Building Construction Types: NFPA 220 and IBC Classifications

Under the International Building Code, every structure falls into one of five building construction types, numbered I through V, that classify the building by whether its structural materials are combustible and how many hours of fire resistance each major component must provide. Type I is the most fire-resistant, requiring non-combustible materials and the highest hourly ratings. Type V is the least restrictive and covers standard wood-frame housing. NFPA 220 runs a parallel classification that uses a three-digit code to spell out the required hours of protection for exterior bearing walls, the structural frame, and floor construction.¹

Reading the NFPA 220 Three-Digit Code

NFPA 220 assigns each construction type a three-digit number in parentheses. The first digit is the fire-resistance rating in hours for exterior bearing walls. The second is the rating for the structural frame, which includes columns, beams, girders, and trusses that carry loads from more than one floor. The third is the rating for floor construction.¹ Type I (442) means 4-hour exterior bearing walls, a 4-hour frame, and 2-hour floors. Type II (000) means no fire-resistance rating is required for any of those elements.

The full menu of sub-types: Type I comes in 442 and 332. Type II comes in 222, 111, and 000. Type III comes in 211 and 200. Type V comes in 111 and 000.² Type IV carries its own designation, 2HH, where “H” stands for heavy timber rather than an hourly number. The IBC’s lettering scheme (I-A, I-B, II-A, II-B, and so on) maps directly onto these NFPA sub-types.

Type I: Fire-Resistive Construction

Type I is the most fire-resistant classification. The IBC requires that all building elements in Type I and Type II structures be made from non-combustible materials, with limited exceptions.³ In practice, reinforced concrete and protected steel form the structural skeleton.

Under IBC Table 601, Type I-A buildings require a 3-hour fire-resistance rating for the primary structural frame and both exterior and interior bearing walls, with a 2-hour rating for floor construction. Type I-B drops those numbers to 2 hours for the frame and bearing walls.³ High-rise residential towers, hospitals, and large commercial centers typically require Type I because their occupants need more time to evacuate. The trade-off is higher material and labor cost, and the payoff is a structural frame designed to remain standing throughout a fully developed fire.

Type II: Non-Combustible Construction

Type II shares the same basic rule as Type I: structural elements must be non-combustible. The difference is how much fire protection those elements need. Type II-A requires a 1-hour rating for the structural frame and floor construction. Type II-B requires none at all.³ Unprotected steel frames, metal roof decks, and concrete masonry walls are the workhorses of this category.

Steel itself will not ignite, but unprotected steel loses structural strength rapidly at high temperatures. Type II buildings tend to be shorter and designed for quick evacuation. Schools, warehouses, and strip malls commonly fall here, getting the durability of non-combustible framing without the cost of wrapping every beam in fireproofing. Type II-B (000) is the most economical non-combustible option, common in single-story retail and light industrial buildings where the exit path is short.

Type III: Ordinary Construction

Type III is called “ordinary construction” because it mixes non-combustible and combustible materials. The IBC requires exterior walls to be non-combustible, but interior building elements can be any code-permitted material, including wood framing.³ Fire-retardant-treated wood framing is also allowed within exterior wall assemblies rated at 2 hours or less.

The idea is straightforward: brick, stone, or concrete-block exterior walls act as firewalls between adjacent buildings, while wood-framed interiors keep construction costs down and floor layouts flexible. Under NFPA 220, the sub-types are 211 and 200.² Type III-A (211) requires a 1-hour rating for the structural frame and interior bearing walls. Type III-B (200) requires a 2-hour rating only for exterior bearing walls, with no hourly requirement for interior elements. This classification dominates older downtown districts where buildings share party walls, and it remains popular for multi-family housing that needs a durable exterior envelope without the budget for a fully non-combustible interior.

Type IV: Heavy Timber and Mass Timber

Type IV has seen the biggest change of any classification in recent code cycles. Traditional Type IV relies on the inherent fire resistance of thick wood members: large cross-sections char slowly on the surface while their cores maintain structural strength.⁴ NFPA 220 designates this construction as 2HH.²

Under the IBC, traditional heavy timber (now designated Type IV-HT) requires exterior walls of non-combustible materials and interior elements of solid wood, laminated heavy timber, or structural composite lumber with no hidden cavities where fire could spread undetected. Columns supporting floor loads must be at least 8 inches nominal in any dimension. Beams and girders must be at least 6 inches wide and 10 inches deep. Floor planks need a minimum thickness of 3 inches, topped with an additional layer of flooring or structural panel. These dimensions let the member char at its surface while its interior stays sound long enough for evacuation and firefighting.

Mass Timber Sub-Types

The 2021 IBC added three new sub-types that bring engineered mass timber products like cross-laminated timber into taller buildings. The additions reflect testing showing that properly designed mass timber assemblies can meet the same fire-performance benchmarks as concrete and steel.

