IBC 2024 Construction Types: Type I–V and Table 601 Explained

IBC 2024 classifies buildings into five construction types—I, II, III, IV, and V—based on the permitted materials and the fire-resistance requirements for building elements. Table 601 represents those five types with 12 subtype columns: IA, IB, IIA, IIB, IIIA, IIIB, IV-A, IV-B, IV-C, IV-HT, VA, and VB.

The previous description of “eight sub-classifications” was obsolete and internally inconsistent because it omitted all four current Type IV columns from the displayed table. Construction type must be recorded at the subtype level; “Type IV” or “noncombustible construction” alone is not enough to select a Table 601 column.

Type V-A vs Type V-B, and the A/B pattern generally

Direct answer: Type V-A requires a 1-hour fire-resistance rating on the primary structural frame, bearing walls, floor construction, and roof construction; Type V-B requires no rating (0 hours) on those same elements. Table 601’s footnotes still apply to Type V-A roofs: roof protection is not required where every part of the roof construction is 20 feet or more above the floor below (except in Groups F-1, H, M and S-1), and heavy timber may be used where a roof needs 1 hour or less (IBC 2024 Table 601, notes b and c). The same pattern holds across every rated type in Table 601: the “A” subtype carries an equal or higher hourly rating than the “B” subtype in the same type (IA 3 hr vs. IB 2 hr for the primary frame; IIA 1 hr vs. IIB 0 hr; IIIA vs. IIIB differs on interior bearing walls). “A” is not automatically fire-resistance-rated in every row, and “B” is not automatically unrated — each building element in Table 601 must be checked on its own row and footnotes.

Type IV is the one type where the code itself uses protected/unprotected language: Type IV-A requires mass timber elements to be fully protected with noncombustible protection, while Type IV-B permits specific, limited areas of mass timber to remain unprotected (up to 100% of ceiling floor area or 40% of wall floor area under Section 602.4.2.2.2, subject to separation and other conditions). That IV-A/IV-B protection split is often the source of the general “A = protected, B = unprotected” shorthand, but it is a Type IV-specific rule, not a definition that applies to Types I, II, III, or V.

IBC 2024 construction-type map

This map describes the Chapter 6 framework. It does not determine that a proposed material, assembly, or building is acceptable without the associated sections, tables, footnotes, listings, and project conditions.

What Table 601 actually contains

Table 601 assigns minimum fire-resistance ratings by building element and construction subtype. It is not one rating for the entire building. The table includes the primary structural frame, exterior and interior bearing walls, interior and exterior nonbearing walls or partitions, floor construction with associated secondary members, and roof construction with associated secondary members.

The following transcription shows the principal rated structural rows. HT means the heavy-timber provisions apply; every value remains subject to the official table notes and other code sections.

Ratings are in hours. This condensed table must be read with the official Table 601 notes. Examples include permitted reductions for members supporting roofs only, conditions where protection of roof members is not required, and use of heavy timber for specified roof construction. Interior nonbearing partitions can also be governed by requirements elsewhere in the code.

Exterior walls need an additional check. Section 602.1 directs exterior walls to Table 705.5, while bearing exterior walls must also satisfy the applicable Table 601 row. See the exterior-wall guide and fire-separation guide for those separate relationships.

A zero-hour Table 601 entry does not mean “no fire ratings”

A zero in Table 601 means that the table does not impose a construction-type fire-resistance rating on that listed element. It does not waive fire-resistance requirements from occupancy separations, fire areas, shafts, corridors, exterior-wall exposure, incidental uses, mixed occupancies, horizontal assemblies, special building provisions, or other code sections.

For example, Type IIB and Type VB have zero-hour entries for several structural rows, but a wall can still require a rating because it is a fire barrier, shaft enclosure, fire partition, exterior wall, or occupancy separation. Conversely, providing one rated assembly in an otherwise lower type does not automatically reclassify the building as a higher construction subtype. Section 602.1.1 says a building is not required to be classified as a higher type merely because selected features exceed the minimum.

Types I and II: noncombustible elements with stated allowances

Section 602.2 describes Types I and II as construction in which the Table 601 building elements are noncombustible, except as permitted by Section 603 and elsewhere in the code. Type I generally carries higher Table 601 ratings than Type II, but both use this material framework.

Do not describe Section 603 as a short or exhaustive list consisting only of fire-retardant-treated wood, finishes, and millwork. Section 603.1 contains numerous specific applications, conditions, and cross-references, including certain FRTW roof construction and partitions, insulation, foam plastics under Chapter 26, classified roof coverings, finishes, blocking, combustible exterior components under other chapters, penetrations, joints, piping, ducts, and electrical components.

The proper test is whether the proposed combustible material is permitted by the applicable Section 603 item or another explicit code provision—not whether it seems incidental to the structure.

Type III: noncombustible exterior walls with a stated FRTW allowance

Section 602.3 defines Type III construction as noncombustible exterior walls with interior building elements of any material permitted by the code. It then expressly permits fire-retardant-treated wood framing and sheathing complying with Section 2303.2 within exterior wall assemblies having a fire-resistance rating of 2 hours or less.

