# IBC Construction Types Explained: Type I, II, III, IV & V

> IBC Chapter 6 sorts every building into five construction types (I through V) with A and B subtypes based on whether the structure is combustible and how many hours of fire resistance its elements carry, and that classification, together with occupancy and sprinklers, sets the allowable height and area.

- Canonical: https://harth.build/resources/assemblies/ibc-construction-types-explained/
- Last updated: 2026-09-03
- Data source: Definitions and the Table 601 values are taken from the [2021 International Building Code, Chapter 6, Types of Construction](https://codes.iccsafe.org/content/IBC2021P1/chapter-6-types-of-construction), with height and area limits in [Chapter 5, General Building Heights and Areas](https://codes.iccsafe.org/content/IBC2021P1/chapter-5-general-building-heights-and-areas) and occupancy groups in [Chapter 3](https://codes.iccsafe.org/content/IBC2021P1/chapter-3-occupancy-classification-and-use). The ISO construction class and [NFPA 220](https://www.nfpa.org/codes-and-standards/nfpa-220-standard-development/220) comparisons are summarized for orientation only. All hours and story counts must be verified against the edition adopted by the authority having jurisdiction and any state or local amendments.

## Key facts

| Fact | Value |
| --- | --- |
| Types I and II | Noncombustible (IBC 2021 Section 602.2; steel and concrete structure) |
| Type III | Noncombustible exterior walls, any permitted interior (Section 602.3; FRTW permitted in exterior walls rated 2 hours or less) |
| Type IV | Heavy timber (IV-HT) and mass timber IV-A, IV-B, IV-C (Section 602.4; the three mass timber subtypes were added in IBC 2021) |
| Type V | Any material permitted by the code (Section 602.5; typical light wood frame) |
| A versus B | A is protected (rated per Table 601), B is unprotected (Type IA primary frame 3 hours; Type IIB, IIIB and VB frames 0 hours) |
| Where type is used | Tables 504.3, 504.4 and 506.2 (Allowable height in feet, stories and area, by occupancy and sprinklers) |

## Direct answer

IBC Section 602 defines five construction types. Types I and II (Section 602.2) use only noncombustible elements such as steel and concrete. Type III (602.3) requires noncombustible exterior walls, with fire-retardant-treated wood permitted in them, while interior elements can be any material the code allows. Type IV (602.4) is heavy timber (IV-HT) plus, since the 2021 edition, the mass timber subtypes IV-A, IV-B and IV-C. Type V (602.5) permits any material, so most wood-frame buildings are Type VA or VB. Within Types I, II, III and V, the A subtype is protected: Table 601 assigns hours to the primary structural frame, bearing walls, floors and roof, from 3 hours for Type IA down to 1 hour for Types IIA, IIIA and VA. The B subtype carries 0 hours for those elements, although exterior bearing walls in Type IIIB still need 2 hours. Type then feeds Tables 504.3, 504.4 and 506.2, which set allowable height, stories and area by occupancy and sprinkler status.

*Content and cited inputs reviewed by [Scott Barrington](https://nz.linkedin.com/in/barrington), Harth founder and architectural technology specialist · Published 2026-09-03 · Updated 2026-09-03.*

## What does this term mean?

**IBC Construction Types Explained**: IBC Chapter 6 sorts every building into five construction types (I through V) with A and B subtypes based on whether the structure is combustible and how many hours of fire resistance its elements carry, and that classification, together with occupancy and sprinklers, sets the allowable height and area.

![Labeled wall-assembly cutaway diagram: IBC Construction Types Explained: Type I, II, III, IV & V](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/assemblies/ibc-construction-types-explained/diagram-1344.webp)

*IBC Chapter 6 sorts every building into five construction types (I through V) with A and B subtypes based on whether the structure is combustible and how man…*

## What are the key takeaways?

- Types I and II: Noncombustible
- Type III: Noncombustible exterior walls, any permitted interior
- Type IV: Heavy timber (IV-HT) and mass timber IV-A, IV-B, IV-C
- Type V: Any material permitted by the code

