Scott Breneman, PhD, SE, WoodWorks – Wood Products Council, Matt Timmers, SE, John A. Martin & Associates, and Dennis Richardson, PE, CBO, CASp, American Wood Council.

In January, the International Code Council (ICC) approved a set of proposals to allow tall wood buildings as part of the 2021 International Building Code (IBC). Based on these proposals, the 2021 IBC will include three new construction types—Type IV-A, IV-B and IV-C—allowing the use of mass timber or noncombustible materials. These new types are based on the previous Heavy Timber construction type (renamed Type IV-HT) but with additional fire-resistance ratings and levels of required noncombustible protection. The code will include provisions for up to 18 stories of Type IV-A construction for Business and Residential Occupancies.

This article is excerpted from a paper summarizing the proposals, including their background, technical research that supported their adoption, and resulting changes to the IBC and product-specific standards.

IBC Definition of Mass Timber

A relatively new category of wood products, mass timber can encompass well known and widely used products such as glue-laminated timber (glulam) framing as well as newer panelized products such as cross-laminated timber (CLT). The definition of mass timber adopted for the 2021 IBC is:

Structural elements of Type IV construction primarily of solid, built-up, panelized or engineered wood products that meet minimum cross section dimensions of Type IV construction.

In practice, mass timber as defined in the IBC has been an umbrella term that includes heavy timber elements, with heavy timber materials and sizes serving as the prerequisite to be considered mass timber. A subtle difference is that most mass timber utilized in Types IV-A, B and C have a minimum required Fire Resistance Rating (FRR) in addition to the intrinsic fire resistance due to the minimum prescriptive size requirements found in Type IV-HT.

Care should be taken to be clear when the minimum wood member size and detailing requirements of Type IV construction apply. When mass timber is used as one of the many current exceptions found in other construction types for “heavy timber” or “Type IV” construction, the heavy timber size and detailing requirements found in Section 2304.11 (IBC 2015 602.4) are applicable. Large format panelized wood products made from solid sawn laminations include: CLT, nail-laminated timber (NLT), dowel-laminated timber (DLT), and glue-laminated timber (GLT). Panelized products made from structural composite lumber include: LVL panels and a proprietary mass plywood panel (MPP) made from laminated plywood. While such products are sometimes called “mass timber,” they will only meet the 2021 IBC definition of mass timber when they meet the specific size and detailing requirements. When such products are used in Type III or Type V construction, as “any material permitted by this code” in Sections 602.4 and 602.5, the Type IV dimension and detailing requirements do not automatically apply.

Fire-Resistance Ratings: The table below contains the minimum FRR requirements for the new construction types found in IBC Table 601. Included for comparison are the requirements for Type I-A and I-B. The new Type IV-A has the same base FRR requirements as Type I-A. Type IV-B and IV-C have the same base FRR as Type I-B.

Noncombustible Protection: A requirement unique to the new construction types is noncombustible protection (for mass timber). This noncombustible material applied to the mass timber helps determine fire behavior by delaying the contribution of the mass timber structure in a fire and has an added benefit of increasing the fire-resistance rating of the mass timber element. The minimum contribution of noncombustible protection was set by the ICC Tall Wood Building Ad Hoc Committee to be no less than 2/3 of the Table 601 required FRR for mass timber elements. All mass timber in Type IV-A construction requires noncombustible protection. Most of the mass timber in Type IV-B requires noncombustible protection with limited exposed mass timber elements. Select mass timber elements in Type IV-C require noncombustible protection. A summary of the noncombustible protection required for building components in the different construction types is shown in the table below.

Three new types: Conceptually, Type IV-A is similar to type I-A with equal or greater FRR requirements and no exposed mass timber. Type IV-C is similar to Type IV-HT with almost all of the interior mass timber permitted to be exposed; however, most structural building components have a 2-hour FRR in addition to minimum heavy timber sizes. In surveying the existing and desired applications of mass timber in the U.S. and around the world, the committee determined that an intermediate construction type between IV-A and IV-C was needed, resulting in Type IV-B.

