How to Determine a Building's Construction Type

Co-authored by Agustin Renoj

Last Updated: July 10, 2024 Approved

Determining a building's construction type is dependent on quite a number of different factors and requires a keen eye for detail. If you would like to be able to identify a building's construction type, start with Step 1 below for an overview of how it's done. You will also find specific information on all six building types.

Method 1

Method 1 of 7:

Overview for Assessing Construction Type

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1
How building class is determined: All buildings must be classified into one of six construction classes (see number 3). Classification of a building class is based on two factors: building elements and fire-resistance rating. These factors may not be included in the submission/documentation, in which case additional information will need to be requested.
- Building elements: The building materials used in the construction of the following elements are the foundation for classification, be they wood, steel, or masonry.
  - Structural frame
  - Exterior bearing walls
  - Interior bearing walls
  - Exterior nonbearing walls and partitions
  - Interior non-bearing walls and partitions
  - Floor construction, including supporting beams and joists
  - Roof construction, including supporting beams and joists, are comprised of
- Fire-resistance rating: This is the other factor in determining construction class. The building materials used in the construction of the building elements above will have a fire-resistance rating. Fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time (ex. 0 hours, 1 hour, or 2 hours), or it may entail a host of other criteria involving other evidence of functionality or fitness for purpose.
  - “Minimum” rule: It is important to remember when selecting the construction class that the building is only as strong as its weakest element. For example, a masonry building may have an unprotected wood roof. The wood roof is the weakest member such that it has no fire-resistance. Thus, the construction class would be Joisted Masonry (see below). Now imagine this same building with a metal deck roof. So long as the supporting members of the building do not contain wood then this building would be Masonry Noncombustible (see below).
2
What to ask: To determine the ISO class of the building, we must therefore know the following composition of the building elements:
- Structural frame
- Bearing walls (interior and exterior)
- Floor construction
- Roof construction
- What the fire rating of the materials
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3
Building classes: All construction types must be classified into the following (all of which are explained extensively below):
- Frame Construction (ISO Class I, IBC Type V)
- Joisted Masonry (ISO Class 2, IBC Type III, IBC Type IV)
- Light Noncombustible (ISO Class 3, IBC Type IIB)
- Masonry Noncombustible (ISO Class 4, IBC Type IIA)
- Modified Fire Resistive (ISO Class 5, IBC Type IB)
- Fire Resistive (ISO Class 6, IBC Type IA)
4
International Building Code (IBC) versus Insurance Services Office (ISO):

These are two major sources identifying construction types, both of which will be addressed in the construction types outlined below. ISO is traditionally what insurance companies use to denote type, whereas IBC is what architects and builders use.^{[1]
X
Research source} While one company may use ISO classifications, many submission documents may reference IBC classifications and it is important to be able to convert this to an ISO classification. (There have been situations where a frame building has been incorrectly classified as fire resistive because the submittal was read incorrectly!) The following explains what is expected under both:
- International Building Code (IBC): This is a model building code developed by the International Code Council (ICC). It has been adopted throughout most of the United States. A large portion of the International Building Code deals with fire prevention. It differs from the related International Fire Code in that the IBC handles fire prevention in regards to construction and design and the fire code handles fire prevention in an ongoing basis. Parts of the code reference other codes including the International Plumbing Code, the International Mechanical Code, the National Electric Code, and various National Fire Protection Association Standards. IBC is more descriptive and also includes A or B types of construction for each class.
  - A is protected, meaning that all structural members of a building or structure have an additional fire rated coating or cover by means of sheetrock, spray on, or other approved method. The additional fire rated coating or cover extends the fire resistance of the structural members by at least 1 hour.
  - B is unprotected, meaning that all structural members of a building or structure have no additional fire rated coating or cover. Exposed members are only fire resistant according to their natural ability, characteristics, and fire rating.
- Insurance Services Office (ISO): This is a provider of data, underwriting, risk management, and legal/regulatory services to property-casualty insurers and other clients.

