Type A Construction: IBC Fire Ratings, Height, and Area

In the International Building Code, Type A construction is the stricter fire-resistance subcategory within each of the code’s main construction types. The IBC sorts every building into one of five types (I through V), and most of those types split into an “A” version and a “B” version based on how many hours the structural frame, bearing walls, floors, and roof must resist fire. The “A” subcategory always carries the higher hourly ratings, and in exchange the code allows taller buildings, larger floor plates, and higher occupant loads. Type I-A sits at the top: a 3-hour rating on the primary structural frame and bearing walls, and no code-imposed limit on height or area for most occupancies.¹ That is why Type I-A dominates high-rise office towers, large hospitals, and dense urban residential projects.

Where Type A Sits in the IBC’s Five Construction Types

Every building the IBC regulates falls into one of five construction types, defined by the materials allowed and the fire-resistance ratings required for the building’s main elements. In every type that has subcategories, the “A” version holds elements to stricter hourly ratings than the “B” version.²

Types I and II require that all elements listed in Table 601 be noncombustible. Type I-A has the highest ratings in the entire code; Type II-B has the lowest among noncombustible types, with several elements requiring zero hours.
Type III requires noncombustible exterior walls, but interior framing can be any code-compliant material, including wood.
Type IV was historically heavy timber, and the 2021 IBC expanded it to add mass timber subcategories IV-A, IV-B, IV-C, and IV-HT.
Type V has no material restrictions and permits conventional wood framing throughout.

So “Type A” is not a single building type. It is the stricter half of whatever type you are in. The practical effect is consistent across the code: moving from the B subcategory to the A subcategory of the same type adds fire-resistance hours and, in return, loosens the limits on how big the building can be.

Fire-Resistance Ratings That Define Type A

Table 601 of the IBC sets the hourly ratings each building element must achieve. The clearest way to see what Type A means in practice is to put the I-A and I-B ratings side by side.

Type I-A

Primary structural frame: 3 hours
Exterior bearing walls: 3 hours
Interior bearing walls: 3 hours
Floor construction: 2 hours
Roof construction: 1½ hours

Type I-B

Primary structural frame: 2 hours
Exterior bearing walls: 2 hours
Interior bearing walls: 2 hours
Floor construction: 2 hours
Roof construction: 1 hour

Both subcategories hold floors to the same 2-hour standard, since a floor failure lets fire jump between stories faster than people can evacuate. Type I-A adds an extra hour on the frame, bearing walls, and roof. That additional hour is the dividing line between a building with height limits and one without.¹

Table 601 does give some relief on roofs. Where every part of the roof construction sits 20 feet or more above the floor below, fire protection on roof framing and decking is not required in most occupancy groups, and fire-retardant-treated wood is permitted for those unprotected members. Heavy timber is allowed for roof construction wherever the required rating is one hour or less.³

Hourly ratings themselves come from standardized fire tests, most commonly ASTM E119, in which an assembly is heated under a prescribed temperature curve and, for load-bearing elements, kept under load. Assemblies that survive without structural failure or excessive heat transfer earn a rating like “1-hour” or “2-hour” that corresponds directly to Table 601.⁴ Builders must follow the certified design listings exactly; even small deviations can void the rating, and building officials verify compliance during construction inspections.

Noncombustible Materials and How Steel Gets Protected

In Types I and II, every building element listed in Table 601 must be made from noncombustible materials.³ In practice that means reinforced concrete, masonry, or structural steel for the frame, walls, and floor systems. None of those materials ignite or add fuel to a fire, which is the central idea: the building itself should not burn.

Steel is noncombustible but loses strength rapidly at high temperatures. An unprotected steel column can begin to fail well before the 3-hour mark that Type I-A demands, so steel members almost always need secondary protection. Two approaches dominate.

Spray-applied fire-resistive material (SFRM) is a thick cementitious coating made from gypsum, vermiculite, or mineral fiber. It insulates the steel from heat and is relatively inexpensive in raw material cost, but it leaves a rough finish best hidden behind ceilings or in concealed spaces. SFRM can absorb moisture, crack, flake, or be damaged by vibration.

Intumescent coatings look like ordinary paint under normal conditions. When exposed to heat they swell to roughly 10 to 50 times their original thickness, forming an insulating char layer. The smooth finish makes them the standard choice when structural steel will be visible in the finished space, such as in airports, museums, or convention centers. They resist moisture and flaking better than SFRM, and shop application before delivery can shorten the construction schedule.

The choice usually comes down to aesthetics and environment. Concealed structure leans toward SFRM; exposed beams and columns use intumescent coatings despite the higher cost per square foot.

