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Over the past 20 years, there has been growing interest in tall buildings constructed from mass timber materials as a means to achieve greater urban density with more sustainable construction. Worldwide, there are now dozens of buildings with more than seven stories of timber construction.
In the U.S., the 2021 and 2024 versions of the International Building Code (IBC) include three new construction types—Type IV-A, IV-B and IV-C—that allow the use of mass timber or noncombustible materials in buildings up to 18, 12 and nine stories (respectively). These construction types are based on the previous Heavy Timber construction type (Type IV, changed to IV-HT in the 2021 IBC) but with additional requirements regarding fire-resistance ratings and the use of noncombustible protection.
Mass timber projects designed under the 2018 or earlier versions of the code are generally constrained by prescriptive building height limits of five to six stories depending on the occupancy group. The term “tall timber” is associated with buildings that exceed the limits of previous codes for number of stories, height, and in some cases area.
If you’re interested in pursuing a tall timber project in the U.S., see below for commonly asked questions and resources. WoodWorks also provides free technical support to developers and design/construction teams. For assistance, contact the WoodWorks Regional Director local to you or email help@woodworks.org.
Visit the WoodWorks Innovation Network for a searchable map of mass timber projects in the U.S. (and elsewhere), and to learn about their project teams. Here’s a link to the map filtered for tall mass timber buildings.
Built or Under Construction:
Follow this link to see which states have adopted the IBC 2021, IBC 2024, or the tall wood code provisions.
Required Noncombustible Protection on Mass Timber Elements by Construction Type
| IV-A | IV-B | IV-C | IV-HT | |
|---|---|---|---|---|
| Timber Exposure Allowances | No exposed timber permitted | Ceilings and integral beams up to an area equaling 20% (per 2021 IBC) or 100% (per 2024 IBC) of fire area or dwelling unit area or Walls and integral columns up to an area equaling 40% of fire area or dwelling unit area or A combination of each, with the sum of ratios (actual exposed divided by allowable exposed) not to exceed 1.0 | Full interior timber exposure permitted* | Full interior timber exposure permitted* |
| Interior Surface of Building Elements | Always required; 2/3 of FRR, 80 minutes minimum | Required with exceptions; 2/3 of FRR, 80 minutes minimum | Not required* | Not required* |
| Exterior Side of Exterior Walls | 40 minutes | 40 minutes | 40 minutes | 15/32″ FRT sheathing or 1/2″ gypsum board or noncombustible material |
| Top of Floor (above Mass Timber) | 1″ minimum | 1″ minimum | Not required* | Not required* |
| Shafts | 2/3 of FRR, 80 minutes minimum, inside and outside | 2/3 of FRR, 80 minutes minimum, inside and outside | 40 minutes minimum, inside and outside | Not required* |
FRR Requirements for Tall Mass Timber Structures (hours)
| Building Element | IV-A | IV-B | IV-C |
|---|---|---|---|
| Primary Frame | 3 | 2 | 2 |
| Exterior Bearing Walls | 3 | 2 | 2 |
| Interior Bearing Walls | 3 | 2 | 2 |
| Roof Construction | 1.5 | 1 | 1 |
| Primary Frame at Roof | 2 | 1 | 1 |
| Floor Construction | 2 | 2 | 2 |
Follow this link for an article that discusses how teams are utilizing the new code provisions to enhance the appearance of their tall mass timber structures with exposed timber framing.
Yes, the 2024 IBC includes new code provisions that allow timber ceiling exposure in Type IV-B construction up to 100%. The 2024 code has been adopted by several states in its entirety, while some states and cities have incorporated just the new Type IV-B provisions. See this page for the status of state adoptions. In jurisdictions where the new limits have not been adopted, several design teams have successfully used the 2024 code provisions to receive a variance allowing for increased timber exposure in Type IV-B construction. The 2024 IBC can be viewed at ICC Digital Codes.
For more information, contact WoodWorks: help@woodworks.org.
Looking for information on the tall wood provisions in the 2021 IBC? This paper summarizes the provisions as well as the background and research that supported their adoption.
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