Method 2

Method 2 of 7:

Frame Construction (ISO Class I, IBC Type V)

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Frame construction is ISO Class 1. ISO Class 1 encompasses IBC Type VA and IBC Type VB. Regardless of whether the IBC classification is A (protected) or B (unprotected) the ISO Class is 1.
2
Building elements:
- Frame buildings are buildings with exterior walls, floors, and roofs with combustible construction — or buildings with exterior walls of noncombustible or slow-burning construction with combustible floors and roofs.
- Frame buildings generally have roof, floor, and supports of combustible material, usually wood, and combustible interior walls.
- Two variations on frame construction don't change the construction class:
  - Masonry veneer (brick veneer)- Masonry veneer is thin layers of brick, stone, or stucco, used for appearance purposes rather than structural support.
  - Metal clad - A building with a metal exterior wall may not look like frame construction, but when the metal skin is attached to wood studs and joists, ISO classifies the building as frame.
- Other conditions that lead to classification as frame construction include:
  - Metal walls or floors sheathed with combustible materials
  - Metal floors or roofs with combustible insulation or ceiling material attached to the underside or within 18 inches (45.7 cm) of horizontal supports
  - Composite assemblies of noncombustible materials with combustible materials
3
Advantages:
- easy to erect and alter
- economical
- versatile
- performs well in Earthquake areas – can move
4
Disadvantages:
- fire can spread rapidly
- highly damageable
- may become unstable in a fire
- may include enclosed spaces where fire can spread undetected

Method 3

Method 3 of 7:

Joisted Masonry (ISO Class 2, IBC Type III, IBC Type IV)

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Joisted Masonry construction is ISO Class 2. ISO Class 2 encompasses IBC Type IIIA and IBC Type IIIB. Regardless of whether the IBC classification is A (protected) or B (unprotected) the ISO Class is 2. IBC Type IV is Heavy Timber construction and is considered ISO Class 2. The reason is that the heavy timbers perform well and do not fail early in a fire.
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Building elements: Joisted Masonry buildings are buildings with exterior walls of masonry or fire-resistive construction rated for not less than one hour and with combustible floors and roofs. There are several types of masonry used in the exterior bearing walls of joisted masonry buildings:
- brick
- concrete — either reinforced or non reinforced
- hollow concrete masonry units
- tile
- stone
- note that exterior bearing walls may also be any noncombustible materials with fire-resistance ratings of not less than one hour
3
Variations: There's one variation on joisted masonry construction that doesn't change the construction class — heavy timber or mill construction. Heavy timber construction uses wood members much larger than those found in frame (Construction Class 1) or other joisted masonry construction. If the building uses steel columns or beams for walls, the beams must be protected so they have a fire-resistance rating of not less than one hour. Heavy Timber Construction (IBC Type IV); ISO classifies the building as heavy timber construction if it meets these requirements:
- walls of masonry construction
- floors of 3 inch (7.6 cm) wood plank or 4 inch (10.2 cm) laminated plank, both surfaced with 1 inch (2.5 cm) flooring
- roof of 2 inch (5.1 cm) wood plank, 3 inch (7.6 cm) laminated plank, or 1-1/8 inch tongue-and-groove plywood deck
- wood column supports not less than 8 inch (20.3 cm) x 8 inch (20.3 cm), wood beams or girders not less than 6 inch (15.2 cm) x 6 inch (15.2 cm), or protected metal
4
Advantages:
- harder to ignite
- consumed more slowly by fire
- more structural stability
- greater salvage value
- lack of concealed spaces (Heavy Timber)
5
Disadvantages:
- floors and roofs of combustible materials subject to damage by fire
- presence of concealed spaces

Method 4

Method 4 of 7:

Light Noncombustible (ISO Class 3, IBC Type IIB)

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Light Noncombustible construction is ISO Class 3. ISO Class 3 encompasses IBC Type IIB (unprotected).
2
Building elements: Light Noncombustible buildings are buildings with exterior walls of light metal or other noncombustible material and with noncombustible floors and roofs:
- buildings with exterior walls, floors, and roofs of noncombustible or slow-burning materials
- building supports of noncombustible or slow-burning materials
- noncombustible or slow-burning roof decks on noncombustible or slow-burning supports — regardless of the type of insulation on the roof surface
3
Advantages:
- easy to erect
- economical to build
- uses materials that don’t readily burn
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Disadvantages:
- contains steel, which loses strength at high temperatures
- highly damageable buildings
- unstable buildings under fire conditions
- uses slow-burning materials that do burn — adding fuel to a fire

Method 5

Method 5 of 7:

Masonry Noncombustible (ISO Class 4, IBC Type IIA)

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Masonry Noncombustible construction is ISO Class 4. ISO Class 4 encompasses IBC Type Type IIA (protected).
2
Building elements: Masonry Noncombustible buildings are buildings with exterior walls of masonry material and with noncombustible or slow burning floors and roofs.
- buildings with exterior walls of masonry — not less than four inches thick, or
- buildings with exterior walls of fire-resistive construction — with a rating of not less than one hour, and
- noncombustible or slow-burning floors and roofs — regardless of the type of insulation on the roof surface
3
Advantages:
- Uses floors and roof supported by superior exterior bearing members that provide for stability and are less likely to collapse during a fire
- Uses materials that don’t readily burn
4
Disadvantages:
- Uses unprotected steel for interior members of floors and roof, and steel loses strength and becomes less stable and more damageable at high temperatures
- Uses slow-burning materials that do burn — adding fuel to a fire

