What Determines a Building’s Construction Type?

A building’s construction type under the International Building Code is determined by two properties of its structural materials — whether they are combustible and how many hours they resist fire — and then forced up or down by what the project needs to do: how tall it will stand, how large each floor will be, and what activity will happen inside. Those two material properties sort every building into one of five types (I through V), each with an A and B subtype. The planned height, floor area, and occupancy group then decide which of those types the project is actually allowed to use. What determines a building’s construction type, in other words, is the intersection of material behavior and intended use, and getting that intersection right at the start of design is where experienced architects earn their fees.

Material Combustibility

The first factor is whether the structural materials feed a fire. The IBC draws a hard line here. Reinforced concrete and structural steel are classified as noncombustible because they will not ignite when exposed to fire or extreme heat. Traditional wood framing is combustible, meaning it adds fuel that helps fire spread through a structure.¹

That single distinction shapes everything that follows. Types I and II require noncombustible materials throughout the structure. Type III requires noncombustible exterior walls but allows combustible interiors. Types IV and V permit wood and other combustible materials under varying conditions. Every component gets evaluated: the structural frame, exterior walls, interior partitions, floor assemblies, and roof construction. Noncombustible materials cost more upfront but shrink insurance premiums and unlock taller, larger designs. Combustible materials keep initial costs down but restrict what can be built and where.

The line is not absolute. The IBC carves out specific situations where fire-retardant-treated wood can be used inside Type I and Type II buildings, including nonbearing interior partitions with a fire-resistance rating of two hours or less, nonbearing exterior walls where fire-rated construction is not required, and roof framing such as trusses, girders, and decking.² For Type IA buildings taller than two stories, treated wood in the roof is only permitted when the distance from the highest occupied floor to the roof is at least 20 feet.

Fire-Resistance Rating in Hours

Beyond whether a material burns, the IBC cares about how long structural components keep doing their job during a fire. Fire-resistance ratings are expressed in hours and represent the tested duration a building element can contain flames, support its load, or both. IBC Table 601 assigns specific hourly ratings to each structural component for every construction type and subtype.

A Type IA building, for example, requires a three-hour fire-resistance rating for its structural frame and a two-hour rating for floor construction. A Type IB building drops those to two hours and two hours. Type IIA only needs one hour on the structural frame. By Type IIB or VB, the requirement falls to zero, meaning no rated fire resistance is required at all.³ This sliding scale is what makes the A-versus-B subtype distinction so important.

The A and B Subtype Split

Each of the five construction types splits into an A and a B subtype. The A subtype requires fire-rated construction throughout; the B subtype either requires no fire-resistance rating or a significantly reduced one. Type IIA requires at least a one-hour rating on structural elements, while Type IIB requires none. The same pattern applies to Types III and V: the A version demands one-hour-rated assemblies, the B version does not.⁴ Type I is the only category where both subtypes require rated construction, with Type IA at three hours and Type IB at two hours for the structural frame.³ That subtype choice ripples through every cost and design decision on a project.

Ratings also have to be maintained in practice. A fire-rated wall or floor assembly loses its value the moment an unprotected hole is cut through it. Any mechanical, electrical, or plumbing penetration must be sealed with a firestop system rated equal to or greater than the assembly, and joints that accommodate building movement must be filled with a tested fire-resistant joint system matching the adjacent rating. Inspectors check these details closely because a single unprotected penetration can undermine an entire fire-rated barrier.

The Five Construction Types

IBC Section 602 combines the combustibility and fire-resistance factors into the five construction types.⁵ The type assigned to a building governs what materials can be used, how fire-resistant each component must be, and ultimately how tall and large it can grow.

Type I (noncombustible, high fire resistance). All structural materials must be noncombustible, typically steel and reinforced concrete. The structural frame in Type IA requires a three-hour rating, making this the standard for high-rises, hospitals, and other large buildings where evacuation takes longer and collapse would be catastrophic. Type IB reduces the structural frame requirement to two hours.¹³
Type II (noncombustible, reduced fire resistance). Materials must still be noncombustible, but the required ratings are lower. Type IIA needs a one-hour rating on structural elements; Type IIB has no hourly rating requirement at all. This makes Type II common for low-rise commercial buildings like strip malls and warehouses where steel framing is used without additional fireproofing.⁴
Type III (noncombustible exterior, combustible interior). Exterior walls must be noncombustible, but interior structural elements can be any code-permitted material, including wood framing. The IIIA subtype requires one-hour-rated interiors; IIIB does not. This hybrid approach is popular for mid-rise apartments and mixed-use buildings in urban settings where fire-resistant exterior walls protect neighboring structures while wood interiors keep costs manageable.
Type IV (mass timber). Traditionally called heavy timber construction, Type IV relies on large wood members that char slowly on the surface during a fire, maintaining structural strength underneath the char layer. The 2021 IBC expanded this category into four subtypes. Type IV-A achieves the highest fire resistance (up to three hours) using mass timber protected by noncombustible coverings and can reach up to 18 stories. Type IV-B allows up to 12 stories, Type IV-C up to 9 stories, with progressively less noncombustible protection required. Type IV-HT retains the traditional heavy timber approach with exposed wood members.⁶
Type V (any permitted material). Both exterior and interior structure can be built from any material the IBC permits, including light wood framing. Type VA requires one-hour fire-rated assemblies throughout; Type VB has no hourly rating requirement. Single-family homes and small apartment buildings almost always fall into this category because it offers the most design flexibility at the lowest cost.¹

Each type represents a trade-off. Moving from Type V toward Type I increases material and labor costs but unlocks larger and taller envelopes. Developers spend serious time early in a project running the math on whether the revenue from additional floors justifies stepping up to a more restrictive type.

