What are the 5 main building construction types?

The International Building Code (IBC) classifies all buildings into one of five main construction types. This classification is fundamental to building design, as it dictates the allowable size of a building and the level of fire safety required. The system is based on two primary factors: the combustibility of building materials (combustible vs. non-combustible) and the fire-resistance rating of structural elements.

The five primary types are ranked from most fire-resistive to least:

• Type I: Non-combustible materials only (e.g., concrete, steel). Highest level of fire resistance.
• Type II: Non-combustible materials only, but with lower fire-resistance requirements than Type I.
• Type III (Ordinary): Non-combustible exterior walls with a combustible (wood) interior structure.
• Type IV (Heavy Timber - HT): Non-combustible exterior walls with a massive, fire-resistant timber interior structure.
• Type V: Combustible materials throughout (e.g., typical wood-frame construction). Lowest level of inherent fire resistance.

Most of these types are further divided into "A" and "B" subtypes:

• "A" (Protected): Structural elements are required to have a specific fire-resistance rating (typically 1-hour, but can be higher for Type I). This is achieved through methods like applying spray-on fireproofing to steel or enclosing elements in rated gypsum board assemblies.

"B" (Unprotected): Structural elements have no added fire-resistance rating required by the construction type itself.

Why Construction Type is a Critical Design Decision

Choosing a building's construction type is one of the first and most consequential decisions an architect makes. This classification, established in IBC Chapter 6, is not an arbitrary label; it's the bedrock of the code's approach to life safety and property protection. It directly governs the maximum allowable building height and floor area, as detailed in IBC Chapter 5, Table 504.3 and 504.4.

Here’s why it matters at every stage:

• Early Design: An architect must balance the project's programmatic needs (size, height, use) with the cost and complexity of different construction types. A large, tall building will almost certainly need to be Type I, while a small retail building might be achievable as a less expensive Type II-B or V-A.
• Structural & MEP Coordination: The construction type dictates material choices and fire-resistance requirements. A structural engineer must design steel beams and columns to accommodate fireproofing for a Type I-A building. MEP engineers must ensure that all pipe, duct, and conduit penetrations through rated walls, floors, and roofs are properly firestopped according to IBC Chapter 7.
• Permitting & Plan Review: The construction type must be clearly stated on the cover sheet of the construction documents. Plan reviewers will meticulously check that every building element—from the primary frame to the roof deck—complies with the fire-resistance ratings specified for that type in IBC Table 601.
• Inspections: Field inspectors will verify that fire-resistance-rated assemblies are built exactly as specified in their tested designs (e.g., from UL or Gypsum Association). They will check the thickness of spray-on fireproofing, the number of gypsum layers, and the integrity of firestopping.

A common pitfall is misunderstanding the relationship between construction type and sprinklers. Installing an automatic sprinkler system does not change a building’s construction type. However, it provides critical trade-offs, allowing for significant increases in height and area and, in some cases, reductions in fire-resistance rating requirements for certain elements.

Based on IBC Table 601, provide a detailed comparison of the required hourly fire-resistance ratings for the primary structural frame, floor construction, and roof construction for a Type I-A, Type II-A, and Type IV-HT building.

The required fire-resistance ratings for these building elements are found directly in IBC 2024 Table 601, "Fire-Resistance Rating Requirements for Building Elements." This table is the definitive source for determining the hourly ratings needed to achieve a specific construction type classification.

Here is a direct comparison for the elements and types you specified:

Deeper Explanation:

• Type I-A: As the most fire-resistive type, it demands very high ratings for its non-combustible structure. A 3-hour rating for the primary frame (columns, girders, trusses) and 2 hours for the floors ensures the building will remain structurally stable for a long time during a fire, allowing for safe evacuation and firefighting operations.
• Type II-A: This "protected non-combustible" type has significantly lower requirements. The 1-hour rating for the frame, floors, and roof is typically achieved by enclosing steel members in 5/8" Type X gypsum board or applying a thin layer of intumescent paint or spray-applied fire-resistive material (SFRM).
• Type IV-HT (Heavy Timber): This type is unique. Instead of an hourly rating, it requires building elements to meet the minimum dimensional requirements for "Heavy Timber" as defined in IBC §2304.11 and referenced in Table 601. The concept is that massive wood members char at a slow, predictable rate. This char layer insulates the unburnt wood, allowing the structure to maintain significant strength for an extended duration in a fire. For example, a heavy timber column might need to be a minimum of 8x8 nominal, and a floor beam a minimum of 6x10 nominal.

