IBC Types of Construction
Summary
Type of construction classifies a building as Type I through V, with A and B subtypes, based on whether its structural elements are noncombustible, mass timber, or any material, and on the fire-resistance rating each element must have. It is the second code decision after occupancy, because the type sets the allowable height and area and the rating of the frame, walls, floors, and roof.
Cols
Scope
- Cites the 2024 IBC, Chapter 6. Always verify the adopted code edition and local amendments for the project jurisdiction
- Covers the classification of the structure and the minimum ratings of its elements, in hours
- Exterior wall ratings by fire separation distance, and opening limits, are in the exterior wall provisions, not in Table 601
- A building need not meet a higher type than the minimum its occupancy, height, and area require (Section 602.1.1)
Terminology
- Type I and II: structural elements noncombustible, with limited combustible materials permitted by Section 603
- Type III: exterior walls noncombustible; interior elements any material the code permits
- Type IV: structure of mass timber or noncombustible materials, with subtypes IV-A, IV-B, IV-C, and heavy timber IV-HT
- Type V: structure and walls of any material the code permits, usually wood frame
- Subtype A and B: A is protected (rated structure); B is unprotected (generally 0-hour)
- Primary structural frame: columns and the members that connect them, and members supporting loads from more than two floors or a floor and roof
- Fire-resistance rating: hours of exposure an assembly withstands in a standard fire test (ASTM E119 or UL 263)
- Noncombustible: passes ASTM E136 or an equivalent test
- Fire-retardant-treated (FRT) wood: wood pressure-treated to resist fire spread, allowed in specific noncombustible-type assemblies
- Assigned protection time: minutes of fire protection a noncombustible covering gives mass timber
- Heavy timber (HT): large solid or laminated wood members meeting minimum dimensions, used in IV-HT and for some roofs
Classification
Type definitions
- IBC 2024, Section 602.2: Types I and II use noncombustible elements, except as permitted by Section 603 and elsewhere
- IBC 2024, Section 602.3: Type III exterior walls are noncombustible; FRT wood framing and sheathing is permitted in exterior wall assemblies rated 2 hours or less
- IBC 2024, Section 602.4: Type IV elements are mass timber or noncombustible; exterior walls are mass timber or noncombustible except in IV-HT
- IBC 2024, Section 602.5: Type V elements may be any material the code permits
- IBC 2024, Section 602.4.1 (IV-A): exterior side of exterior mass timber walls has noncombustible protection of at least 40 minutes; interior faces have protection totaling at least 80 minutes
- IBC 2024, Section 602.4.2 (IV-B): like IV-A, but unprotected mass timber is allowed in limited amounts: ceilings up to 100 percent of the floor area per dwelling unit or fire area per story, and walls up to 40 percent, with exposed walls at least 15 ft apart
- IBC 2024, Section 602.4.3 (IV-C): interior mass timber may be unprotected; combustible exterior walls have 40-minute protection; concealed spaces have 40-minute noncombustible protection
- IBC 2024, Section 602.4.4 (IV-HT): exterior walls noncombustible, FRT wood, or CLT at least 4 in. thick; interior elements solid wood, glued-laminated timber, or structural composite lumber; concealed spaces sprinklered, filled with noncombustible insulation, or lined with 5/8 in. Type X gypsum board
- IBC 2024, Section 602.4: in buildings over 75 ft above fire department vehicle access, mass timber exit and elevator enclosures are protected, and above 12 stories or 180 ft they are noncombustible
Required ratings
- IBC 2024, Table 601: minimum fire-resistance rating of structural elements, in hours
| Type | Primary frame | Exterior bearing walls | Interior bearing walls | Floor | Roof |
|---|---|---|---|---|---|
| I-A | 3 | 3 | 3 | 2 | 1-1/2 |
| I-B | 2 | 2 | 2 | 2 | 1 |
| II-A | 1 | 1 | 1 | 1 | 1 |
| II-B | 0 | 0 | 0 | 0 | 0 |
| III-A | 1 | 2 | 1 | 1 | 1 |
| III-B | 0 | 2 | 0 | 0 | 0 |
| IV-A | 3 | 3 | 3 | 2 | 1-1/2 |
| IV-B | 2 | 2 | 2 | 2 | 1 |
| IV-C | 2 | 2 | 2 | 2 | 1 |
| IV-HT | HT | 2 | 1 or HT | HT | HT |
| V-A | 1 | 1 | 1 | 1 | 1 |
| V-B | 0 | 0 | 0 | 0 | 0 |
- IBC 2024, Table 601, nonbearing walls: exterior nonbearing walls take their rating from the fire separation distance table in the exterior wall provisions; interior nonbearing walls and partitions are 0 hours for all types except IV-HT
- IBC 2024, Table 601, roof supports: the rating of roof supports for the frame and bearing walls may be reduced by 1 hour where they support only the roof
- IBC 2024, Table 601, roof 20 ft rule: in groups other than F-1, H, M, and S-1, roof structural members need no fire protection where every part of the roof is 20 ft or more above any floor or mezzanine below; FRT wood is allowed for those members
