Finished Interior of a Residential Kitchen — Carbon12 / Kaiser+Path / photo Andrew Pogue

Over the past 10 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 International Building Code (IBC) includes 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 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.

Ascent building at sunset in Autumn by Korb + Associates Architects — Ascent / Korb + Associates Architects

Notable Tall Timber Buildings

Visit the WoodWorks Information 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 projects that are seven stories and higher.

Notable projects that go beyond the prescriptive limits of previous versions of the IBC include:

Built or Under Construction:

1510 Webster – Oakland, CA – 19 stories, 16 of which are mass timber.
Carbon12 – Portland, OR – Eight stories of mass timber.
INTRO – Cleveland, OH – Eight stories of mass timber over a concrete podium.
Ascent – Milwaukee, WI – Nineteen stories of mass timber over six stories of concrete.
80M – Washington, DC – Two new mass timber floors + habitable penthouse over an existing seven-story structure.
Apex Clean Energy – Charlottesville, VA – Six stories of mass timber over two stories of concrete.
Baker’s Place – Madison, WI – 12 stories of mass timber over three stories of concrete podium.
11 E Lenox – Boston, MA – Seven stories of mass timber.
Heartwood – Seattle, WA – Eight stories of mass timber.
Minnesota Places – Portland, OR – Seven stories of mass timber over a concrete podium.
TimberView – Portland, OR – Eight stories of mass timber.
Bunker Hill Housing Redevelopment Building M – Boston, MA – 6 stories of mass timber

Tall Timber FAQs

1. What is the current status of tall mass timber buildings in the building code?

Follow this link for the status of the 2021 IBC and jurisdictions that have adopted the tall wood code provisions.

2. What are the height limits for the new construction types?

3. What are the timber exposure limits for Types IV-A, IV-B, IV-C and IV-HT construction

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*

*Not required by construction type. Other code requirements may apply.
5/8″ Type X gypsum = 40 minutes

4. What are the fire-resistance rating requirements for tall mass timber?

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

Source: 2021 IBC Table 601

5. How are design teams leveraging tall mass timber code provisions to maximize the amount of timber exposure?

Follow this link for an article that discusses how teams are utilizing the new code provisions to enhance the appearance of their tall mas timber structures with exposed timber framing.

6. I’ve heard that the 2024 IBC will allow 100% timber ceiling exposure in type IV-B, up to 12 stories tall. Is that code language finalized?

Yes, the 2024 IBC will include new code changes, which have been approved and will be incorporated, which allow timber ceiling exposure in Type IV-B construction up to 100%. The new code language as it will read in the 2024 IBC is available here. Several jurisdictions such as the City of Denver, City of Dallas, State of Oregon and State of Washington are already in the process of incorporating these new timber exposure limits in their building codes, and several design teams are looking to utilize the new limits in project-specific discussions with their local building officials. Reach out to your local WoodWorks Regional Director to see how projects in your area can approach these design topics.

Technical Design Guidance from WoodWorks

Solution Papers Tall Wood Buildings in the 2021 IBC – Up to 18 Stories of Mass Timber

Looking for information on the tall wood provisions in the 2021 International Building Code? This paper summarizes the provisions as well as the background and research that supported their adoption.

View Resource

Inventory of Acoustically-Tested Mass Timber Assemblies

Find an acoustically-tested assembly that’s right for your project. Browse assemblies tested in the U.S., including STC and ICC ratings and test reports.

Designing Mass Timber Floors for Acoustics

Inventory of Fire-Tested Mass Timber Assemblies & Penetrations

Browse this inventory of mass timber assemblies and penetration fire stopping systems in mass timber assemblies that have been tested for fire resistance.

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Tall Mass Timber