The former claim that Type III’s exterior-wall requirement was “absolute” and never permitted FRTW was therefore incorrect. The correct analysis is more precise:

1. Start with the noncombustible exterior-wall rule.
2. Determine whether the proposed FRTW framing and sheathing meet Section 2303.2.
3. Confirm the exterior wall assembly is within the Section 602.3 rating limit.
4. Check Table 601 for bearing walls and Table 705.5 for exterior-wall exposure.
5. Verify every other applicable wall, opening, fireblocking, material, and assembly provision.

Type IIIA and IIIB differ in several Table 601 rows, but both show 2 hours for exterior bearing walls before the table notes and other provisions are applied.

Type IV: four distinct mass-timber and heavy-timber subtypes

Section 602.4 defines Type IV around mass timber or noncombustible building elements with the ratings in Table 601. The code then separates four systems that must not be treated as interchangeable:

• Type IV-A: mass timber elements receive the protection required by Sections 602.4.1.1 through 602.4.1.6. Interior faces of mass timber elements are protected, with at least 80 minutes assigned to the noncombustible interior protection under Section 602.4.1.2.1.
• Type IV-B: protected mass timber with specifically limited unprotected areas. Section 602.4.2.2.2 permits qualifying unprotected ceiling portions up to 100 percent of the floor area or wall portions up to 40 percent of the floor area, with additional rules where both are exposed, separation requirements, and an allowance for certain standalone columns and beams.
• Type IV-C: Section 602.4.3.2 permits mass timber elements to be unprotected on the interior, while exterior protection, concealed-space protection, shaft protection, fire-resistance ratings, and the rest of the subtype still apply.
• Type IV-HT: the heavy-timber subtype. Section 602.4.4 governs permitted solid wood, glued-laminated timber, structural composite lumber, CLT, minimum dimensions, walls, concealed spaces, and exterior structural members.

Cross-laminated timber used under Section 602.4 must be labeled as conforming to ANSI/APA PRG 320 as referenced by Section 2303.1.4. Type IV-A, IV-B, and IV-C also have special enclosure provisions where occupied floors exceed the stated fire-department-access height. Above 12 stories or 180 feet, the identified interior exit and elevator hoistway enclosures must be noncombustible.

Do not select a Type IV subtype from a desired exposed-timber aesthetic alone. The protection scheme, building-element ratings, concealed spaces, floor and roof construction, shafts, allowable height and stories, and material standards must be evaluated together.

Type V: permitted materials do not erase other requirements

Section 602.5 permits the structural elements and exterior and interior walls of Type V construction to use any materials allowed by the code. Type VA carries 1-hour entries for the primary frame, bearing walls, floors, and roofs in the condensed Table 601 rows above; Type VB has zero-hour entries for those rows.

“Any permitted material” does not mean any material in any configuration. The proposed assembly still must meet structural, exterior-wall, fire-protection, interior-finish, weather-protection, energy, accessibility, and other applicable provisions. A Type VB label also does not predict how many stories a particular occupancy can have.

Construction type and allowable height or area are separate steps

Construction type identifies the Chapter 6 material and fire-resistance framework. Allowable building height, stories, and area are then evaluated under Chapter 5 using the occupancy, construction subtype, sprinkler condition, frontage, mixed-occupancy approach, and other project variables.

Do not use claims such as “Type I-B is always unlimited” or “Type V-B cannot be multistory.” The applicable height and area analysis must use the exact editioned tables and formulas. Likewise, sprinkler increases under Chapter 5 do not automatically replace Table 601 ratings.

The related roof-assembly guide applies the separate roof-covering classification table. Unlike Table 601, that table groups all Type IV subtypes under one Type IV column, so the two tables must not be transcribed as if their column structures were identical.

A reliable classification workflow

1. Determine the adopted code and edition.
2. Establish the proposed building elements and materials.
3. Select a candidate subtype under Sections 602.2 through 602.5.
4. Apply every relevant Table 601 row and note.
5. Apply exterior-wall ratings under Table 705.5 and any bearing-wall requirement from Table 601.
6. Check fire-resistance requirements imposed elsewhere in the code.
7. Evaluate occupancy, mixed-use approach, height, stories, and area under Chapter 5.
8. Verify the selected construction type against the drawings, specifications, listed assemblies, and structural/fire-protection details.

Start with the occupancy-classification guide, then continue to the IBC resource hub for the surrounding code sequence.

Primary source and scope

• 2024 International Building Code, Chapter 6 — Types of Construction

This guide describes the unamended 2024 model IBC. It does not determine an adopted code, local amendment, construction type, listed assembly, material approval, or allowable building size for a specific project.

Correction history

• 2026-08-31: Replaced the obsolete eight-subtype model with all 12 Table 601 columns; added the missing Type IV ratings; corrected the Type III FRTW allowance and Type IV-A/B/C/HT scope; removed unsupported evacuation, jurisdiction-comparison, and “most common error” claims; clarified zero-hour entries, exterior-wall ratings, and the separate Chapter 5 analysis.