## What are the five IBC construction types?

IBC Chapter 6 requires every building to be classified into one of five types of construction, defined in Section 602 by what the building elements are made of and how much fire resistance they carry. The types run from most to least restrictive:

| Type | Materials (IBC 2021 Section 602) | Subtypes | Typical examples |
| --- | --- | --- | --- |
| I | Noncombustible only (602.2) | IA, IB | High-rise office and residential towers, hospitals, large public buildings |
| II | Noncombustible only (602.2) | IIA, IIB | Steel-frame warehouses, big-box retail, tilt-up concrete industrial, low-rise steel offices |
| III | Noncombustible exterior walls; interior elements any permitted material (602.3) | IIIA, IIIB | Masonry-walled mixed-use with wood floors, many older main-street buildings, wood-over-podium exteriors |
| IV | Heavy timber (IV-HT) or mass timber (IV-A, IV-B, IV-C) (602.4) | IV-A, IV-B, IV-C, IV-HT | Mill buildings, CLT and glulam offices and residential mid-rises |
| V | Any material permitted by the code (602.5) | VA, VB | Light wood-frame houses, townhouses, small apartment and commercial buildings |

The classification is a property of the whole building. Where a building contains more than one type, Section 602.1 and the special provisions of Section 510 govern how the parts are treated, and the least restrictive type that still satisfies the height and area limits for the occupancy is normally the economic choice.

## What do combustible, noncombustible and protected mean?

**Noncombustible** materials are those that pass ASTM E136 or meet the composite criteria in Section 703; structural steel, concrete and masonry qualify. Types I and II require noncombustible building elements, with limited combustible allowances listed in Section 603 (for example fire-retardant-treated wood in some non-bearing partitions and roof construction, and combustible finishes and blocking). Types III, IV and V allow combustible structure to varying degrees.

**Protected** is shorthand for the A subtype. Table 601 assigns a fire-resistance rating in hours to five categories of building element: the primary structural frame, exterior bearing walls, interior bearing walls, floor construction and roof construction. An A building carries those hours; the corresponding B building carries 0 hours for the frame, interior bearing walls, floors and roof. The one place B is not zero is the exterior bearing wall of Type IIIB, which still needs 2 hours because that wall is what defines Type III.

Ratings are achieved with tested assemblies (gypsum encasement, spray-applied fire-resistive material, concrete cover, membrane ceilings) chosen from UL or the prescriptive tables in Section 721, and they are separate from the exterior wall ratings driven by fire separation distance in Table 705.5 and from the fire barriers and partitions of Chapter 7 that respond to occupancy rather than type.

## IBC 2021 Table 601 fire-resistance ratings by construction type

The hours below are reproduced from the 2021 IBC Table 601. They must be verified against the adopted edition and local amendments, and the table's footnotes (which, for example, modify roof requirements in some conditions and address the members supporting rated elements) apply. Types IV-A, IV-B and IV-C carry their own column entries in the 2021 table and are not reproduced here.

| Building element | IA | IB | IIA | IIB | IIIA | IIIB | IV-HT | VA | VB |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Primary structural frame | 3 | 2 | 1 | 0 | 1 | 0 | HT | 1 | 0 |
| Exterior bearing walls | 3 | 2 | 1 | 0 | 2 | 2 | 2 | 1 | 0 |
| Interior bearing walls | 3 | 2 | 1 | 0 | 1 | 0 | 1/HT | 1 | 0 |
| Floor construction and associated secondary members | 2 | 2 | 1 | 0 | 1 | 0 | HT | 1 | 0 |
| Roof construction and associated secondary members | 1.5 | 1 | 1 | 0 | 1 | 0 | HT | 1 | 0 |

HT means heavy timber conforming to the member sizes in Section 2304.11; 1/HT for Type IV-HT interior bearing walls means 1 hour or heavy timber. Values are hours.

## What changed with mass timber Types IV-A, IV-B and IV-C?

The 2021 IBC added three mass timber subtypes to Type IV, following the ICC Ad Hoc Committee on Tall Wood Buildings. All three require mass timber elements (CLT, glulam, NLT, DLT and similar) meeting Section 602.4 and Chapter 23, and all three are fully sprinklered by definition. The differences are in how much timber may be exposed and how tall the building can go:

- **Type IV-A**: all mass timber is protected with noncombustible material; the tallest allowance of the three, up to 18 stories for common business and residential occupancies in Table 504.4.
- **Type IV-B**: limited areas of exposed ceiling or wall timber are permitted; up to 12 stories for the same occupancies.
- **Type IV-C**: exposed mass timber is generally permitted; up to 9 stories.
- **Type IV-HT**: the traditional heavy timber type retained from earlier editions, with minimum solid-sawn or glulam member sizes rather than hourly ratings.