Type IV-A requires a 3-hour fire-resistance rating, equivalent to Type I-A performance. Every mass timber surface must be fully protected by non-combustible material on both inside and outside, leaving no wood exposed.⁵
Type IV-B requires a 2-hour rating. Limited interior mass timber surfaces may be left exposed if they are spatially separated from one another, but exterior faces and concealed spaces must be covered with non-combustible protection.
Type IV-C also requires a 2-hour rating, but relies on the inherent fire resistance of the mass timber for interior elements, with no non-combustible covering required inside. Concealed spaces, shafts, and stairway enclosures still need non-combustible protection, and exterior faces must be covered.

A sprinklered Type IV-A building can reach up to 18 stories for certain occupancy groups, and Type IV-B can reach 12 stories, putting mass timber into a height range previously reserved for concrete and steel.⁶ Type IV-HT remains capped at lower heights. Getting the sub-type wrong during design can derail a project.

Type V: Wood-Frame Construction

Type V is the least restrictive classification and the backbone of residential construction. The IBC defines it simply: structural elements, exterior walls, and interior walls can be any code-permitted material.⁵ In practice, that means light wood framing using standard dimensional lumber or engineered wood joists.

Type V-A (NFPA 220 sub-type 111) requires a 1-hour rating for exterior bearing walls, the structural frame, and floor construction. Type V-B (sub-type 000) requires no fire-resistance rating for structural elements at all.³ Most single-family homes and small apartment buildings are Type V-B. The cost and speed advantages are large: a wood-frame house can go from foundation to framing in days using widely available lumber and standard labor.

The trade-off is fire vulnerability. Modern codes offset this with gypsum board on walls and ceilings, fire-blocking between stud cavities, and automatic sprinkler systems in many multi-family applications. Type V buildings also face the tightest limits on height and floor area, which is why wood-frame buildings rarely rise above three or four stories over a podium.

Why the Construction Type Choice Matters

Construction type is not just a fire-safety label. It directly controls how tall and how large a building can be. IBC Tables 504.3 and 504.4 set the maximum height in feet and the maximum number of stories above grade for every combination of construction type and occupancy group. Type I-A buildings have unlimited height and unlimited stories for most occupancy groups. A Type V-B building used for mercantile purposes is capped at 40 feet and one story without a sprinkler system.⁶

Floor area works the same way. IBC Table 506.2 sets a maximum allowable area per floor by construction type and occupancy. Type I again gets unlimited area for most uses. A Type V-B business occupancy is limited to 9,000 square feet per floor without modifications.⁷ Construction type selection is one of the earliest decisions in any project because of these limits. Picking Type V to save on materials, then discovering the building you need is two stories too tall for that type, is an expensive fix on paper and a devastating one in the field.

Sprinkler System Trade-Offs

Automatic sprinklers can substantially expand what a given construction type allows. A building equipped with an NFPA 13 sprinkler system generally qualifies for a height increase of 20 feet and one additional story beyond the base limits in the IBC tables. The area increase is more dramatic: up to 200 percent for multi-story buildings and 300 percent for single-story buildings, depending on the occupancy group. Buildings with NFPA 13R systems (a lighter residential sprinkler standard) get more limited benefits, typically capped at 60 feet and four stories. These trade-offs are how developers build five-story wood-frame apartment buildings that would otherwise be limited to three stories under Type V-A.

Combustible Materials Allowed in Type I and Type II

The rule that Type I and Type II buildings use non-combustible structural materials does not mean every scrap of material must be non-combustible. IBC Section 603 lists a long menu of combustible materials permitted in otherwise non-combustible construction types.³

Fire-retardant-treated wood is permitted for roof construction in Type I and II buildings, including girders, trusses, and roof decking, as long as the roof structure is 20 feet or more above the floor below. Type I-A buildings taller than two stories carry an additional restriction: if the roof is less than 20 feet above the top occupied floor, fire-retardant-treated wood cannot be used for roof framing. The exceptions also include thermal and acoustical insulation with a flame-spread index of 25 or less, foam plastics installed per code, interior wall and ceiling finishes, millwork like door frames and window sashes, finish flooring, and trim. These exceptions matter because they affect cost and design flexibility. A designer who assumes a Type I-A building must be 100 percent steel and concrete from roof to floor will overbuild and overspend.

1
National Fire Protection Association (NFPA). Types of Construction and Material Combustibility
2
Intertek Inform. NFPA 220:2024 Standard on Types of Building Construction
3
International Code Council. IBC Chapter 6 Types of Construction
4
International Code Council. 2015 IBC Significant Changes
5
International Code Council. IBC 2021 Chapter 6 Types of Construction
6
International Code Council. IBC 2021 Chapter 5 General Building Heights and Areas
7
International Code Council. IBC Chapter 5 General Building Heights and Areas