Method 6

Method 6 of 7:

Modified Fire Resistive (ISO Class 5, IBC Type IB)

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Modified Fire Resistive construction is ISO Class 5. ISO Class 5 encompasses IBC Type IB.
2
Building elements: Modified Fire Resistive Buildings are buildings where the exterior bearing walls and load-bearing portions of exterior walls must be of noncombustible materials or of masonry, but exterior nonbearing walls and wall panels may be slow-burning, combustible, or with no fire- resistance rating.
- Buildings with exterior walls, floors, and roofs of masonry materials described in the definition of fire resistive (Construction Class 6) — less thick than required for fire-resistive structures but not less than four inches thick, or
- Fire-resistive materials with a fire-resistance rating less than two hours but not less than one hour
3
Variations:
- Structural steel protection: Note that modified fire-resistive buildings also include structural steel protection techniques — fire-protection material applied to steel. Materials include:
  - concrete
  - plaster
  - clay tile
  - brick or other masonry units
  - gypsum block
  - gypsum wallboard
  - mastic coatings
  - mineral and fiberboard
  - mineral wool
- Ceilings protecting steel beams or joists: What happens when there is no fire-protection material applied to steel beams or joists that support floors or roofs? ISO still considers a building modified fire resistive if it has a suitable ceiling. Ceilings can be plaster or gypsum wallboard or suspended mineral tile. The entire floor-ceiling (a fire-resistive ceiling protecting a floor) or roof-ceiling (a fire-resistive ceiling protecting roof supports) should conform to construction details in a UL-listed or Factory Mutual (FM)-approved design. ISO individually evaluates each approved design.
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Advantages:
- uses noncombustible materials
- allows greater height and area than other construction classes
- uses load-bearing members or assemblies that resist damage from fire
5
Disadvantages:
- expensive to construct and repair
- provides a false sense of security

Method 7

Method 7 of 7:

Fire Resistive (ISO Class 6, IBC Type IA)

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Fire Resistive construction is ISO Class 6. ISO Class 6 encompasses IBC Type IA.
2
Building elements: The exterior bearing walls and load-bearing portions of exterior walls must be of noncombustible materials or of masonry, but exterior nonbearing walls and wall panels may be slow-burning, combustible, or with no fire-resistance rating.
- Walls:
  - solid masonry, including reinforced concrete not less than four inches thick
  - hollow masonry not less than 12 inches (30.5 cm) thick
  - hollow masonry less than 12 inches (30.5 cm) thick, but not less than eight inches thick with a listed fire-resistance rating of not less than two hours
  - assemblies with not less than a two-hour fire-resistance rating
- Floors and roofs:
  - reinforced concrete not less than four inches thick
  - assemblies with not less than a two-hour fire-resistance rating
- Structural metal supports:
  - Horizontal and vertical load-bearing protected metal supports — including pre-stressed and post-tensioned concrete units — with not less than a two-hour fire-resistance rating
Both pre- and post-tensioned concrete units have steel cables installed in the concrete to provide tensile strength. With pre-stressed concrete units, builders pull the cables tight before pouring the concrete and release them as the concrete cures. With post-tensioned concrete units, builders pull one end of the cable tight after pouring the concrete.
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Advantages:
- uses noncombustible materials
- allows greater height and area than other construction classes
- uses load-bearing members or assemblies that resist damage from fire
5
Disadvantages:
- expensive to construct and repair
- provides a false sense of security

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Question

I bought a house. How do I determine which ISO class it is?

Community Answer

Most residential homes are wood framed which would designate them as IBC class V or ISO class 1, however a home constructed with 4" + thick solid masonry exterior walls such as filled block or solid brick could be classified as IBC type II or ISO class 4 dependent on if the home has a concrete floor and the ceiling is 5/8" drywall or plaster.

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What does the B mean in a 5 B structure?

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It means it is an unprotected construction. Members rely on their natural fire resistance, rather than a cladding or other protection added to them to provide fire resistance.

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What construction type would a mobile/manufactured home be?

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Mobile homes are not regulated by IBC codes, but they do have their own construction codes through the federal Department of Housing and Urban Development (HUD). Some newer mobile homes can be classified as similar to type-V construction.

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