Jurisdictions still on the 2018 IBC or earlier do not recognize IV-A, IV-B or IV-C, and several states adopted the 2021 tall mass timber provisions with amendments, so confirm the adopted edition before designing to them.

## How does construction type interact with occupancy, height, area and sprinklers?

Construction type is never chosen in isolation. The code reads three tables together, each with occupancy group down the side and construction type across the top, split into sprinklered and non-sprinklered columns:

| Table (IBC 2021) | What it limits | Sprinkler effect |
| --- | --- | --- |
| 504.3 | Allowable building height in feet above grade plane | Separate NS and S columns; an NFPA 13 system (903.3.1.1) generally adds height |
| 504.4 | Allowable number of stories above grade plane | Separate NS, S13R, S13D and S columns; NFPA 13 typically adds a story |
| 506.2 | Allowable area factor per story | NS, S1 (single story) and SM (multistory) columns, then adjusted for frontage per Section 506.3 |

The workflow is: fix the occupancy group under Chapter 3, decide whether the building will be fully sprinklered, then read across the tables to find the least restrictive type that still fits the intended height, stories and floor area. If Type VB fits, use it; if not, move to VA, IIIA or IIB, and so on toward Type I. High-rise buildings (occupied floors more than 75 feet above fire department access, Section 403) and Group H occupancies push the answer toward Types I and II regardless of area.

## How do ISO classes and NFPA 220 differ from IBC types?

Two other systems use similar words and cause confusion. ISO (Insurance Services Office) grades buildings for insurance rating into six classes: 1 Frame, 2 Joisted Masonry, 3 Noncombustible, 4 Masonry Noncombustible, 5 Modified Fire Resistive and 6 Fire Resistive. NFPA 220, Standard on Types of Building Construction, also uses Types I through V but adds a three-digit code, such as I(442), II(000) or III(211), giving the required hours for exterior bearing walls, structural frame and floor construction. Both roughly track the IBC but neither is the permitting basis; use the IBC classification in the code analysis and translate to ISO or NFPA 220 only when an insurer or a referenced NFPA standard asks for it. Verify every value against the adopted edition and local amendments.

## IBC 2021 Table 601 fire-resistance matrix (hours)

| Building element | IA | IB | IIA | IIB | IIIA | IIIB | IV-HT | VA | VB |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Primary structural frame | 3 | 2 | 1 | 0 | 1 | 0 | HT | 1 | 0 |
| Exterior bearing walls | 3 | 2 | 1 | 0 | 2 | 2 | 2 | 1 | 0 |
| Interior bearing walls | 3 | 2 | 1 | 0 | 1 | 0 | 1/HT | 1 | 0 |
| Floor construction and associated secondary members | 2 | 2 | 1 | 0 | 1 | 0 | HT | 1 | 0 |
| Roof construction and associated secondary members | 1.5 | 1 | 1 | 0 | 1 | 0 | HT | 1 | 0 |

Reproduced from the 2021 IBC Table 601 for reference; verify against the adopted edition, its footnotes and local amendments before use. HT indicates heavy timber sizing under Section 2304.11. Types IV-A, IV-B and IV-C have separate column entries in the 2021 table and must be read there directly.

## Choose-your-construction-type decision worksheet

Work down the rows in order. The answer to each row narrows the candidate types before you open Tables 504.3, 504.4 and 506.2.

| Step | Question | Answer to record | Effect on candidate types |
| --- | --- | --- | --- |
| 1 | What is the occupancy group (Chapter 3), including any mixed-occupancy strategy under Section 508? | Group and subgroup, e.g. R-2, B, S-1 | Sets the row you will read in the height and area tables |
| 2 | Will the building be fully sprinklered under NFPA 13, NFPA 13R or NFPA 13D (Section 903.3.1)? | NS, S, S13R or S13D | Selects the column set in Tables 504.3, 504.4 and 506.2 |
| 3 | Is any occupied floor more than 75 feet above the lowest fire department vehicle access (Section 403 high-rise)? | Yes or no | Yes generally requires Type I or II and high-rise provisions |
| 4 | Target height in feet, number of stories and largest story area? | Values from the massing study | Compared against each candidate type in Tables 504.3, 504.4 and 506.2 |
| 5 | Is the structure intended to be steel or concrete (noncombustible), masonry-walled with combustible interior, mass timber, or light wood frame? | Material intent | Points to Type I/II, III, IV or V respectively |
| 6 | Does the site or owner require exposed timber, or a podium (Section 510.2) with a different type below? | Exposed timber; podium yes or no | Exposed timber implies IV-B, IV-C or IV-HT; a podium allows a Type IA lower portion under a Type III, IV or V upper portion |
| 7 | Which is the least restrictive type whose limits in all three tables exceed the targets in step 4? | Selected type and subtype | Record it, then confirm the Table 601 hours and any Chapter 7 exterior wall ratings it triggers |
| 8 | Which adopted IBC edition and local amendments apply? | Edition year and amendment references | Governs whether IV-A, IV-B and IV-C exist and whether Table 601 matches the values above |