Height, Floor Area, and Occupancy Push the Type Up

Material behavior is only half the equation. The IBC does not let a designer pick a construction type in isolation. Chapter 5 sets maximum allowable heights in feet, number of stories, and floor area per story for each construction type, cross-referenced against the building’s occupancy group.⁷ If a project exceeds the limits for a given type, the choice is to shrink the building or upgrade to a more restrictive construction type.

The range is dramatic. A Type VB building used for business occupancy might be limited to two stories and 40 feet without sprinklers. A Type IIIA building with the same occupancy could reach five stories and 65 feet. Step up to Type IV-A with sprinklers, and the same occupancy group can go to 18 stories and 270 feet. Taller buildings demand more fire resistance because fires on upper floors are harder to reach, evacuation takes longer, and structural failure affects more people.

Occupancy Group

What happens inside a building is just as decisive as what it is built from. The IBC organizes building uses into ten occupancy groups: Assembly (A), Business (B), Educational (E), Factory (F), High-Hazard (H), Institutional (I), Mercantile (M), Residential (R), Storage (S), and Utility (U). Several groups have subgroups that further refine the risk level. A warehouse storing inert goods presents far less danger than a chemical plant or a crowded nightclub, and the code reflects that.

The occupancy group interacts directly with construction type through the Chapter 5 tables. High-hazard occupancies face the strictest limits, which often requires Type I or II construction even for smaller buildings. Assembly spaces need wider egress and higher fire-resistance ratings for structural elements because large crowds are slow to evacuate. Residential buildings prioritize compartmentalization between dwelling units to keep a fire in one apartment from spreading to the next. Getting the occupancy classification wrong at the design stage can force expensive redesigns when the building department catches the mismatch during plan review.

Mixed-Use Buildings

When a building contains more than one occupancy, the IBC provides three handling methods, and the choice affects which construction type the project can live with. Accessory occupancies are small secondary uses occupying no more than 10 percent of any story’s floor area and need no fire-rated separation from the main occupancy.⁸ Nonseparated occupancies allow different groups to coexist without fire barriers, but the entire building must comply with the most restrictive height, area, and fire protection requirements of any occupancy present. Separated occupancies use fire barriers to compartmentalize each group, and each is then evaluated independently for height and area, which often permits a larger or taller building overall. High-hazard occupancies in Groups H-2 through H-5 generally cannot use the nonseparated approach.

Sprinklers and Frontage Shift the Limits

The base floor area allowed by the IBC tables is not a hard ceiling. Section 506 provides a formula for increasing allowable area based on two factors: automatic sprinkler protection and frontage on public streets or open spaces.⁹ The formula is Aa = At + (At × If) + (At × Is), where At is the base tabular area, If is the frontage increase factor, and Is is the sprinkler increase factor.

For the frontage bonus, the building must have more than 25 percent of its perimeter facing a public way or open space at least 20 feet wide. More qualifying perimeter and wider open space mean a larger bonus. An NFPA 13 sprinkler system can increase the allowable floor area by up to 200 percent for multistory buildings and 300 percent for single-story buildings. Sprinklers also allow an additional 20 feet of building height and one extra story beyond the base tables.

Sprinklers do more than lift height and area caps. Throughout the IBC, installing an NFPA 13 system unlocks reductions in fire-resistance requirements that lower construction costs. Corridor fire-resistance ratings can be eliminated entirely. Fire partitions between dwelling or sleeping units can drop from one hour to half an hour. Fire barriers separating different occupancy groups can often be reduced by one hour. Vertical shaft enclosures in high-rise buildings can drop from a two-hour to a one-hour rating when sprinklers are installed at the top of the shaft and at alternate floor levels. The cumulative savings often exceed the cost of the sprinkler system itself, which is why sprinkler installation is one of the most common voluntary design decisions even in buildings small enough to avoid the sprinkler mandate. The catch: once installed, the system becomes a permanent life-safety obligation that must be maintained, inspected, and kept operational for the life of the building.

Why the Type Can Change Later

Construction type is not locked in forever. When an existing building changes its use, the International Existing Building Code requires an evaluation of whether the new occupancy triggers stricter construction standards. Converting a warehouse (Storage occupancy) into a restaurant (Assembly occupancy), for example, almost certainly means the building must now meet tougher fire protection, sprinkler, and possibly construction type requirements that the original design never contemplated.

The IEBC uses a tiered system for renovations based on how much of the building is affected. Alterations touching up to 50 percent of a floor’s area trigger moderate upgrades like smoketight stair enclosures and improved interior finishes in corridors. Once the work exceeds 50 percent of the floor area, the entire floor must generally meet current code standards for means of egress.¹⁰ When the change of use moves to a higher-hazard occupancy category, the building typically must comply with the height and area limits in IBC Chapter 5, which can force an upgrade to a more restrictive construction type or sprinklers throughout.

Developers who buy existing buildings for adaptive reuse sometimes underestimate these triggers. The purchase price may be attractive, but bringing a Type VB wood-framed warehouse up to Type IIA noncombustible standards for a new assembly use can erase the savings. Getting a code consultant involved before closing the deal is worth every dollar of their fee.

1
ICC NTA. IBC Building Construction Types for Combustibility
2
UpCodes. Combustible Material in Types I and II Construction
3
Corada. IBC Table 601 – Fire-Resistance Rating Requirements for Building Elements
4
UpCodes. Types of Construction
5
UpCodes. Construction Classification
6
ICC. 2021 Code Conforming Wood Design and the IBC
7
ICC. IBC 2021 Chapter 5 – General Building Heights and Areas
8
ICC. General Building Heights and Areas
9
UpCodes. Section 506 Building Area Modifications
10
UpCodes. IEBC 2021 Chapter 8 – Alterations Level 2