What's the difference between Type II-A and Type II-B construction?

The only difference between Type II-A and Type II-B construction is the presence of fire-resistance ratings on the structural elements. Both types require the building elements to be constructed of non-combustible materials like steel, concrete, or masonry.

• Type II-A (Protected): Requires key structural elements to have a 1-hour fire-resistance rating, as specified in IBC Table 601. This includes the primary structural frame, bearing walls, and floor/roof construction.
• Type II-B (Unprotected): Has no fire-resistance rating requirements for any building element (0 hours in Table 601). The elements are still non-combustible, but they are left unprotected. A typical example is a warehouse with an exposed steel bar joist roof and unprotected steel columns.

This difference has a major impact on allowable building size. According to IBC Table 504.4, a Group B (Business) occupancy in a Type II-A building can be up to 4 stories tall, while the same building in Type II-B is limited to 2 stories.

If a project changes from Type V-B to Type V-A, does this automatically trigger a requirement for 1-hour rated interior bearing walls, or only the elements listed in IBC Table 601?

Changing a project from Type V-B to Type V-A only triggers the 1-hour rating requirement for the specific building elements listed in IBC Table 601 for Type V-A construction. It does not automatically apply a 1-hour rating to every element in the building.

The elements that must be 1-hour rated in Type V-A are:

• Primary Structural Frame
• Bearing Walls (both exterior and interior)
• Floor Construction
• Roof Construction

Therefore, to answer your question directly: yes, interior bearing walls must become 1-hour rated. However, interior non-bearing walls or partitions are not required to be rated by Table 601. Those elements only require a rating if mandated by another section of the code, such as for corridor walls (IBC §1020.1), shaft enclosures (IBC §713), or fire partitions separating different tenant spaces or dwelling units (IBC §708).

Is a brick building considered Type III construction?

Not necessarily, but it's a classic example. The defining characteristic of Type III construction (IBC §602.3) is having exterior walls made of non-combustible materials (like brick, concrete block, or precast concrete) while the interior building elements, including floors, roofs, and framing, are permitted to be of any material allowed by the code, which is almost always combustible wood.

So, a building with brick exterior walls and a wood-framed interior is the textbook definition of Type III construction. However, a building with brick exterior walls and a non-combustible interior steel frame would be classified as Type II, not Type III. The classification depends on the combination of both the exterior walls and the interior structure.

Is all concrete and steel construction considered Type I-A?

No, this is a common and critical misunderstanding. While Type I construction must be built of non-combustible materials like concrete and steel, the specific classification (I-A or I-B) depends entirely on achieving the high fire-resistance ratings listed in IBC Table 601.

• A steel-framed building where the columns and beams are left exposed and unprotected would likely be classified as Type II-B (Unprotected Non-combustible).
• The same steel building with 1-hour fire protection applied to its frame and floors would be Type II-A.
• Only when that steel frame is protected to achieve a 3-hour rating, and the floors are protected to achieve a 2-hour rating, can it be classified as Type I-A.

The material itself (steel/concrete) makes it non-combustible, but the level of fire protection applied to it determines whether it is Type I, Type II-A, or Type II-B.

Is 'combustible' vs 'non-combustible' the main difference between Type II and Type V?

Yes, that is the primary and fundamental difference between these two construction types.

• Type II Construction: As defined in IBC §602.2, all building elements (walls, floors, roofs, frame) must be of non-combustible materials. The IBC defines non-combustible materials in §703.5.
• Type V Construction: As defined in IBC §602.5, the building elements may be of any materials permitted by the code, which functionally means combustible wood-frame construction.

