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How Mass Timber Continues to Grow: Understanding the Significance of the ICC’s Decision to Add Subtypes of Category IV Construction

How Mass Timber Continues to Grow: Understanding the Significance of the ICC’s Decision to Add Subtypes of Category IV Construction

There’s no question that heavy and mass timber construction is (literally) on the rise. The public’s desire for sustainable, natural construction elements is as strong as it’s ever been, as is the passion for architects to build with beautiful, durable, and efficient timber material. In anticipation of the demand for more and taller timber buildings, the International Code Council (ICC) updated its building and code requirements in 2021 to ensure the fire protection safety of occupants and first responders as well as the overall performance of these structures.

Construction Codes

From its beginnings in 1927 to its most recent update in 2021, the International Building Code (IBC) has classified five construction types by their fire resistance, with noncombustible materials in Types I-III, timber construction in Type IV, and wood framing in Type V.

Due to the expanding market for mass timber structures, the IBC update of 2021 added subtypes to Type IV, dividing it into three new categories of fire resistance:

  • Type IV-A is for buildings with all structural elements, internal and external, completely protected with noncombustible protection, such as Type X gypsum board. This is the highest fire resistance level in this type.
  • Type IV-B is for buildings with some interior structural elements exposed, but with all concealed spaces and shafts fully protected with noncombustible protection.
  • Type IV-C is for buildings with most structural elements left unprotected, relying instead on the inherent fire-resistant nature of the mass timber itself.

The Mass Timber Effect

Mass timber, also known as engineered wood, glulam (glued-laminated timber), or CLT (cross-laminated timber), is a composite of multiple timber layers held together with adhesives to create a strong, relatively light material that’s well-suited for modern construction. Mass timber can form long spans allowing for large open spaces and vaulted interiors, and it has a remarkable ability to safely flex in high-wind and seismic scenarios.

Maybe most surprisingly, mass timber is naturally quite fire-resistant. When exposed to flame — even over long periods — only the outer layer chars, protecting the structurally sound interior layers.

Building Boom

The ICC’s recognition of the need for subtypes within the construction code affirms that modern mass timber is here to stay, and that it’s a safe, viable alternative to traditional concrete and steel.

Additional factors to support the increased use of mass timber include:

  • A small fraction of carbon pollution is produced in production versus concrete and steel.
  • Timber naturally stores carbon dioxide even after it is used for construction.
  • It has natural insulating qualities that concrete and steel can’t match.
  • Timber is a sustainable resource from responsibly harvested forests.
  • The components can be prefabricated in an off-site facility.
  • The lighter weight of the material brings efficiencies in shipping and construction.

So, how high can mass timber go? The IBC allows for tremendous heights to be built, reaching up to 18 stories for Type IV-A, 12 stories for Type IV-B, and 9 stories for Type IV-C. Tall mass timber structures have already been built all over the world, from Norway to Australia and Singapore to Milwaukee.

Safety First

The safety of occupants is ultimately the most important aspect of any structure. With the ICC’s updated requirements, they have laid the foundation for mass timber construction to follow strict rules that ensure fire safety while also satisfying the public’s demand for safe structures.

At Mid-Atlantic Timberframes, we approach every project with safety first and foremost in our minds. Our use of mass timber construction and strict adherence to ICC standards guarantee the strength, safety, and integrity of the final product will last for generations to come. Contact us to start a conversation today.

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