## Where can you explore related guidance?

- [Explore all assemblies guidance](https://harth.build/resources/assemblies.md)
- [Fire-Rated Wall Assemblies Explained (1-Hour vs 2-Hour)](https://harth.build/resources/assemblies/fire-rated-wall-assemblies-explained.md)
- [Typical Exterior Wall Assemblies (Layer by Layer)](https://harth.build/resources/assemblies/typical-exterior-wall-assemblies.md)
- [Understanding R-Value and U-Factor](https://harth.build/resources/assemblies/understanding-r-value-and-u-factor.md)
- [IBC Occupancy Classifications Explained](https://harth.build/resources/design-standards/ibc-occupancy-classifications-explained.md)

## Sources and method

Definitions and the Table 601 values are taken from the [2021 International Building Code, Chapter 6, Types of Construction](https://codes.iccsafe.org/content/IBC2021P1/chapter-6-types-of-construction), with height and area limits in [Chapter 5, General Building Heights and Areas](https://codes.iccsafe.org/content/IBC2021P1/chapter-5-general-building-heights-and-areas) and occupancy groups in [Chapter 3](https://codes.iccsafe.org/content/IBC2021P1/chapter-3-occupancy-classification-and-use). The ISO construction class and [NFPA 220](https://www.nfpa.org/codes-and-standards/nfpa-220-standard-development/220) comparisons are summarized for orientation only. All hours and story counts must be verified against the edition adopted by the authority having jurisdiction and any state or local amendments.

### Source links

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IBC2021P1/chapter-6-types-of-construction)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IBC2021P1/chapter-5-general-building-heights-and-areas)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IBC2021P1/chapter-3-occupancy-classification-and-use)

- [nfpa.org](https://www.nfpa.org/codes-and-standards/nfpa-220-standard-development/220)

## Frequently asked questions

### What is the difference between Type 2A and Type 2B construction?

Both are noncombustible under IBC Section 602.2. Type IIA is protected, so Table 601 requires 1-hour fire resistance for the primary structural frame, bearing walls, floors and roof. Type IIB is unprotected: those elements carry 0 hours, which is why most single-story steel warehouses and retail boxes are built as Type IIB.

### What is Type III construction?

Type III (Section 602.3) has noncombustible exterior walls, which may include fire-retardant-treated wood framing where the required rating is 2 hours or less, while interior elements can be any material permitted by the code. Table 601 requires 2-hour exterior bearing walls in both IIIA and IIIB, with IIIA adding 1 hour on the frame, floors and roof.

### What is Type IV construction and what are Types IV-A, IV-B and IV-C?

Type IV (Section 602.4) covers heavy timber (IV-HT) sized per Section 2304.11, plus three mass timber subtypes added in IBC 2021. IV-A is fully protected with noncombustible coverings, IV-B allows limited exposed mass timber, and IV-C allows exposed mass timber with a lower height limit. Each carries its own Table 601 hours; confirm the adopted edition.

### What is Type 5 construction?

Type V (Section 602.5) permits any material allowed by the code for the structure, so it covers conventional light wood framing. Type VA is protected with 1-hour ratings on the frame, bearing walls, floors and roof per Table 601; Type VB is unprotected at 0 hours. Type V has the lowest allowable heights and areas of any type.

### Are ISO construction classes and NFPA 220 the same as IBC types?

No. ISO uses six insurance classes, from Class 1 (Frame) to Class 6 (Fire Resistive), to rate property risk. NFPA 220 uses Types I to V with three-digit designations such as II(000) or III(211) giving hours for exterior walls, structural frame and floors. The IBC classification governs permitting; the others do not.

### How does construction type affect building height and area?

Construction type is one axis of Tables 504.3 (height in feet), 504.4 (stories) and 506.2 (area per story); occupancy group is the other, and the columns split by sprinkler status. A more protected, noncombustible type allows more height and area for the same occupancy, and an NFPA 13 sprinkler system usually increases both.

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