Wildfire “season” has become nearly year-round in the western U.S., with wildfire risk expanding eastward. A 2026 U.S. Forest Service study projects that wildfire occurrence could increase 56% nationally by 2060, with the largest percentage increases in the eastern U.S., creating increased risk for commercial property owners.
When planning new real estate construction, it’s helpful to understand common construction material types. Beyond material cost and long-term maintenance, material choices affect insurance underwriting in the event of a fire. A little knowledge and advanced planning will help you anticipate and manage insurance costs—now and in the future.
Insurance underwriters rely on the insurance tools and analytics provided by the Insurance Services Office, Inc. (ISO). For commercial construction, ISO grades buildings and assigns a construction “class” based on the impact of a fire loss. Construction classes consider materials used, the percentage of each type of material used and the amount of damage a building is likely to sustain if exposed to a fire.
ISO construction classes rank from 1 to 6 based on combustibility or fire resistance. Class 1 is the least fire-resistive; Class 6 is the most fire-resistive. The following is a high-level description of the six classes of commercial real estate construction established by ISO to rate and insure commercial property.
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Class 1: Wood frame
ISO defines this class as structures with exterior walls, floors and roofs made of combustible construction and buildings with outer walls of non-combustible and/or slow-burning construction with combustible floors and roofs (including brick veneer or metal-clad exterior used for appearance versus support). While frame construction is economical and easy to erect and alter, it is most susceptible to fire damage and allows fire to spread quickly.
Class 2: Jointed masonry
This class includes buildings with exterior walls of masonry or a fire-resistive construction rating of not less than one hour, and flammable or combustible flooring and roofs, including heavy timber or mill construction. (Fire-resistive time ratings refer to the number of hours a structure can withstand a fire simulation test.) Construction material types include brick, tile, stone and concrete. This construction class is harder to ignite but still highly susceptible to fire damage.
Class 3: Non-combustible
As the name suggests, this class describes buildings with outer walls, floors and roofs made of non-combustible or slow-burning materials. Like Class 1 wood frames, non-combustible construction is economical and easy to erect. Conversely, this construction type typically contains steel, which loses strength at high temperatures, creating unstable situations and uses slow-burning materials that serve as fuel for the fire.
Class 4: Masonry non-combustible
ISO defines this class of buildings as having exterior walls constructed of masonry—not less than four inches thick—or buildings having exterior walls with a fire-resistive rating of not less than one hour. Also included are slow-burning or non-combustible flooring and roofs, irrespective of the insulation type used for the roof’s surface. Masonry non-combustible is highly stable and less likely to collapse during a fire. Like Class 3 non-combustible, unprotected steel becomes less stable and more damageable at high temperatures, and slow-burning materials fuel the fire.
Class 5: Modified fire-resistive
Class 5 is building construction that uses fire-resistive materials such as masonry and protected steel materials at least 4 inches thick. It has a fire-resistive rating of more than one hour but less than two hours. Many high- and mid-rise buildings fall into this class. Non-combustible materials allow greater height than other classes and use load-bearing assemblies that resist fire damage. These materials are more expensive to construct and repair.
Class 6: Fire-resistive
The highest class of ISO-rated construction types includes solid or hollow masonry and reinforced concrete with varying thicknesses and fire-resistance ratings of at least two hours. Typical Class 6 structures include high-rise buildings and parking garages. While Class 6 is considered the safest, it is expensive to construct and repair and may give occupants a false sense of security, as fire risk exists with any commercial construction.
Commercial building construction FAQ
- Why do building codes classify commercial construction into different types?
Building codes classify commercial construction to mitigate fire risk and specify which materials and assemblies prevent fire spread or building collapse. Insurance companies use building classifications to determine their property-loss exposure. - Which commercial construction type is best for high-rise buildings?
Non-combustible structural materials—typically reinforced concrete and/or protected steel—with substantial fire-resistance ratings are especially suitable for high-rise construction because they resist fire for extended periods, reduce the likelihood of collapse and limit fire and heat transfer between floors. While more fire-resistant, these materials can be more expensive and add structural weight (see next question). - How do commercial construction types impact project costs and timelines?
Commercial construction type can significantly affect both cost and schedule because it determines the structural system, materials, fire-resistance requirements, foundation requirements, construction methods and the availability of labor and materials. Together, these factors affect the structure’s real estate economics (e.g., cost per square foot). - What are the most commonly used commercial construction types?
While some mix of fire-resistive, masonry and non-combustible materials is most common for new construction exteriors, the mix varies substantially by building height, occupancy and location. - How do you determine the right construction type for a project?
The right commercial construction type balances building-code requirements with the project’s intended use, size, location, cost, schedule and long-term performance goals. In many cases, the building code substantially narrows the choices before the developer can compare economics. Insurance is an increasingly important consideration. In areas subject to severe weather events, it may be useful to involve a risk consultant before deciding on construction materials.
GNP’s commercial real estate development team recommends engaging the architect and an insurer early in a project to assess the financial impact of construction materials. While an architect will adhere to building codes, zoning and structural requirements, there is some flexibility in the choice of building materials. Understanding the likely impact on insurance underwriting, construction and operating costs helps inform decision-making in commercial real estate construction. Contact us today to learn more about commercial property construction.
Updated September 2026: This article has been revised and expanded with updated information about wildfire risk, commercial construction types and factors to consider when choosing building materials.
