TYPE IA (ISO 6): The main structural elements are noncombustible. Examples of these materials would be masonry and concrete.Basically, all concrete construction.

TYPE IB (ISO 5): The main structural elements are noncombustible. Examples of these materials are heavy steel with spray-on insulation or enclosed in double layers of sheetrock. Basically, protected steel construction.

TYPE IIA (ISO 4): The main structural elements of the walls are masonry or concrete. The roof support is unprotected metal beams or bar joists.

TYPE IIB (ISO 3): The main structural elements of the walls and roof are unprotected steel. These are lightweight metal buildings like “Butler Buildings” and Quonset Huts. Caution - if metal siding is affixed to wood members, construction is TYPE VB.

TYPE IIIA (ISO 2): A combination of building construction comprised of exterior walls of masonry or concrete and roof/floors of combustible material that has been treated to have at least a 1-hr fire resistance rating.

TYPE IIIB (ISO 2): Same as TYPE IIIA, except that the roof/floors of combustible materials have no fire resistance rating. This is the most common of ISO 2 construction where the underside of the roof is seen with wood construction but the walls are masonry or concrete.

TYPE IV (ISO 2):Known as heavy timber or mill construction in which the exterior walls are of noncombustible materials. Interior building elements utilize wood structural members and heavy wood decking. Building elements do not contain concealed spaces and contain systems that prevent a fire from the exterior into unprotected openings.

TYPE VA (ISO 1): A wood-framed building. Protected construction having all major building elements with at least a 1-hour fire-resistance rating. Exception: non-load bearing interior walls and partitions having no rating.

TYPE VB (ISO 1): A wood-framed building having no fire-resistance ratings. This is the most commonly seen ISO 1 construction.

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Building Types NFPA