Combustible Materials That Are Still Allowed

The noncombustible rule for Types I and II is not absolute. Section 603 of the IBC lists exceptions, and first-time builders on Type I-A projects often get this wrong by assuming every single component has to be steel, concrete, or masonry.

Fire-retardant-treated wood is permitted for nonbearing partitions with a required fire-resistance rating of two hours or less, nonbearing exterior walls where fire-rated construction isn’t required, and roof construction including trusses, framing, and decking.⁵ One important limit: in Type I-A buildings taller than two stories, fire-retardant-treated wood is not allowed in roof construction when the vertical distance from the top occupied floor to the roof is less than 20 feet.

Beyond fire-retardant-treated wood, the exceptions cover thermal and acoustical insulation (flame spread index of 25 or less), foam plastics that comply with Chapter 26, interior wall and ceiling finishes, floor coverings, millwork such as doors and window frames, roof coverings with an A, B, or C classification, trim, and blocking for handrails and cabinets. The code also permits combustible exterior wall coverings, balconies, and the sprayed fire-resistive materials themselves, which often contain organic binders.² The structural skeleton must be noncombustible; many secondary components and finishes can use combustible materials within defined limits.

Why Developers Build to Type A: Height and Area

Under IBC Tables 504.3, 504.4, and 506.2, Type I-A construction is designated “UL” (unlimited) for allowable height, number of stories, and floor area in nearly every occupancy classification.⁶ An office tower, apartment building, or hospital built to Type I-A faces no code-imposed cap on how tall it can be or how much floor space each story can hold.

The contrast with Type I-B is sharp. Most occupancy groups in Type I-B are capped at 11 stories, and several sit far lower. Group A-1 assembly occupancies like large theaters drop to 5 stories, Group I-2 hospitals are limited to 4 stories, and certain hazardous occupancies are restricted to a single story regardless of construction type.⁷

That is why developers accept the higher cost of Type I-A on urban high-rises. The extra hour of protection on the frame and bearing walls, compared with Type I-B, effectively removes the height ceiling. For a 40-story residential tower on expensive land, there is no cheaper path to legal compliance than building to Type I-A from the start.

Type IV-A: The Mass Timber Equivalent

The 2021 IBC added three new subcategories within Type IV construction (IV-A, IV-B, and IV-C) alongside the traditional heavy timber designation IV-HT. Type IV-A is the most heavily protected of these and matches Type I-A’s fire-resistance ratings: 3 hours on the structural frame and bearing walls, 2 hours on floors, and 1½ hours on roofs.³

The difference is the material. Type IV-A achieves those ratings using mass timber such as cross-laminated timber and glued-laminated timber, rather than relying solely on steel and concrete. Every mass timber element must be shielded with noncombustible protection applied directly to the wood surface. The noncombustible layer on interior faces must provide at least 80 minutes of protection, exterior faces of mass timber walls require at least 40 minutes, and floor assemblies need at least one inch of noncombustible material above the mass timber.³ The ratings are identical to Type I-A, but the encapsulation requirements mean the mass timber is never exposed in the finished building.

Keeping the Rating After the Building Opens

Meeting Type A requirements during construction is only half the obligation. The International Fire Code requires ongoing maintenance of every fire-resistance-rated element after occupancy. Section 701 covers structural fire resistance, fire-rated separations, and construction designed to resist smoke passage.⁸

Protected elements include structural members, exterior walls, fire walls and fire barriers, horizontal assemblies, and shaft enclosures. Owners cannot hang merchandise, signage, or decorative materials from acoustical ceiling systems that form part of a fire-rated horizontal assembly, since doing so can compromise the rating. Spray-applied fireproofing that gets scraped, chipped, or water-damaged during tenant buildouts needs prompt repair back to the originally tested thickness.⁸

This is where many buildings quietly fall out of compliance. A renovation contractor who strips fireproofing from a beam to run new ductwork and doesn’t restore it has downgraded the building’s actual fire resistance, regardless of what the certificate of occupancy says. Fire marshals can issue violations during routine inspections, and insurers sometimes condition coverage on proof that fire-rated assemblies remain intact.

1
International Code Council. 2021 International Building Code – Table 601 Fire-Resistance Rating Requirements for Building Elements
2
International Code Council. 2018 International Building Code – Chapter 6 Types of Construction
3
International Code Council. 2021 International Building Code – Chapter 6 Types of Construction
4
ICC Evaluation Service. ASTM E119 – Fire Tests of Building Construction and Materials
5
International Code Council. International Building Code Interpretation 63-13
6
International Code Council. 2021 International Building Code – Chapter 5 General Building Heights and Areas
7
International Code Council. 2018 International Building Code – Section 504.4 Number of Stories
8
International Code Council. 2021 International Fire Code – Chapter 7 Fire and Smoke Protection Features