarios, the fire burned itself out after three hours and the char layer resulting from an intentionally exposed area of a CLT ceiling never extended beyond the first lamination.


The architectural design community quickly endorsed the new allowance for taller mass timber buildings. Beginning in 2021, mass timber products manufactured from renewable and sustainable forests provide an alternative to less environmentally friendly materials for tall buildings. Most of the energy needed to manufacture mass timber building elements is produced using energy derived from carbon neutral biomass combustion.

Additionally, mass timber buildings store enormous amounts of carbon captured from the atmosphere and stored indefinitely as a wood product. Lower embodied energy from the manufacturing process combined with the sequestering of atmospheric carbon makes mass timber a viable option for reducing a buildings impact on the environment.


More TMT buildings will be planned and constructed, and the use of CLT in the United States will expand now that the TMT code provisions are destined to be in the 2021 edition of the IBC. Oregon and Washington have already taken steps to permit the construction of TMT that complies with the approved ICC code changes. Additionally, tall mass timber buildings have been proposed in Cleveland, Ohio; Newark, N.J.; Milwaukee, Wis.; and Washington, D.C.

Now is the time to become familiar with the coming code updates.

by Kenneth Bland

The AWC represents the interests of the North American wood products industry. For more information, download the 2015 Wood Frame Construction Manual.

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