Types of Construction: Type I to Type V Explained
The building code sorts every building into one of five types of construction, Type I through Type V, based on what the main structure is made of and how fire-resistant it is. Types I and II are noncombustible, Type III has noncombustible exterior walls, Type IV is mass timber or heavy timber, and Type V can be any permitted material, usually wood. Each type is split into a protected (A) and an unprotected (B) version. The type, together with the occupancy and sprinklers, decides how tall and how big the building can be. Here is each type under the 2025 Oregon building code, with the actual ratings and story limits.
| Type | Main materials (OSSC 602) | Frame rating, A / B (Table 601) |
|---|---|---|
| Type I | Noncombustible, the most fire-resistant | 3 hours / 2 hours |
| Type II | Noncombustible | 1 hour / none |
| Type III | Noncombustible exterior walls, any permitted material inside | 1 hour / none |
| Type IV | Mass timber (IV-A, IV-B, IV-C) or heavy timber (IV-HT) | 3, 2 and 2 hours for IV-A, B, C; heavy timber sizes for HT |
| Type V | Any permitted material, usually wood framing | 1 hour / none |
Ratings are for the primary structural frame from Table 601 of the 2025 Oregon Structural Specialty Code. Bearing walls, floors and roofs have their own columns, below.
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The five types, one by one
- Type I. The building elements are noncombustible, and the ratings are the highest in the code: 3 hours on the frame for I-A, 2 hours for I-B.
- Type II. Also noncombustible, with lower ratings: 1 hour on the frame for II-A and none for II-B.
- Type III. Exterior walls are noncombustible; interior elements can be any material the code allows. Fire-retardant-treated wood framing is permitted inside the exterior walls under the code's conditions. Exterior bearing walls need 2 hours in both III-A and III-B.
- Type IV. Mass timber or noncombustible materials. IV-A, IV-B and IV-C are mass timber with set fire ratings and protection rules; IV-HT is traditional heavy timber, with noncombustible exterior walls and solid or laminated timber inside.
- Type V. The structure, exterior walls and interior walls can be any material the code allows. V-A needs 1 hour on the frame, bearing walls, floors and roof; V-B needs no rating at all.
A vs B: protected vs unprotected
Within each type, A means the structure carries fire-resistance ratings and B means less or none. Table 601 sets the hours for each part of the building:
| Element | I-A | I-B | II-A | II-B | III-A | III-B | V-A | V-B |
|---|---|---|---|---|---|---|---|---|
| Primary structural frame | 3 | 2 | 1 | 0 | 1 | 0 | 1 | 0 |
| Exterior bearing walls | 3 | 2 | 1 | 0 | 2 | 2 | 1 | 0 |
| Interior bearing walls | 3 | 2 | 1 | 0 | 1 | 0 | 1 | 0 |
| Floors | 2 | 2 | 1 | 0 | 1 | 0 | 1 | 0 |
| Roofs | 1.5 | 1 | 1 | 0 | 1 | 0 | 1 | 0 |
Hours, from Table 601 of the 2025 Oregon Structural Specialty Code (Type IV columns left out for space). Footnotes allow some reductions, such as for elements that support only a roof.
The B versions need less fire protection, but they are allowed to be smaller and shorter. That trade-off is why the type gets chosen early, in schematic design.
How the type limits height
Table 504.4 sets the number of stories allowed above grade for each occupancy and construction type. A few sprinklered examples from the 2025 Oregon code:
| Occupancy (sprinklered) | II-B | III-A | V-A | V-B |
|---|---|---|---|---|
| Group B (offices, clinics) | 4 | 6 | 4 | 3 |
| Group M (retail) | 3 | 5 | 4 | 2 |
| Group R-2 (apartments) | 5 | 5 | 4 | 3 |
Without sprinklers the limits drop: a Group B building in V-A goes from 4 stories to 3, and in V-B from 3 to 2. Floor area has its own limits in the same chapter. Which group a building falls in is covered in our guide to occupancy classification.
Type III vs Type V
Both can have wood inside, so they are often compared. The difference is the outside wall. In Type III the exterior walls are noncombustible and carry a 2-hour rating as bearing walls, in both III-A and III-B. In Type V the exterior walls can be wood, with a 1-hour rating in V-A and none in V-B. That heavier exterior buys height. Under Table 504.4, a sprinklered apartment building (Group R-2) can go 5 stories in Type III-A or III-B, against 4 in V-A and 3 in V-B. A sprinklered office (Group B) can go 6 stories in III-A and 4 in III-B, against 4 in V-A and 3 in V-B. When a design is one story over the limit for Type V, Type III is a common next option to look at.
Why the type matters to an owner
- It sets the cost of the structure. Moving from wood framing to noncombustible walls or protected steel changes the price of the whole building.
- It sets how far you can grow. Height and area limits come from the type, so a future addition may not fit the type you build today.
- It drives the rated assemblies. The hours in Table 601 mean rated walls, floors and roofs, and every pipe and duct through them needs firestopping.
- It matters in a change of use. An existing building's type can limit what new use fits inside it; see change of use permits.
- You do not have to overbuild. The code says a building does not have to meet the details of a higher type than its occupancy requires, even if parts of it would qualify.
Frequently asked questions
What are the 5 types of construction?
Type I and Type II use noncombustible materials for the main building elements. Type III has noncombustible exterior walls with any permitted material inside. Type IV is mass timber or heavy timber. Type V allows any permitted material, which in practice is usually wood framing.
What does the A or B mean, like Type V-A or Type V-B?
A is the protected version with fire-resistance ratings on the structure, and B is the unprotected version. In the Oregon code, Type V-A needs a 1-hour rating on the frame, bearing walls, floors and roof; Type V-B needs none.
What is Type V construction?
A building where the structure, exterior walls and interior walls can be any material the code allows. Wood-framed commercial and apartment buildings are often Type V. A sprinklered office (Group B) in Type V-A can go up to 4 stories under Table 504.4.
What is Type III construction?
A building with noncombustible exterior walls, such as masonry or concrete, and interior elements of any permitted material, often wood. Fire-retardant-treated wood framing is also allowed in the exterior walls under the code's conditions.
Who decides the construction type?
The architect picks it during design, based on the occupancy, the height and the floor area, and shows it on the construction documents. The building department checks it at plan review.
The bottom line
Types of construction run from Type I, noncombustible with 3-hour ratings, to Type V-B, any permitted material with no rating. The type, the occupancy and sprinklers together set how tall and how large a building can be under Table 504.4. It is one of the first decisions in design, and one of the hardest to change later.
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