Type IV-B has similar FRR requirements as IV-C; however, all the mass timber requires noncombustible protection except the following:

  • Area of ceilings not greater than 20% of the floor area
  • Area of walls not greater than 40% of the floor area

A “sum of the ratios” type equation allows for a combination of the exposed ceilings and walls within a dwelling unit or fire area—e.g., area of exposed ceilings equal to 10% of the floor area plus area of exposed walls equal to 20% of the floor area. Unprotected areas of ceilings and walls also need be located with at least 15 feet horizontal separation. This requirement is to avoid re-radiation of heat between adjacent exposed mass timber elements which could extend the duration or severity of an uncontrolled fire. As an aside, the most frequent application of exposed mass timber in buildings to date is a portion of the ceiling alone.

Common Requirements: Additional requirements applicable to Type IV-A, IV-B and IV-C were included in the G108-18 proposal and include: No exposed mass timber in concealed spaces; concealed space permitted only with noncombustible protection as required for the interior mass timber • Exterior side of exterior walls protected by a non-combustible material—e.g., 5/8″ Type X gypsum sheathing • No combustible exterior wall coverings except for certain water-resistant barriers • No exposed mass timber on the inside and outside surfaces of exit enclosures and elevator hoistways in high-rise buildings (occupied floor > 75 feet from lowest fire department access) • Noncombustible construction only for exit enclosures and elevator hoistways greater than 12 stories or 180 feet.

Allowable Building Sizes

As the ICC Tall Wood Building Ad Hoc Committee developed and tested requirements for the new construction types, the allowable heights and areas were developed with the following performance objectives as the basis:

  • No collapse under reasonable scenarios of complete burnout of fuel without automatic sprinkler protection being considered
  • No unusually high radiation exposure from the subject building to adjoining properties to present a risk of ignition under reasonably severe fire scenarios
  • No unusual response from typical radiation exposure from adjacent properties to present a risk of ignition of the subject building under reasonably severe fire scenarios
  • No unusual fire department access issues
  • Egress systems designed to protect building occupants during the design escape time, plus a factor of safety
  • Highly reliable fire suppression systems to reduce the risk of failure during reasonably expected fire scenarios; the degree of reliability should be proportional to evacuation time (building height) and risk of collapse

The comprehensive package of approved changes met these objectives, and allowable heights and areas for select occupancies in the 2021 IBC are shown in the table below.

To help visualize the building size associated with these tabular limits, the figure below shows representative buildings of equal area per floor with the maximum building sizes for a Business occupancy. As Business occupancies have the largest allowable areas and share the tallest height limits with other occupancies, these are the largest building sizes permitted by the new construction types, excluding minor additional area increases allowed by the frontage increase of IBC 506.3.

Note that in the current IBC, the use of NFPA 13 sprinklers is mandatory for all high-rise buildings defined with an occupied floor more than 75 feet above the lowest level of fire department access.  For existing construction types, the use of NFPA 13 sprinklers typically provides for a one-story and 20-foot increase in the allowable building height but a substantial increase in allowable area. However, the new construction types are highly conservative when allowable area is compared with common A, B and R occupancy groups where unlimited area is allowed for existing Type I buildings


The proposals from the ICC Tall Wood Building Ad Hoc Committee led to a comprehensive set of code changes developed using a rational performance-based approach to establish new size allowances for mass timber buildings in the IBC. The three new construction types have been crafted to support a variety of exposed and protected mass timber design systems which have been validated through fire testing.

The American Wood Council’s 2018 National Design Specification® (NDS®) for Wood Construction and Technical Report 10 have been published and together include updated provisions for structural fire-resistance calculations and protection of connections, as well as a summary of prior and recent fire testing providing substantiation for the design criteria.

For more information, including a summary of the proposals, requirements for the new construction types, fire protection features, results of fire testing and other research, and an update to the CLT product standard, the complete paper can be found at

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