This distinction is the reason a building with metal studs is Type II, while an identical building with wood studs is Type V.

If a building is Type V-A, does that mean the whole thing has to be rated for 1 hour?

No. Only the specific structural elements listed for Type V-A in IBC Table 601 must be 1-hour rated. This requirement applies to the load-bearing components of the building to ensure structural stability during a fire.

The elements requiring a 1-hour rating are:

• Primary Structural Frame
• Bearing Walls (Interior and Exterior)
• Floor Construction
• Roof Construction

Items like non-bearing partitions, doors, windows, millwork, and finishes do not get their rating requirements from Table 601. Their fire-resistance or fire-protection rating requirements are determined by other code sections based on their specific location and function (e.g., a door in a rated corridor wall).

What do the letters 'A' and 'B' stand for in construction types like IIIA and IIIB?

The letters designate the level of fire protection applied to the structural elements.

• 'A' stands for "Protected." In a Type A building, the key structural elements listed in Table 601 are required to be protected to achieve a specific fire-resistance rating (usually 1-hour).
• 'B' stands for "Unprotected." In a Type B building, the structural elements have no added fire-resistance rating requirement. They are left "unprotected" beyond their inherent material properties.

This A/B designation applies to Types I, II, III, and V. Type IV (Heavy Timber) does not have this distinction; it is simply Type IV-HT.

Which construction type is the most fireproof?

Type I-A is the most fire-resistive construction type defined by the IBC. It requires the highest hourly fire-resistance ratings for its structural components, such as a 3-hour rating for the primary structural frame and a 2-hour rating for floor construction, all built from non-combustible materials. This robust construction is designed to contain a fire for a long duration and prevent structural collapse.

What construction type is a typical high-rise office building?

A typical high-rise office building is Type I-A construction. The IBC defines a high-rise in §202 as a building with an occupied floor more than 75 feet above the lowest level of fire department vehicle access. IBC §403.2.1 explicitly requires all high-rise buildings to be of Type I construction. Due to the extreme life-safety risk, the most stringent requirements of Type I-A are almost universally applied.

Is my single-family house a Type V-B building?

Yes, a typical single-family house in the United States built with light wood framing is classified as Type V-B construction. It is built with combustible materials (wood), and its structural elements (stud walls, joists, rafters) are not required to have a fire-resistance rating under Table 601. While some assemblies like the wall between an attached garage and the house require a rating (IRC §R302.6), this does not change the overall classification of the building.

Does adding sprinklers to my building change its construction type?

No, adding an automatic sprinkler system does not change a building's construction type. A building constructed as Type V-A remains Type V-A even after a full NFPA 13 sprinkler system is installed.

However, the presence of a sprinkler system provides enormous benefits and "trade-offs" within the code. IBC Chapter 5 allows for significant increases in allowable height and area if a building is sprinklered. For example, a multi-story building that would be too large as a Type V-A might be permissible as a Type V-A with sprinklers. Sprinklers can also lead to reductions in rating requirements for some elements, like corridors, but the base classification from Chapter 6 remains the same.

What's the fire rating on a standard wood stud wall in a Type V-B house?

In a Type V-B building, a standard interior non-load-bearing wood stud wall has no required fire-resistance rating based on the construction type itself. Table 601 lists "0" hours for all elements in Type V-B.

However, a typical assembly of 2x4 wood studs with 1/2-inch gypsum wallboard on each side can inherently provide a fire-resistance rating of about 30 minutes when tested. An assembly with 5/8-inch Type X gypsum wallboard on each side is a common way to achieve a 1-hour fire-resistance rating, as prescribed in IBC Table 721.1(2) or by a listed UL design. This rating might be required for specific conditions (like separating dwelling units) but not because the building is Type V-B.

Is there a Type I-C or Type II-C construction?

No. The International Building Code only uses the designations 'A' (Protected) and 'B' (Unprotected) to subdivide the main construction types. There are no "C" subtypes or other letter designations in the IBC classification system.