- IBC 2024, Table 601, heavy timber roof: heavy timber roof construction (Section 2304.11) is allowed where a 1-hour or lower rating is required
- IBC 2024, Table 601, heavy timber bearing walls: at least 1 hour when they support more than two floors, or a floor and a roof
- IBC 2024, Section 703.2: ratings are determined by ASTM E119 or UL 263, without credit for sprinklers
- IBC 2024, Section 703.3: noncombustible means passing ASTM E136
- IBC 2024, Section 703.6: protection time of a covering equals the rating of the protected assembly minus the rating of the unprotected assembly
Combustibles in Types I and II
- IBC 2024, Section 603.1: lists categories of combustible material permitted in Type I and II buildings, each with conditions; examples are FRT wood in nonbearing partitions rated 2 hours or less, roof coverings Class A through C, foam plastics under Chapter 26, finishes under Chapter 8, millwork, and light-transmitting plastics
- IBC 2024, Section 603.1: insulation other than foam plastic with a flame spread index of 25 or less is permitted; 100 or less between noncombustible layers without air space; 200 or less under finished floors on solid decking
Worked examples
Choosing a type for a four-story office
Group B, four stories above grade, about 52 ft tall, 15,000 sq ft per floor, sprinklered throughout (IBC 2024 Tables 504.3, 504.4, and 506.2).
| Type | Stories allowed (sprinklered) | Height allowed (sprinklered) | Area per story (sprinklered, multi-story) | Result |
|---|---|---|---|---|
| II-B | 4 | 75 ft | 69,000 sq ft | Allowed; no structural ratings |
| V-A | 4 | 70 ft | 54,000 sq ft | Allowed; 1-hour frame, walls, floors, roof |
| V-B | 3 | 60 ft | 27,000 sq ft | Not allowed; fourth story exceeds 3 |
Type II-B uses noncombustible framing with no rating and is permitted without reductions. Type V-A allows wood framing but requires 1-hour protection on the frame, bearing walls, floors, and roof, which usually means added gypsum board.
Roof framing in an unrated type
Group B, Type II-A, roof deck 28 ft above the second floor below.
| Step | Check | Result |
|---|---|---|
| Group | B, not F-1, H, M, or S-1 | 20 ft rule available |
| Roof height | 28 ft above the highest floor below | Exceeds 20 ft |
| Roof members | Table 601 requires 1 hour for II-A roof | No fire protection needed on roof members |
Coordination
- Structural engineer: select the type with the engineer before design development; the type drives member protection, connection details, and cost
- Height and area: use the heights and areas note to check the proposed type against the story, height, and area program
- Exterior walls: the rating of nonbearing exterior walls and the openings in them follow the fire separation distance provisions
- Mass timber: the protection plan (noncombustible covering, char, concealed spaces) is a design item, not a finish; coordinate sprinklers, MEP routing, and exposed surface areas
- Material submittals: rated assemblies come from listed designs; confirm the listing matches the type's required hours
Common issues
- Assuming sprinklers reduce Table 601 hours: ratings are determined without sprinkler credit and stay unless the code specifically allows a reduction
- Missing exterior wall rating: Table 601 lists bearing walls only; nonbearing exterior walls follow fire separation distance
- Type V-B limits: unrated wood construction is limited to a few stories and small areas in many groups
- Mass timber exposure: exposed mass timber is limited by type (IV-B and IV-C allow more); extra protection is required elsewhere
- Roof 20 ft rule: the rule excludes F-1, H, M, and S-1 and needs every part of the roof, including low points, to be 20 ft above the floors below
- Combustibles in Types I and II: wood furring, backing, and finishes are limited to Section 603 categories
Standards
- ASTM E119 and UL 263: fire tests of building construction and materials, the basis of all rated assemblies
- ASTM E136: noncombustibility of materials
- NFPA 285: exterior wall assemblies with combustible components, relevant where combustible cladding is used on Types I through IV
Sources
- UpCodes: 2024 IBC Chapter 6 types of construction (Department of Defense adoption), https://up.codes/viewer/department-of-defense/ibc-2024/chapter/6/types-of-construction
- UpCodes: 2024 IBC Chapter 7 (Sections 703 and 705, Department of Defense adoption), https://up.codes/viewer/department-of-defense/ibc-2024/chapter/7/fire-and-smoke-protection-features
- GoCodebook: 2025 California Building Code, Sections 504 and 506 (based on the 2024 IBC), https://gocodebook.com/library/us/ca/ca-building-code/chapter-5-general-building-heights-and-areas/section-504-building-height-and-number-of-stories