Additional Considerations for Design Professionals

Common Mistakes and Misinterpretations

• Confusing Materials with Type: Assuming a steel building is automatically Type I. The fire-resistance rating, not just the material, determines the final classification.
• Applying Table 601 Ratings Globally: Believing that in a Type V-A building, all walls must be 1-hour rated. The rating only applies to the specific elements listed in the table (frame, bearing walls, floors, roof).
• Sprinkler Trade-off Errors: Incorrectly assuming sprinklers allow you to change the construction type (e.g., build a Type V-B building to the height limits of a Type V-A). Sprinklers increase the allowable area/height for a given construction type; they don't change the type itself.
• Ignoring Mixed Occupancies: Forgetting that when a building contains multiple occupancies, the allowable height and area must be calculated based on the most restrictive combination, often requiring a more fire-resistive construction type.

Coordination Between Disciplines

The construction type is a team decision with cascading effects:

• Architect: Selects the type based on program, cost, and code-allowed size. This decision is documented on the code analysis sheet of the drawings.
• Structural Engineer: Designs the structural system to meet the material (combustible/non-combustible) and fire-resistance rating requirements. This includes specifying beam sizes for Heavy Timber, concrete cover for rebar, or the thickness of SFRM on steel.
• MEP & Fire Protection Engineers: Must design systems that respect the fire-resistance ratings. All penetrations through rated floors, walls, and roofs must be detailed with approved firestop systems (IBC §714). Ductwork passing through rated walls may require fire or smoke dampers (IBC §717).
• Inspector/Plan Reviewer: Verifies that the design documents and field installation align with the declared construction type and all its associated rating requirements.

Jurisdictional Variations

While the IBC model code is used almost everywhere, local jurisdictions can and do amend it. Major cities like New York and Chicago have their own building codes with different classification systems or requirements. For example, the Chicago Building Code has historically used a different numbering system for construction types. It is absolutely essential to verify the adopted code and any local amendments for the specific project location before finalizing the construction type.

Frequently Asked Questions (FAQ)

What is "Heavy Timber" (Type IV-HT) construction?

Heavy Timber construction uses massive wood members for the interior structure. The IBC prescribes minimum cross-sectional dimensions for these members. The principle is that these thick timbers char at a slow, predictable rate in a fire, maintaining structural integrity long after light-frame construction would have failed.

Can you mix construction types in one building?

Yes, under specific conditions. IBC §510 allows for mixed types, most commonly by placing a building of a lighter construction type (e.g., Type V-A) on top of a more robust "podium" (e.g., Type I-A). This is common in mid-rise residential buildings with a concrete ground floor for retail/parking and wood-framed apartments above.

How does occupancy classification relate to construction type?

Occupancy classification (e.g., Assembly, Business, Residential) and construction type are the two key variables used in IBC Chapter 5 to determine the maximum allowable building height and area. A high-hazard occupancy will be severely restricted in a less fire-resistive construction type like V-B, while a low-hazard occupancy can be much larger.

Where is the construction type listed on construction documents?

The construction type must be clearly stated on the cover sheet or in the general notes of the architectural drawings, typically as part of a "Code Analysis" or "Building Data" block that also lists the occupancy, sprinkler status, and allowable vs. actual height and area.

Does the exterior siding or cladding material determine the construction type?

No. The construction type is determined by the fire-resistance and combustibility of the load-bearing structure, not the exterior veneer. For example, a wood-framed Type V building can have non-combustible brick veneer siding without changing its classification.

What is the code definition of "non-combustible"?

‍IBC §703.5 defines a non-combustible material as one that, in the form in which it is used and under the conditions anticipated, will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. Materials passing ASTM E136 are considered non-combustible. Steel, concrete, and masonry are primary examples.

How do I determine the required rating for a specific building element?

First, identify your building's construction type. Then, find that type in the columns of IBC Table 601. Finally, find the specific building element (e.g., "Floor construction," "Interior bearing walls") in the rows of the table. The cell where the row and column intersect gives the required hourly fire-resistance rating.

‍